Chemistry

Ferrofluids

2012-07-26T00:09:00.000-07:00

Magnetic colloids, also known as a ferrofluids (FFs), are colloidal suspensions of single-domain magnetic nanoparticles (ferro- or ferri-magnetic), with typical dimensions of about 10 nm, dispersed in a polar or non-polar liquid carriers like water, benzene, toluene or hexane¹.These are often coated with surfactants to prevent agglomeration due to magnetic forces or van der Waals forces. The magnetic properties of these magnetically controllable nanofluids is of great scientific interest and is intensively researched^2-4.

Ferrofluids contain about 10¹⁷ particles/mL and is opaque to visible light⁴. Ferrofluids posses magnetism and fluid behavior incorporated in them⁵. Since the rheological behavior of ferrofluids can be controlled by means of magnetic field, they can instantaneously change from a liquid to a solid like state and vice versa. They do not retain magnetism in the absence of an externally applied field and also form spikes in the magnetic field due to induced dipoles in the superparamagnetic nanoparticles aligning in the direction of applied magnetic field which can be observed only at or above a certain concentration typically beyond 10 vol %⁴.

The important applications of ferrofluids include semiactive shock absorbers, dampeners for seismic damage controls in civil engineering, in seals, valves, robotics and microelectronic devices^5,6. Many of the ferrofluid applications require high magnetization capacity and long-term stability. The dimension of the nanoparticles and the steric and coulombic interactions between these particles are of prime importance in determining the colloidal stability⁷. According to Rosensweig (1985), an ideal ferrofluid is a colloidal suspension of nanoparticles of ferro- or ferrimagnetic materials dispersed in to a liquid carrier that does not settle out, even after long exposure to a force field (gravitational or magnetic)⁸.

Reference

1. C. Scherer and A. M. F. Neto, Brazilian Journal of Physics 2005, 35, 718.

2. P. Berger, N. B. Adelman, K. J. Beckman, D. J. Campbell, A. B. Ellis and G. C. Lisensky, J. Chem. Edu 1999, 76, 943.

3. P. Bain and T. J. McCarthy, Langmuir 2010, 26, 6145.

4. N. Jain, Y. Wang, S. K. Jones, B. S. Hawkett and G. G. Warr, Langmuir 2010, 26, 4465.

5. N. Jain, X. Zhang, B. S. Hawkett and G. G. Warr, ACS Appl. Mater. Interfaces 2011, 3, 662.

6. J. Fang, H. Wang, Y. Xue, X. Wang and J. Lin, ACS Appl. Mater. Interfaces 2010, 2, 1449.

7. J. A. Gomes, M. H. Sousa, F. A. Tourinho, R. Aquino, G. J. da Silva and R. Perzynski, J. Phys. Chem. C 2008, 112, 6220.

8. R. E. Rosensweig. Ferrohydrodynamics, Cambridge University Press, Cambridge. 1985.

Resrarch on Hydrogen Energy- On Progress

2012-04-30T23:58:00.001-07:00

Indian Association for Hydrogen Energy and Advanced Materials (IAHEAM)is one of the professional societies registered in 2011, which is catalysing the research on Hydrogen Energy. Young researchers are given immense opportunities through the various programs like HEAM Young scientist 2012, HEAM Scholar 2012, HEAM Thesis 2012 and many others. For more information, you can visit their website at : http://www.iaheam.org/ and download the news letters. Researchers and eminent personalities working in this field are given opportunities to express their in-depth views and inovative ideas related to this field.

Laws Of Thermodynamics

2011-11-21T23:51:00.000-08:00

Thermodynamics is the branch of science which deals with the study of the flow of heat or any other forms of energy into or out of a system as it undergoes a physical or chemical change. The study of thermodynamics is based on three important laws or generalisations which is confirmed by well established experimental results. These laws are known as the Zeorth, First, Second and Third Laws of Thermodynamics.

Zeroth Law of Thermodynamics states that : ''If two systems are each in thermal equilibrium with a third, they are also in thermal equilibrium with each other.''

A system is said to be in thermal equilibrium when it experiences no net change in thermal energy. If A, B, and C are distinct thermodynamic systems, the zeroth law of thermodynamics can be expressed as:

"If A and C are each in thermal equilibrium with B, A is also in equilibrium with C."

The zeroth law was not initially recognized as a law, as its basis in thermodynamical equilibrium was implied in the other laws. Once the importance of the zeroth law for the definition of temperature was realized, it was impracticable to renumber the other laws, hence it was numbered the zeroth law.

First law of thermodynamics states that : A change in the internal energy of a closed thermodynamic system is equal to the difference between the heat supplied to the system and the amount of work done by the system on its surroundings. E = q - W

The first law is in fact, an application of the principle known as the Law of Conservation of Energy to thermodynamic systems. Other statements of the first law are:

1. The total energy of an isolated system remains constant though it may change from one form to another.

2. Energy can neither be created nor destroyed, although it can be changed from one form to another.

3. Total energy of a system and surroundings remains constant.

4. It is not possible to construct a perpetual motion machine, i.e., a machine which can produce work without expenditure of of energy.

Second law of thermodynamics states that : ''Heat cannot spontaneously flow from a colder location to a hotter location.''

The need for the second law arise from the fact that for a particular process or change, the first law helps us to balance the internal energy, heat released and work done on the system or by the system. But, it does not sy anything about the thermodynamic possibility of the process to occur.

The second law explains that ''whenever a spontaneous or irreversible process takes place, it is accompanied by an increase in the total entropy of the universe.'' All spontaneous processes take place in the direction of increasing entropy. Entropy is a state quantity that is a measure of the randomness or disorder of the molecules of the system.

Third law of thermodynamics states that : As a system approaches absolute zero, all processes cease and the entropy of the system approaches a minimum value.

The third law does not give any any new concept. It only places a limitation to the value of entropy of a crystalline solid. The entropy of a substance varies directly with temperature. If we increase the temperature of a system, for example water, the molecules attain kinetic energy and starts moving restlessly resulting in an increasing entropy of the system. But, if we cool the system, the vibration of molecules slow down limiting the freedom of movement thereby decreasing the entropy. Finally, at absolute zero all molecular vibrations ceases resulting in nil disorder and the entropy of the system will be zero. i.e., S=0 at T=0K.

Hydrogen Energy

2011-11-03T04:40:00.000-07:00

INTRODUCTION

Hydrogen is the simplest element known to exist having one proton and one electron. It has the highest energy content of any common fuel by weight, but the lowest energy content by volume. It is the lightest element and a gas at normal temperature and pressure. It is also the most abundant gas in the universe, and the source of all the energy we receive from the sun through the process of nuclear fusion. Hydrogen is always present in the compound form in combination with elements like oxygen (H2O), carbon (CH4), etc. It is one of the most abundant elements on the earth’s crust.

Hydrogen is one of the most promising energy carriers in future. It is a high efficiency, low polluting fuel that can be used for transportation, heating, and power generation in places where it is difficult to use electricity. Hydrogen was observed and collected long before it was recognized as a unique gas by Robert Boyle in 1671, who dissolved iron in diluted hydrochloric acid.

HYDROGEN PRODUCTION AND BENEFITS

Since molecular hydrogen does not occur abundantly on earth’s atmosphere, it must be manufactured. There are several ways for producing hydrogen for commercial purposes. Hydrogen can be produced using diverse, domestic resources including fossil fuels, such as coal and natural gas; nuclear; and biomass and other renewable energy technologies, such as wind, solar, geothermal, and hydroelectric power. Some methods for hydrogen production are natural gas reforming, renewable electrolysis, photo electrochemical processes, high temperature thermochemical water splitting etc many of which are only in the early stages of development. Hydrogen can generate power without exhaust emission in fuel cells. Environmental benefits and health benefits are even greater when hydrogen is produced from low or zero-emission sources such as solar energy, wind and and nuclear energy. Hydrogen combustion produces only water as the by-product. It can also compensate for the intermittency of renewable energy production.

FOOD ADDITIVES

2011-04-27T01:43:00.000-07:00

Food additives are substances added to food to preserve their freshness, improve their visual appeal or taste, enhance their flavor etc. Humans have been using food additives since long time. For example, preserving food by adding vinegar, oil, sugar and salt. Since more number of food additives, both artificial and natural ones, have been introduced with the advent of processed food, it is important to know the type of chemicals used as food additives.

There are various types of food additives used with different purposes. Some of them are :-

Food Acids: Added in order to add tartness to the flavor of foods, also act as anti-oxidants and preservatives. Example:- citric acid, lactic acid, fumaric acid, malic acid, tartaric acid, phosphoric acid (in colas), and vinegar.

Food coloring: Colorings are added to food to replace colors lost during preparation, or to make food look more attractive. Example:- Caramel coloring (E150), made from caramelized sugar. The great bulk of artificial colorings used in food are synthetic dyes.

Flavors: These are added in order to enhance the flavors of foods, which can be either artificial or made from natural sources. Some of the common flavoring agents are monosodium glutamate (MSG), maltol, and disodium guanylate. Some of the compounds used to produce artificial flavors are;

Chemical	Odor
Ethylvanillin	Vanilla
Isoamyl acetate	Banana
Benzaldehyde	Bitter almond
Cinnamic aldehyde	Cinnamon
Ethyl propionate	Fruity
Methyl anthranilate	Grape
Limonene	Orange

Preservatives: These are chemicals that are added to food products in order to prevent food from spoiling due to the growth of micro-organisms like bacteria and fungi. Apart from being anti-microbial, there are also preservatives that are anti-oxidants. Common antimicrobial preservatives include calcium propionate, sodium nitrate, sodium nitrite, sulfites (sulfur dioxide, sodium bisulfite, potassium hydrogen sulfite, etc.) and disodium EDTA. Antioxidants include BHA and BHT.

Sodium nitrite and sodium nitrate are two closely related chemicals used for centuries to preserve meat. Nitrate is harmless but, it is readily converted to nitrite where it can form nitrosamines, extremely powerful cancer-causing chemicals. The chemical reaction occurs most readily at the high temperatures of frying. Nitrite has long been suspected as being a cause of stomach cancer.

Antioxidants: Antioxidants such as vitamin C act as preservatives by inhibiting the effects of oxygen on food or preventing them from becoming rancid, and can be beneficial to health.

Stabilizers: Stabilizers, thickeners and gelling agents, like agar or pectin (used in jam) give foods a firmer texture.

Emulsifiers: These are added in order to enable oils and water to emulsify, or remain combined together, such as homogenized milk, mayonnaise, and ice-cream. Examples of food emulsifiers are egg yolk where the main emulsifying agent is lecithin, honey, and mustard, where a variety of chemicals in the mucilage surrounding the seed hull act as emulsifiers.

There has been significant controversy associated with the risks and benefits of food additives. Some artificial food additives have been linked with cancer, digestive problems, neurological conditions, ADHD, heart disease or obesity. Natural additives may be similarly harmful or be the cause of allergic reactions in certain individuals.To regulate these additives, and inform consumers, each additive is assigned a unique number, termed as "E numbers", which is used in Europe for all approved additives.

Associative and Dissociative Substitution

2011-02-17T06:44:00.000-08:00

Associative substitution describes a pathway by which coordination and organometallic complexes interchange ligands. Associative mechanism resembles the SN2 mechanism in organic chemistry. The opposite pathway is dissociative substitution, being analogous to SN1 pathway. Intermediate pathways exist between the pure associative and pure dissociative pathways, these are called interchange mechanisms.

Associative mechanism

Metal size should be large
Ligand size should be small
Incoming group should have pi bonding ability (CN- is a pi acid ligand)
Rate depend on the nature of nucleophile
Trigonal bipyramidal intermediate

    L5MX + Y =   L5MXY    (SLOW)
    L5MXY =    L5MY + X        (FAST)

    Rate = k [L5MX] [Y]

Dissociative mechanism

Metal size should be small
Ligand size should be large
Rate does not depend on nature of the nucleophile

    L5MX = L5M + X   (SLOW)
    L5M + Y = L5MY   (FAST)

    Rate = k [L5MX]

Zinc Oxide Nanomaterials

2011-02-11T19:08:00.000-08:00

Zinc oxide is one of the most important II-VI semiconductor material with direct wide band gap (3.37 eV or 375 nm), good transparency, high electron mobility (>100 cm2/Vs) photoconductivity, strong room-temperature luminescence large exciton and biexciton energies of 60 meV and 15 meV respectively. It has attracted increasing attention due to its excellent optical and electrical properties, inexpensiveness, relative abundance, chemical stability towards air, ability to produce significant quantum confinement effect.

Since ZnO is an important trace element for humans, it is environment friendly and suitable for in vivo applications. Zinc oxide has high refractive index, high thermal conductivity, antibacterial and UV-protection properties. Consequently, it is added in various materials and products including plastics, ceramics, glass, cement, rubber, lubricants, paints, ointments, adhesives, sealants, pigments, foods, batteries, ferrites, fire retardants etc. and non-toxicity, making it a suitable additive for textiles and surfaces that come in contact with humans.

Nanosized ZnO has immense importance due to its multifunctionality. Thermal stability, irradiation resistance and flexibility to form different nanostructures are the advantages that highlight its promising applications in solar cells and electronic devices, ultraviolet-light detectors and photo diodes and in catalysis.Structures like nanowires, nanobelts and nanorings are of great interest in photonics research, optoelectronics, nanotechnology, and biomedicine. Therefore, the controlled synthesis of various ZnO nanostructures such as nanocrystals, nanowires, nanobelts and other complex nano architectures has been extensively explored.

Zinc oxide nanostructure growth is heavily researched presently. The substance is likely to have the largest variety of nanostructures (and their associated properties) among all known materials. Its hexagonal lattice can easily match catalyst lattice structure and facilitate controlled growth patterns. Positive zinc surfaces and negative oxygen surfaces create electric dipoles that facilitate polarization growth along certain directions and planes under applied voltage and temperature. Different techniques such as sol-gel, spray pyrolysis, thermal evaporation, wet chemical processes etc are used for the synthesis of ZnO nanomaterials.

ZnO quantum dots exhibit emission bands in the ultraviolet and visible regions as shown by its photoluminescence spectra. The UV emission band at around 370 nm is usually attributed to the interband transition or the exciton combination in ZnO . Even though the emissions in the visible region are associated with the electronic defects due to surface states or trapping effects in the QDs, there are still
controversies related to the unambiguous electron transitions. In the visible region, the blue emission in ZnO QDs has considerable importance in biological fluorescence labeling. Several studies indicate that ZnO is one of the most efficient oxide-based phosphors in both photoluminescence (PL) and electroluminescence (EL).

Considerable studies have been done on the properties of metal incorporated fluorescent materials like Co, Ni and Fe doped ZnO aiming to develop efficient magnetic semiconductors. The properties of inner-transition metal doped ZnO nanoparticles have also been subjected to tremendous amount of studies. The combination of magnetic and optical properties provides the composites significantly important applications in biomedical fields including drug targeting, bioseparation and diagnostic analysis.

ENDOSULFAN

2011-02-06T23:05:00.000-08:00

Endosulfan is an organochlorine insecticide and acaricide with a cyclodiene sub-group. It is highly toxic and can bioaccumulate in organisms. It can also act as an endocrine disruptor i.e., it interferes with the synthesis, secretion, transport, binding, action, or elimination of natural hormones in the body that are responsible for the maintenance of homeostasis (normal cell metabolism), reproduction, development, and behavior.

Endosulfan has been used in agriculture around the world to control insect pests including whiteflys, aphids, leafhoppers, Colorado potato beetles and cabbage worms. Because of its non-specificity it impacts many beneficial insects also. It is also used as a wood preservative.

Comercial names : Beosit, Thiodan, Cyclodan, Malix, Thifor, Endocide etc.

Specifically, it is produced by the Diels-Alder reaction of hexachlorocyclopentadiene with cis-butene-1,4-diol and subsequent reaction of the adduct with thionyl chloride.Technical endosulfan is a 7:3 mixture of stereoisomers, designated α and β. α- and β-endosulfan are conformational isomers arising from the pyramidal stereochemistry of sulfur. α-Endosulfan is the more thermodynamically stable of the two, thus β-endosulfan irreversibly converts to the α form, although the conversion is slow.

Endosulfan breaks down into endosulfan sulfate and endosulfan diol, both of which, according to the EPA, have "structures similar to the parent compound and are also of toxicological concern. Since it neither dissolves in water easily nor stick to soil particles readily, its transport to other regions is easier.

India the world's largest user of endosulfan, and a major producer with three companies—Excel Crop Care, H.I.L., and Coromandal Fertilizers—producing 4,500 tonnes annually for domestic use and another 4,000 tonnes for export.

In 2001, in Kerala, India, endosulfan spraying became suspect when linked to a series of abnormalities noted in local children. Initially endosulfan was banned, yet under pressure from the pesticide industry this ban was largely revoked.

Endosulfan is acutely neurotoxic to both insects and mammals, including humans. Symptoms of acute poisoning include hyperactivity, tremors, convulsions, lack of coordination, staggering, difficulty breathing, nausea and vomiting, diarrhea, and in severe cases, unconsciousness. Doses as low as 35 mg/kg have been documented to cause death in humans, and many cases of sub-lethal poisoning have resulted in permanent brain damage. Farm workers with chronic endosulfan exposure are at risk of rashes and skin irritation.

Quantum tunnelling

2011-01-11T07:22:00.000-08:00

Quantum tunnelling refers to the quantum mechanical phenomenon where a particle tunnels through a barrier that it classically could not surmount because its total mechanical energy is lower than the potential energy of the barrier.

Quantum tunnelling is where a particle is found outside a confining potential despite it having insufficient energy to cross the barrier classically.

Particles are confined to certain regions of space because they do not have enough energy to escape from that region. These regions are defined by potential energy curves.

The effect arises from the fact that a wavefunction does not fall abruptly to zero at the walls of a container (unless the potential is infinite), but decays exponentially inside the barrier. The result of this is that the wavefunction may be non-zero on the far side of the potential barrier and hence, by the Born interpretation of the wavefunction, there is some probability of finding the particle there.

Quantum tunnelling occurs on an extremely small scale. We cannot directly perceive what a particle does when it tunnels so much of our understanding of the process is shaped by the language and imagery of the macroscopic world.

Quantum Tunneling has a number of applications like, it is used to explain the Alpha Decay of the Uranium Nucleus, it is used in fabricating high speed transistors and new age cooling equipments.

STEREOCHEMISTRY

2010-12-02T07:52:00.000-08:00

Stereochemistry involves the study of the relative spatial arrangement of atoms within molecules. It is also known as 3D chemistry because the prefix "stereo-" means "three-dimensionality.

Some basic concepts are;

Isomers are molecules that have the same molecular formula but different arrangements of their constituent atoms.
Stereoisomers are molecules with identical connectivity but different spatial arrangements of their constituent atoms that cannot be interconverted by bond rotation.

In order to categorise stereoisomers it is necessary to prioritise different atomic substituents using the Cahn–Ingold and Prelog sequence rules.

Chirality (cheir, Greek for "hand") refers to objects which are related as non–superimposable mirror images and the term derives from the fact that left and right hands are examples of chiral objects.

An enantiomer is one of a pair of stereoisomers that are related as non–superimposable mirror images.
A solution of a single enantiomer will rotate the plane of plane–polarised light and is referred to as optically active; although this physical property cannot be directly related to absolute configuration of the molecule. An enantiomer is given the prefix (+)– if the rotation is clockwise (dextrorotatory) and (–)– if the rotation is anticlockwise (levorotatory).

An equal mixture of opposite enantiomers is a racemate and solutions of racemic mixtures do not rotate the plane of plane–polarized light.

Enantiomeric Excess = (%Enantiomer A –% Enantiomer B)%

A chiral molecule is a type of molecule that lacks an internal plane of symmetry and has a non-superimposable mirror image. The feature that is most often the cause of chirality in molecules is the presence of an asymmetric carbon atom.

Diastereoisomers are stereoisomers with a different relative configuration and are not related as mirror images. They have different chemical and physical properties.

Cis-trans isomerism or geometric isomerism or configuration isomerism or E-Z isomerism is a form of stereoisomerism describing the orientation of functional groups within a molecule. In general, such isomers contain double bonds, which cannot rotate, but they can also arise from ring structures, wherein the rotation of bonds is greatly restricted.

Importance of Carbon

2010-08-14T08:15:00.000-07:00

Carbon is the chemical element with symbol C and atomic number 6. It is a member of group 14 on the periodic table. The name "carbon" comes from Latin language carbo, coal.

Carbon is Non-metallic
Tetravalent
Has three naturally occurring isotopes ( ¹²C and ¹³C is stable, ¹⁴C is radioactive)
Has many allotropes of which the best known are graphite, diamond, and amorphous carbon

Carbon is the 15th most abundant element in the Earth's crust, and the fourth most abundant element in the universe by mass after hydrogen, helium, and oxygen. It is present in all known lifeforms, and in the human body carbon is the second most abundant element by mass (about 18.5%) after oxygen. This abundance, along with the unique diversity of organic compounds and their unusual catenation ability at the temperatures commonly encountered on Earth, make this element the chemical basis of all known life.

Formation of the carbon atomic nucleus requires a nearly simultaneous triple collision of alpha particles (helium nuclei) within the core of a giant or supergiant star. This happens in conditions of > 100 megakelvin temperature and helium concentration that the rapid expansion and cooling of the early universe prohibited, and therefore no significant carbon was created during the Big Bang.

Carbon is essential to all known living systems, and without it life as we know it could not exist. The major economic use of carbon other than food and wood is in the form of hydrocarbons, most notably the fossil fuel methane gas and crude oil (petroleum). Crude oil is used by the petrochemical industry to produce, amongst others, gasoline and kerosene, through a distillation process, in refineries.

Cellulose is a natural, carbon-containing polymer produced by plants in the form of cotton, linen, and hemp. Cellulose is mainly used for maintaining structure in plants. Commercially valuable carbon polymers of animal origin include wool, cashmere and silk. Plastics are made from synthetic carbon polymers, often with oxygen and nitrogen atoms included at regular intervals in the main polymer chain. The raw materials for many of these synthetic substances come from crude oil.

Organometallic compounds by definition contain at least one carbon-metal bond. A wide range of such compounds exist; major classes include simple alkyl-metal compounds (e.g. tetraethyllead), η²-alkene compounds (e.g. Zeise's salt, and η³-allyl compounds (e.g. allylpalladium chloride dimer; metallocenes containing cyclopentadienyl ligands (e.g. ferrocene); and transition metal carbene complexes.

Carbon black is used as the black pigment in printing ink, artist's oil paint and water colours, carbon paper, automotive finishes, India ink and laser printer toner. Carbon black is also used as a filler in rubber products such as tyres and in plastic compounds. Activated charcoal is used as an adsorbent in filter material in applications as diverse as gas masks, water purification and kitchen extractor hoods and in medicine to absorb toxins, poisons, or gases from the digestive system. Carbon is used in chemical reduction at high temperatures.

Coke is used to reduce iron ore into iron. Case hardening of steel is achieved by heating finished steel components in carbon powder. Carbides of silicon, tungsten, boron and titanium, are among the hardest known materials, and are used as abrasives in cutting and grinding tools. Carbon compounds make up most of the materials used in clothing, such as natural and synthetic textiles and leather, and almost all of the interior surfaces in the built environment other than glass, stone and metal.

HPLC

2010-08-14T07:44:00.000-07:00

High-performance liquid chromatography (or high-pressure liquid chromatography, HPLC) is a chromatographic technique that can separate a mixture of compounds, and is used in biochemistry and analytical chemistry to identify, quantify and purify the individual components of the mixture.

High performance liquid chromatography is basically a highly improved form of column chromatography. Instead of a solvent being allowed to drip through a column under gravity, it is forced through under high pressures of up to 400 atmospheres. That makes it much faster.

It also allows you to use a very much smaller particle size for the column packing material which gives a much greater surface area for interactions between the stationary phase and the molecules flowing past it. This allows a much better separation of the components of the mixture.

The other major improvement over column chromatography concerns the detection methods which can be used. These methods are highly automated and extremely sensitive.

HPLC utilizes different types of stationary phase (typically, hydrophobic saturated carbon chains), a pump that moves the mobile phase(s) and analyte through the column, and a detector that provides a characteristic retention time for the analyte. The detector may also provide other characteristic information (i.e. UV/Vis spectroscopic data for analyte if so equipped). Analyte retention time varies depending on the strength of its interactions with the stationary phase, the ratio/composition of solvent(s) used, and the flow rate of the mobile phase.

The output will be recorded as a series of peaks - each one representing a compound in the mixture passing through the detector and absorbing UV light.

Plasma

2010-06-30T08:15:00.000-07:00

Plasma was first identified in a Crookes tube, and so described by Sir William Crookes in 1879 (he called it "radiant matter").
Plasma, in physics, fully ionized gas of low density, containing approximately equal numbers of positive and negative ions . It is electrically conductive and is affected by magnetic fields. The study of plasma, called plasma physics, is especially important in research efforts to produce a controlled thermonuclear reaction . Such a reaction requires extremely high temperatures; it has been computed that a temperature of about 10 million degrees Celsius would be needed to initiate the reaction between deuterium and tritium.

By passing a very high electric current through plasma great heat is produced and, simultaneously, an electromagnetic field is created, causing the plasma to withdraw from the walls of its container. The contraction of the plasma, called the pinch effect, prevents the container from being destroyed, but the effect may become unstable too quickly for the fusion reaction. The properties of plasma are distinct from those of the ordinary states of matter, and for this reason many scientists consider plasma a fourth state of matter. Interstellar gases, as well as the matter inside stars, are thought to be in the form of plasma, thus making plasma a common form of matter in the universe.

Like gas, plasma does not have a definite shape or a definite volume unless enclosed in a container; unlike gas, in the influence of a magnetic field, it may form structures such as filaments, beams and double layers. Some common plasmas are stars and neon signs.

Chemistry: Lab-on-a-chip

2010-06-30T08:03:00.000-07:00

Chemistry: Lab-on-a-chip

Chemistry: Diffuse interstellar bands.

2010-06-30T08:02:00.000-07:00

Chemistry: Diffuse interstellar bands.

Diffuse interstellar bands.

2010-06-30T08:00:00.000-07:00

Diffuse interstellar bands (DIBs) are absorption features seen in the spectra of astronomical objects in our galaxy. They are caused by the absorption of light by the interstellar medium. More than 200 bands are seen, in ultraviolet, visible and infrared wavelengths.

The origin of DIBs was unknown and hotly disputed for many years, and the DIBs were long believed to be due to polycyclic aromatic hydrocarbons and other large carbon-bearing molecules. However, no agreement of the bands could be found with laboratory measurements or with theoretical calculations.

The great problem with DIBs, apparent from the earliest observations, was that their central wavelengths did not correspond with any known spectral lines of any ion or molecule, and so the material which was responsible for the absorption could not be identified. A large number of theories were advanced as the number of known DIBs grew, and determining the nature of the absorbing material (the 'carrier') became a crucial problem in astrophysics.

One important observational result is that the strengths of most DIBs are not correlated with each other. This means that there must be many carriers, rather than one carrier responsible for all DIBs. Also significant is that the strength of DIBs is broadly correlated with the extinction. Extinction is caused by dust in the interstellar medium, and so DIBs are likely to be also due to dust or something related to it.

The existence of sub-structure in DIBs supports the idea that they are caused by molecules. Substructure results from band heads in the rotational band contour and from isotope substitution. In a molecule containing, say, three carbon atoms, some of the carbon will be in the form of the carbon-13 isotope, so that while most molecules will contain three carbon-12 atoms, some will contain two C¹² atoms and one C¹³ atom, much less will contain one C¹² and two C¹³s, and a very small fraction will contain three C¹³ molecules. Each of these forms of the molecule will create an absorption line at a slightly different rest wavelength.

The most likely candidate molecules for producing DIBs are thought to be large carbon-bearing molecules, which are common in the interstellar medium. Polycyclic aromatic hydrocarbons, long carbon-chain molecules, and fullerenes are all potentially important.

In recent years, very high resolution spectrographs on the world's most powerful telescopes have been used to observe and analyse DIBs . Spectral resolutions of 0.005 nm are now routine using instruments at observatories such as the European Southern Observatory at Cerro Paranal, Chile, and the Anglo-Australian Observatory in Australia, and at these high resolutions, many DIBs are found to contain considerable sub-structure.

Lab-on-a-chip

2010-03-19T00:10:00.000-07:00

A lab-on-a-chip (LOC) is a device that integrates one or several laboratory functions on a single chip of only millimeters to a few square centimeters in size. LOCs deal with the handling of extremely small fluid volumes down to less than pico liters. Lab-on-a-chip devices are a subset of MEMS devices and often indicated by "Micro Total Analysis Systems" (µTAS) as well.

Microfluidics is a broader term that describes also mechanical flow control devices like pumps and valves or sensors like flowmeters and viscometers. However, strictly regarded "Lab-on-a-Chip" indicates generally the scaling of single or multiple lab processes down to chip-format, whereas "µTAS" is dedicated to the integration of the total sequence of lab processes to perform chemical analysis. The term "Lab-on-a-Chip" was introduced later on when it turned out that µTAS technologies were more widely applicable than only for analysis purposes.

Chip materials and fabrication technologies
The basis for most LOC fabrication processes is photolithography. Initially most processes were in silicon, as these well-developed technologies were directly derived from semiconductor fabrication. Because of demands for e.g. specific optical characteristics, bio- or chemical compatibility, lower production costs and faster prototyping, new processes have been developed such as glass, ceramics and metal etching, deposition and bonding, PDMS processing (e.g., soft lithography), thick-film- and stereolithography as well as fast replication methods via electroplating, injection molding and embossing. Furthermore the LOC field more and more exceeds the borders between lithography-based microsystem technology, nano technology and precision engineering.

Advantages of LOCs
LOCs may provide advantages, which are specific to their application. Typical advantages are:

low fluid volumes consumption (less waste, lower reagents costs and less required sample volumes for diagnostics)
faster analysis and response times due to short diffusion distances, fast heating, high surface to volume ratios, small heat capacities.
better process control because of a faster response of the system (e.g. thermal control for exothermic chemical reactions)
compactness of the systems due to integration of much functionality and small volumes
massive parallelization due to compactness, which allows high-throughput analysis
lower fabrication costs, allowing cost-effective disposable chips, fabricated in mass production
safer platform for chemical, radioactive or biological studies because of integration of functionality, smaller fluid volumes and stored energies

For the chips to be used in areas with limited resources, many challenges must be overcome. In developed nations, the most highly valued traits for diagnostic tools include speed, sensitivity, and specificity; but in countries where the healthcare infrastructure is less well developed, attributes such ease of use and shelf life must also be considered. The reagents that come with the chip, for example, must be designed so that they remain effective for months even if the chip is not kept in a climate-controlled environment. Chip designers must also keep cost, scalability, and recyclability in mind as they choose what materials and fabrication techniques to use.

One active area of LOC research involves ways to diagnose and manage HIV infections. Around 40 million people are infected with HIV in the world today, yet only 1.3 million of these people receive anti-retroviral treatment. Around 90% of people with HIV have never been tested for the disease. Measuring the number of CD4+ T lymphocytes in a person’s blood is an accurate way to determine if a person has HIV and to track the progress of an HIV infection. At the moment, flow cytometry is the gold standard for obtaining CD4 counts, but flow cytometry is a complicated technique that is not available in most developing areas because it requires trained technicians and expensive equipment.

LASER

2010-03-13T05:41:00.000-08:00

Light amplification by stimulated emission of radiation (LASER or laser) is a mechanism for emitting electromagnetic radiation, typically light or visible light, via the process of stimulated emission. The emitted laser light is usually a spatially coherent, narrow low-divergence beam, that can be manipulated with lenses. In laser technology, "coherent light" denotes a light source that produces light of in-step waves of identical frequency, phase, and polarization.

Design

A laser consists of a gain medium inside a highly reflective optical cavity, as well as a means to supply energy to the gain medium. The gain medium is a material with properties that allow it to amplify light by stimulated emission. In its simplest form, a cavity consists of two mirrors arranged such that light bounces back and forth, each time passing through the gain medium. Typically one of the two mirrors, the output coupler, is partially transparent. The output laser beam is emitted through this mirror.

Light of a specific wavelength that passes through the gain medium is amplified (increases in power); the surrounding mirrors ensure that most of the light makes many passes through the gain medium, being amplified repeatedly. Part of the light that is between the mirrors (that is, within the cavity) passes through the partially transparent mirror and escapes as a beam of light.

The process of supplying the energy required for the amplification is called pumping. The energy is typically supplied as an electrical current or as light at a different wavelength. Such light may be provided by a flash lamp or perhaps another laser. Most practical lasers contain additional elements that affect properties such as the wavelength of the emitted light and the shape of the beam.

Types and operating principles

Gas lasers

Gas lasers using many gases have been built and used for many purposes.

The helium-neon laser (HeNe) emits at a variety of wavelengths and units operating at 633 nm are very common in education because of its low cost.

Carbon dioxide lasers can emit hundreds of kilowatts[14] at 9.6 µm and 10.6 µm, and are often used in industry for cutting and welding. The efficiency of a CO2 laser is over 10%.

Argon-ion lasers emit light in the range 351-528.7 nm. Depending on the optics and the laser tube a different number of lines is usable but the most commonly used lines are 458 nm, 488 nm and 514.5 nm.

A nitrogen transverse electrical discharge in gas at atmospheric pressure (TEA) laser is an inexpensive gas laser producing UV light at 337.1 nm.

Metal ion lasers are gas lasers that generate deep ultraviolet wavelengths. Helium-silver (HeAg) 224 nm and neon-copper (NeCu) 248 nm are two examples. These lasers have particularly narrow oscillation linewidths of less than 3 GHz (0.5 picometers),[16] making them candidates for use in fluorescence suppressed Raman spectroscopy

Chemical lasers

Chemical lasers are powered by a chemical reaction, and can achieve high powers in continuous operation. For example, in the Hydrogen fluoride laser (2700-2900 nm) and the Deuterium fluoride laser (3800 nm) the reaction is the combination of hydrogen or deuterium gas with combustion products of ethylene in nitrogen trifluoride. They were invented by George C. Pimentel

Excimer lasers

Excimer lasers are powered by a chemical reaction involving an excited dimer, or excimer, which is a short-lived dimeric or heterodimeric molecule formed from two species (atoms), at least one of which is in an excited electronic state. They typically produce ultraviolet light, and are used in semiconductor photolithography and in LASIK eye surgery. Commonly used excimer molecules include F2 (fluorine, emitting at 157 nm), and noble gas compounds (ArF [193 nm], KrCl [222 nm], KrF [248 nm], XeCl [308 nm], and XeF [351 nm]).

Solid-state lasers

Solid-state laser materials are commonly made by "doping" a crystalline solid host with ions that provide the required energy states. For example, the first working laser was a ruby laser, made from ruby (chromium-doped corundum). The population inversion is actually maintained in the "dopant", such as chromium or neodymium. Formally, the class of solid-state lasers includes also fiber laser, as the active medium (fiber) is in the solid state. Practically, in the scientific literature, solid-state laser usually means a laser with bulk active medium, while wave-guide lasers are caller fiber lasers.

"Semiconductor lasers" are also solid-state lasers, but in the customary laser terminology, "solid-state laser" excludes semiconductor lasers, which have their own name.

Neodymium is a common "dopant" in various solid-state laser crystals, including yttrium orthovanadate (Nd:YVO4), yttrium lithium fluoride (Nd:YLF) and yttrium aluminium garnet (Nd:YAG). All these lasers can produce high powers in the infrared spectrum at 1064 nm. They are used for cutting, welding and marking of metals and other materials, and also in spectroscopy and for pumping dye lasers. These lasers are also commonly frequency doubled, tripled or quadrupled to produce 532 nm (green, visible), 355 nm (UV) and 266 nm (UV) light when those wavelengths are needed.

Ytterbium, holmium, thulium, and erbium are other common "dopants" in solid-state lasers. Ytterbium is used in crystals such as Yb:YAG, Yb:KGW, Yb:KYW, Yb:SYS, Yb:BOYS, Yb:CaF2, typically operating around 1020-1050 nm. They are potentially very efficient and high powered due to a small quantum defect. Extremely high powers in ultrashort pulses can be achieved with Yb:YAG. Holmium-doped YAG crystals emit at 2097 nm and form an efficient laser operating at infrared wavelengths strongly absorbed by water-bearing tissues. The Ho-YAG is usually operated in a pulsed mode, and passed through optical fiber surgical devices to resurface joints, remove rot from teeth, vaporize cancers, and pulverize kidney and gall stones.

Titanium-doped sapphire (Ti:sapphire) produces a highly tunable infrared laser, commonly used for spectroscopy as well as the most common ultrashort pulse laser.

Thermal limitations in solid-state lasers arise from unconverted pump power that manifests itself as heat and phonon energy. This heat, when coupled with a high thermo-optic coefficient (dn/dT) can give rise to thermal lensing as well as reduced quantum efficiency. These types of issues can be overcome by another novel diode-pumped solid-state laser, the diode-pumped thin disk laser. The thermal limitations in this laser type are mitigated by using a laser medium geometry in which the thickness is much smaller than the diameter of the pump beam. This allows for a more even thermal gradient in the material. Thin disk lasers have been shown to produce up to kilowatt levels of power

Dye lasers

Dye lasers use an organic dye as the gain medium. The wide gain spectrum of available dyes allows these lasers to be highly tunable, or to produce very short-duration pulses (on the order of a few femtoseconds)

Free electron lasers

Free electron lasers, or FELs, generate coherent, high power radiation, that is widely tunable, currently ranging in wavelength from microwaves, through terahertz radiation and infrared, to the visible spectrum, to soft X-rays. They have the widest frequency range of any laser type. While FEL beams share the same optical traits as other lasers, such as coherent radiation, FEL operation is quite different. Unlike gas, liquid, or solid-state lasers, which rely on bound atomic or molecular states, FELs use a relativistic electron beam as the lasing medium, hence the term free electron.

Uses

The first application of lasers visible in the daily lives of the general population was the supermarket barcode scanner, introduced in 1974. The laserdisc player, introduced in 1978, was the first successful consumer product to include a laser, but the compact disc player was the first laser-equipped device to become truly common in consumers' homes, beginning in 1982, followed shortly by laser printers.

Some of the other applications include:

Medicine: Bloodless surgery, laser healing, surgical treatment, kidney stone treatment, eye treatment, dentistry

Industry: Cutting, welding, material heat treatment, marking parts

Defense: Marking targets, guiding munitions, missile defence, electro-optical countermeasures (EOCM), alternative to radar, blinding enemy troops.

Research: Spectroscopy, laser ablation, laser annealing, laser scattering, laser interferometry, LIDAR, laser capture microdissection

Product development/commercial: laser printers, CDs, barcode scanners, thermometers, laser pointers, holograms, bubblegrams.

Laser lighting displays: Laser light shows

Cosmetic skin treatments: acne treatment, cellulite and striae reduction, and hair removal.

BLOOM ENERGY

2010-02-28T07:29:00.000-08:00

Bloom Energy Corporation, a Silicon Valley-based

company committed to changing the way people generate and consume energy, announced

today the availability of the Bloom Energy Server, a patented solid oxide fuel cell (SOFC)

technology that provides a cleaner, more reliable, and more affordable alternative to both today’s

electric grid as well as traditional renewable energy sources. The Bloom Energy Server provides

distributed power generation, allowing customers to efficiently create their own electricity onsite.

The company introduced its groundbreaking technology at an event hosted today at eBay Inc.

headquarters along with California Governor Arnold Schwarzenegger, General Colin Powell, and

several of its early customers.

Built using abundant and affordable materials, Bloom’s fuel cell technology is fundamentally

different from the legacy “hydrogen” fuel cells most people are familiar with. The Bloom Energy

Server is distinct in four primary ways: it uses lower cost materials, provides unmatched efficiency

in converting fuel to electricity, has the ability to run on a wide range of renewable or traditional

fuels, and is more easily deployed and maintained.

Unlike traditional renewable energy technologies, like solar and wind, which are intermittent,

Bloom’s technology can provide renewable power 24/7.

Each Bloom Energy Server provides 100 kilowatts (kW) of power in roughly the footprint of a

parking space. Each system generates enough power to meet the needs of approximately 100

average U.S. homes or a small office building. For more power, customers simply deploy multiple

Energy Servers side by side. The modular architecture allows customers to start small and “pay

as they grow”.

Powder to Power – How It Works

Founded in 2001, Bloom Energy can trace its roots to the NASA Mars space program. For NASA,

Sridhar and his team were charged with building technology to help sustain life on Mars using

solar energy and water to produce air to breath and fuel for transportation. They soon realized

that their technology could have an even greater impact here on Earth and began work on what

would become the Bloom Energy Server.

The Bloom Energy Server converts air and nearly any fuel source – ranging from natural gas to a

wide range of biogases – into electricity via a clean electrochemical process, rather than dirty

combustion. Even running on a fossil fuel, the systems are approximately 67% cleaner than a

typical coal-fired power plant. When powered by a renewable fuel, they can be 100% cleaner.

Each Energy Server consists of thousands of Bloom's fuel cells – flat, solid ceramic squares

made from a common sand-like "powder."

About Bloom Energy

Bloom Energy is a provider of breakthrough solid oxide fuel cell technology that generates clean,

highly-efficient power onsite from virtually any fuel source. Bloom Energy’s mission is to make

clean, reliable energy affordable for everyone in the world. The Bloom Energy Server is currently

producing power for several Fortune 500 companies.

Chemicals-Risk

2009-12-10T23:19:00.000-08:00

The 20th century has seen the birth of three Ages, each with profound social implications. These have been called the Nuclear Age, the Electronic Age and the Chemical Age. The latter is the oldest (beginning ca. 1930), and although its impact has been less dramatic than the other two, its consequences have more thoroughly and deeply permeated our day-to-day lives. Our local grocery, hardware, garden and drug stores carry an impressive array of commonly used chemical "tools", such as detergents, adhesives, lubricants, fabrics, pesticides, pharmaceutical drugs, vitamins and a multitude of fabricated plastic items. Industrial applications of chemical tools include explosives, heat-transfer gases and liquids, specialized coatings, fire retardants and high-performance plastic components.

Despite our widespread use of chemical tools, indeed some might say because of our reliance on them, many people fear exposure to these materials, and have deep concerns regarding the use, storage and disposal of chemicals. Paradoxically, we find our desires for more abundant consumer goods, energy and personal mobility in conflict with maintenance of a healthful environment. To be sure, environmental degradation, with accompanying threats to health and disruption of ecosystems, is not a new phenomenon. From the earliest recorded history, human disturbance of the environment by deforestation, air pollution from cooking and heating fires, and careless sewage and waste disposal has been noted. Today, as global populations grow and per capita energy use and material consumption increases, pollution problems are exacerbated, and previously unnoticed secondary effects manifest themselves.

Every day we take risks and avoid others. About 250 people in the U.S. are electrocuted every year in accidents involving home wiring or appliances. This represents a risk of death of about 8*10-7 per year (250 divided by the U.S. population) or 6*10-6 per lifetime (75 yr.). Nevertheless, most of us choose to live in electrically wired homes, and make extensive use of electrical appliances. Likewise, many people would be unwilling to live within 20 miles of a nuclear power plant, yet accept (even request) a 4,000 times greater radiation dose from medical x-rays or 6,500 times greater cosmic radiation at altitudes of a mile or more.

The concept of risk and the notion of uncertainty are closely related. The lifetime risk of dying from cancer is roughly 22%, and is somewhat greater for those who smoke. However, even if an individual is a heavy smoker, we cannot say with certainty he(she) will die of lung cancer. On the other hand, if that individual is dying as the result of a serious automobile accident, the risk of dying from cancer drops to nearly zero.

Using Chemicals
As with any other kind of tool, chemicals must be handled correctly, with proper care and precaution. Although chemicals vary in the hazards they present, it is generally wise to treat all chemicals as though they are potentially dangerous. Among the recognized hazardous properties of chemicals are: explosiveness, flammability, corrosiveness, irritation, sensitivity, toxicity and radioactivity. One of the most useful sources of information about chemical hazards is the material safety data sheet (MSDS). Information about these data sheets is available at MSDSonline. It is an interesting excercise to examine the MSDS for common chemicals such as acetic acid (vinegar) and naphthalene (mothballs).

Of all the hazardous properties noted above, toxicity seems to constitute the greatest concern in the minds of the public. Contrary to popular belief, the fact that a substance is toxic does not mean it will always kill people or animals exposed to it. Virtually all substances are lethal if taken in sufficient amount. As noted by the Swiss Physician Paracelsus, It is the dose that makes the poison! Thus, 1.5 grams of arsenic trioxide will kill a 180 pound man; 2 milligrams will not. Small amounts of vitamin D (ca. 10 micrograms per day) are necessary for good health, but in larger amounts it is more toxic than arsenic compounds.

Humans vary considerably in their sensitivity to natural and synthetic chemicals. Strawberry, peanut and latex allergies are relatively common, and reports of asthma-like symptoms on exposure to synthetic plasticizers exist. A more complex and less well-defined syndrome, known as multiple chemical sensitivity, is the subject of medical controversy, although it is very real to those who suffer its effects. One thing is certain. If you wish to avoid exposure to chemicals, the planet earth is a poor place to live.

CLAYS

2009-12-10T23:06:00.000-08:00

Clay minerals are hydrous aluminium phyllosilicates, sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths and other cations. Clays have structures similar to the micas and therefore form flat hexagonal sheets. Clay minerals are common weathering products (including weathering of feldspar) and low temperature hydrothermal alteration products. Clay minerals are very common in fine grained sedimentary rocks such as shale, mudstone and siltstone and in fine grained metamorphic slate and phyllite.

Clays are ultra fine grained (normally considered to be less than 2 micrometres in size on standard particle size classifications) and so require special analytical techniques. Standards include x-ray diffraction, electron diffraction methods, various spectroscopic methods such as Mossbauer spectroscopy, infrared spectroscopy, and EDS or energy dispersive spectroscopy. These methods should always augment standard polarized light microscopy, a technique which is sometimes overlooked but often where fundamental occurrences or petrologic relationships are established.

Clays are commonly referred to as 1:1 or 2:1. Clays are fundamentally built of tetrahedral sheets and octahedral sheets, as described in the Structure section below. A 1:1 clay would consist of one tetrahedral sheet and one octahedral sheet, and examples would be kaolinite and serpentine. A 2:1 clay consists of an octahedral sheet sandwiched between two tetrahedral sheets, and examples are illite, smectite, attapulgite, and chlorite (although chlorite has an external octahedral sheet often referred to as "brucite").

Structure

Like all phyllosilicates, clay minerals are characterised by two-dimensional sheets of corner sharing SiO4 and AlO4 tetrahedra. These tetrahedral sheets have the chemical composition (Al,Si)3O4, and each tetrahedron shares 3 of its vertex oxygen atoms with other tetrahedra forming a hexagonal array in two-dimensions. The fourth vertex is not shared with another tetrahedron and all of the tetrahedra "point" in the same direction (i.e. all of the unshared vertices are on the same side of the sheet).

In clays the tetrahedral sheets are always bonded to octahedral sheets formed from small cations, such as aluminium or magnesium, coordinated by six oxygen atoms. The unshared vertex from the tetrahedral sheet also form part of one side of the octahedral sheet but an additional oxygen atom is located above the gap in the tetrahedral sheet at the center of the six tetrahedra. This oxygen atom is bonded to a hydrogen atom forming an OH group in the clay structure. Clays can be categorised depending on the way that tetrahedral and octahedral sheets are packaged into layers. If there is only one tetrahedral and one octahedral group in each layer the clay is known as a 1:1 clay. The alternative, known as a 2:1 clay, has two tetrahedral sheets with the unshared vertex of each sheet pointing towards each other and forming each side of the octahedral sheet.

Bonding between the tetrahedral and octahedral sheets requires that the tetrahedral sheet becomes corrogated or twisted, causing ditrigonal distortion to the hexagonal array, and the octahedral sheet is flattened. This minimizes the overall bond-valence distortions of the crystallite.

Depending on the composition of the tetrahedral and octahedral sheets, the layer will have no charge, or will have a net negative charge. If the layers are charged this charge is balanced by interlayer cations such as Na+ or K+. In each case the interlayer can also contain water. The crystal structure is formed from a stack of layers interspaced with the interlayers.

Named Reactions

2009-10-07T10:45:00.001-07:00

Link: http://www.chem.wisc.edu/areas/reich/syntheses/syntheses.htm

Link: http://www.organic-chemistry.org/namedreactions/

Chemistry: Organic synthesis

2009-10-07T10:40:00.000-07:00

Chemistry: Organic synthesis

Organic synthesis

2009-10-07T10:37:00.001-07:00

Organic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has developed into one of the most important branches of organic chemistry. There are two main areas of research fields within the general area of organic synthesis: total synthesis and methodology.

Total synthesis

A total synthesis is the complete chemical synthesis of complex organic molecules from simple, commercially available (petrochemical) or natural precursors. In a linear synthesis there is a series of steps which are performed one after another until the molecule is made- this is often adequate for a simple structure. The chemical compounds made in each step are usually referred to as synthetic intermediates.
The "father" of modern organic synthesis is regarded as Robert Burns Woodward, who received the 1965 Nobel Prize for Chemistry for several brilliant examples of total synthesis such as his 1954 synthesis of strychnine.

Asymmetric synthesis

Asymmetric synthesis, also called chiral synthesis, enantioselective synthesis or stereoselective synthesis, is organic synthesis which introduces one or more new and desired elements of chirality.This is important in the field of pharmaceuticals because the different enantiomers or diastereomers of a molecule often have different biological activity.

There are three main approaches to asymmetric synthesis:
chiral pool synthesis
chiral auxiliaries
asymmetric catalysis

In practice, a mixture of all three is often used in order to maximize the advantages of each method.

Methodology

Each step of a synthesis involves a chemical reaction, and reagents and conditions for each of these reactions need to be designed to give a good yield and a pure product, with as little work as possible. A method may already exist in the literature for making one of the early synthetic intermediates, and this method will usually be used rather than "trying to reinvent the wheel". However most intermediates are compounds that have never been made before, and these will normally be made using general methods developed by methodology researchers. To be useful, these methods need to give high yields and to be reliable for a broad range of substrates. Methodology research usually involves three main stages- discovery, optimisation, and studies of scope and limitations. The discovery requires extensive knowledge of and experience with chemical reactivities of appropriate reagents. Optimisation is where one or two starting compounds are tested in the reaction under a wide variety of conditions of temperature, solvent, reaction time, etc., until the optimum conditions for product yield and purity are found.

Link: http://www.chem.wisc.edu/areas/reich/syntheses/syntheses.htm

Organometallic chemistry

2009-09-29T23:15:00.001-07:00

Organometallic chemistry is the study of chemical compounds containing bonds between carbon and a metal.Since many compounds without such bonds are chemically similar, an alternative may be compounds containing metal-element bonds of a largely covalent character. Organometallic chemistry combines aspects of inorganic chemistry and organic chemistry.

Organometallic compounds are also known as organo-inorganics, metallo-organics and metalorganics. Organometallic compounds are distinguished by the prefix "organo-" e.g. organopalladium compounds. Examples of such organometallic compounds include all Gilman Reagents, which contain lithium and copper. Tetracarbonyl nickel, and ferrocene are examples of organometallic compounds containing transition metals. Other examples include organomagnesium compounds like iodo(methyl)magnesium MeMgI, diethylmagnesium (Et2Mg), and all Grignard reagents; organolithium compounds such as butyllithium (BuLi), organozinc compounds such as chloro(ethoxycarbonylmethyl)zinc (ClZnCH2C(=O)OEt); and organocopper compounds such as lithium dimethylcuprate (Li+[CuMe2]–).

In addition to the traditional metals, lanthanides, actinides, and semimetals, elements such as boron, silicon, arsenic, and selenium are considered to form organometallic compounds, e.g. organoborane compounds such as triethylborane (Et3B).

Structure and properties

The metal-carbon bond in organometallic compounds is generally of character intermediate between ionic and covalent. Primarily ionic metal-carbon bonds are encountered either when the metal is very electropositive (as in the case of the alkali metals) or when the carbon-containing ligand exists as a stable carbanion. Carbanions can be stabilized by resonance (as in the case of the aromatic cyclopentadienyl anion) or by the presence of electron-withdrawing substituents (as in the case of the triphenylmethyl anion). Hence, the bonding in compounds like sodium acetylide and triphenylmethylpotassium is primarily ionic. On the other hand, the ionic character of metal-carbon bonds in the organometallic compounds of transition metals, poor metals, and metalloids tends to be intermediate, owing to the middle-of-the-road electronegativity of such metals.

Organometallic compounds with bonds that have characters in between ionic and covalent are very important in industry, as they are both relatively stable in solutions and relatively ionic to undergo reactions. Two important classes are organolithium and Grignard reagents. In certain organometallic compounds such as ferrocene or dibenzenechromium, the pi orbitaStructure and properties.

Organometallic compounds undergo several important reactions:

oxidative addition and reductive elimination
transmetalation
carbometalation
Hydrometalation
electron transfer
beta-hydride elimination
organometallic substitution reaction
carbon-hydrogen bond activation
cyclometalation
Migratory insertion