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ALDOL CONDENSATION

Lokender Pal, group 23. Science adviser is Olga Levashova.


An aldol condensation is an organic reaction in which an enol or an enolate ion reacts with a carbonyl compound to form a β-hydroxyaldehyde or β-hydroxyketone, followed by a dehydration to give a conjugated enone.

Aldol condensations are important in organic synthesis, providing a good way to form carbon–carbon bonds. The Robinson annulation reaction sequence features an aldol condensation; theWieland-Miescher ketone product is an important starting material for many organic syntheses. Aldol condensations are also commonly discussed in university level organic chemistry classes as a good bond-forming reaction that demonstrates important reaction mechanisms.In its usual form, it involves the nucleophilic addition of a ketone enolate to an aldehyde to form a β-hydroxy ketone, or "aldol" (aldehyde + alcohol), a structural unit found in many naturally occurring molecules and pharmaceuticals.

The name aldol condensation is also commonly used, especially in biochemistry, to refer to just the first (addition) stage of the process—the aldol reaction itself—as catalyzed by aldolases. However, the aldol reaction is not formally a condensation reaction because it does not involve the loss of a small molecule.

The reactions between an aldehyde/ketone and a carbonyl compound lacking an alpha-Hydrogen(Cross Aldol condensation) is called Claisen-Schmidt condensation. These reactions are named after two of its pioneering investigators Rainer Ludwig Claisen and J. G. Schmidt, who independentlypublished on this topic in 1880 and 1881. An example is the synthesis ofdibenzylideneacetone.




Mechanism;-The first part of this reaction is an aldol reaction, the second part a dehydration—an elimination reaction(Involves removal of a water molecule or an alcohol molecule). Dehydration may be accompanied by decarboxylation when an activated carboxyl group is present. The aldol addition product can be dehydrated via two mechanisms; a strong base like potassium t-butoxide,potassium hydroxide or sodium hydride in an enolate mechanism, or in an acid-catalyzed enol mechanism.



CANNIZZARO REACTION

Shahab Zakir, group 23. Science adviser is Olga Levashova.


The Cannizzaro reaction, named after its discoverer Stanislao Cannizzaro, is a chemical reaction that involves the base-induced disproportionation of an aldehyde lacking a hydrogen atom in the alpha position.


Cannizzaro first accomplished this transformation in 1853, when he obtained benzyl alcohol and potassium benzoate from the treatment of benzaldehyde with potash (potassium carbonate). More typically, the reaction would be conducted with sodium or potassium hydroxide:

2C6H5CHO + KOH → C6H5CH2OH + C6H5CO2K

The oxidation product is a salt of a carboxylic acid and the reduction product is an alcohol. For aldehydes with a hydrogen atom alpha to the carbonyl, i.e. R2CHCHO, the preferred reaction is an aldol condensation, originating from deprotonation of this hydrogen.

Mechanism

The reaction begins with nucleophilic attack of hydroxide on the carbonyl center. The resulting anion attacks another molecule of aldehyde, transferring hydride.In the final step of the reaction, the acid and alkoxide ions formed exchange a proton. In the presence of a very high concentration of base, the aldehyde first forms a doubly charged anion from which a hydride ion is transferred to the second molecule of aldehyde to form carboxylate and alkoxide ions. Subsequently, the alkoxide ion acquires a proton from the solvent.

Overall, the reaction follows third-order kinetics. It is second order in aldehyde and first order in base:

rate = k[RCHO]2[OH-]

At very high base a second path (k') becomes important that is second order in base:

rate = k[RCHO]2[OH-] + k'[RCHO]2[OH-]2

The k' pathway implicates a reaction between the doubly charged anion (RCHO22-) and the aldehyde. The direct transfer of hydride ion is evident from the observation that the recovered alcohol does not contain any deuterium attached to the α-carbon when the reaction is performed in the presence of D2O.

Scope

The reaction is limited to aldehydes lacking alpha hydrogen centers. Under ideal conditions the reaction produces only 50% of the alcohol and the carboxylic acid. The latter is obtained only after acidification of the highly basic reaction mixture, typically 30% base.To avoid the low yields, it is more common to conduct the crossed Cannizzaro reaction with a sacrificial aldehyde. In this variation, the reductant is formaldehyde,



Which to sodium formate and the corresponding alcohol is obtained in a high yield, although the atom economy is still low.

A solvent-free reaction has been reported involving grinding liquid 2-chlorobenzaldehyde with potassium hydroxide in a mortar and pestle.

GREAT CHEMIST

AbegundeYetunde Catherine, group 23. Science adviser is Olga Levashova.


A chemist is a scientist trained in the study of chemistry. Chemist study the composition of matter and its properties. The word chemist is derived from New Latin noun Chimista, an abbreviation of alchimista (alchemist). Alchemist discovered many chemical processes that led to the development of modern chemistry. Chemistry bas we know it today, was invented by Antoine Lavoisier(the father of modern chemistry) with his law of conservation of mass in 1783.

The foundations of modern chemistry were laid in the 18th and 19th centuries and further extended in the 20thcentury. They encompassed the development of theoretical framework for understanding and explaining the physical and chemical properties of atoms and molecules, together with the invention of increasingly sophisticated techniques for interacting with the entities in order to study and influence their structures and behaviours. These developments have given humanity a degree of mastery over its physical environment that surpasses the sum of achievements over the entire previous period of human history.

Throughout the modern period of it development, chemistry has contributed enormously both to broad improvements in human wellbeing (including enhancement of health and quality and to wealth creation for individuals and nations.

Here are some great chemist that contributed to the development of modern chemistry and medicine of pharmaceutical science.

Alessandro Volta (1800): Discovered that a continuous of electricity was generated when using certain fluids as conductors to promote a chemical reaction between the metals or electrodes and this made mass production of portable power sources enabled a vast range of applications from automobiles to radios.

William Cruickshank (1800): first description of electrolysis of brine.

Michael Faraday (1833): formulation of the laws that govern the electrolysis of aqueous solution.

Ernst Solvay (1861): patented Solvay process for manufacture of industrial soda using carbon dioxide, brine, and ammonia. The contribution of these chemists made electrolysis become an extremely important method of transforming materials and especially for the production ofv inorganic chemicals and compounds, either for use in their own right or as a source of feed stocks for the manufacture of other compounds, including organics.

Charles Gerhardt (1853): first synthesis of acetylsalicylic acid.

Felix Hoffman (1897): Investigated acetylsalicylic acid as a less irritating replacement for salicylate medicine, e.g.for treating rheumatism.

Louis Pasteur (1860-1864): Demonstrated that fermentation is caused by specific microorganisms and formulated the germ theory of disease providing the basis for biotechnology and biotechnology and anti-microbial chemotherapy.

Alexander Fleming, Howard Florey, Ernst Chain (1928-1940): discovery of penicillin the first family of B –Lactam antibiotics, and development of large-scale process of its production.

James Watson Francis Crick (1953): Discovery of double helix structure of DNA the foundation of molecular biology.

Frederick Sanger (1955): first determination of the complete amino acid sequence of protein – insulin.

Linus Paultng, Harvey S.J Singer, Ibert Wells (1949): Publication of “Sickle Cell Anaemia, a molecular disease”. The first proof of a human disease caused by an abnormal protein and the dawn of molecular genetics.

These chemists studied microorganisms and the physiological effect of chemicals and work on the structural modification of natural products and synthetic chemicals in the 19th century laid the foundation for pharmaceutical industry in the 20th century. Major classes of therapeutic agents soon emerged, including analgesics, anaesthetics, anti-infective and anti-tumor agents.




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