aldehydes and ketones

Aldehydes are characterized by the presence of a carbonyl group, in which at least two of the remaining bonds to the carbon atom is to a hydrogen atom. The carbonyl group is highly polar. In other words, it exhibits an uneven distribution of electrons, which renders the carbon electrically positive and the oxygen electrically negative. This increases the chemical activities of aldehydes and affects their physical properties. Ketones are characterized by the presence of a carbonyl atom in which the carbon is covalently bonded to an oxygen atom. The remaining two bonds are to other carbon atoms or hydrocarbon radicals. Although highly reactive, ketones are less so than aldehydes, to which they are related. Much of their chemical activity results from the nature of the carbonyl group. Several aldehydes have characteristic, pleasant odors. Prepared synthetically by a variety of methods, they are derived, in principle, from alcohols by dehydrogenation (removal of hydrogen), from which process came the term aldehyde. Easily oxidized to acids and reduced to alcohols, aldehydes undergo a variety of chemical reactions. The tendency for many nucleophiles (chemical species that seek positively charged centers) to interact with the carbonyl carbon atom forms the basis for important synthetic methods. In one example, a hydrogen atom attached to the carbon atom immediately adjacent to the carbonyl group (the so-called alpha carbon atom) can be removed by acids or bases, resulting in a nucleophilic carbon atom that causes many useful reactions. Aldehydes also undergo polymerization. Such simpler aldehydes as acetaldehyde and formaldehyde readily link together into chains containing tens of thousands of the molecule. The combination of these aldehydes with other types of molecules produces such familiar plastics as Bakelite, made from formaldehyde and phenol, and Formica, made from formaldehyde and urea. Many aldehydes, manufactured on a large scale, are useful as solvents, polymer components, perfume ingredients, and intermediates in the production of dyes, plastics, and pharmaceuticals. Some of the aldehydes involved in physiological processes include pyridoxal phosphate, a form of vitamin B6, and retinal (vitamin A aldehyde), important in human vision. Because formaldehyde prevents deterioration of substances rich in protein, it is used as a preservative. Glucose and other so-called reducing sugars are aldehydes, as are several natural and synthetic hormones.Ketones, which can be synthesized by a variety of methods, are nearly ideal chemical intermediates because of their ease of preparation, relative stability, and high reactivity. Ketones are used as building blocks in the synthesis of many organic compounds. Only a small number of ketones are manufactured on a large scale in industry. They find widest employment as solvents, especially in industries manufacturing explosives, lacquers, paints, and textiles. They are also employed in tanning, as preservatives, and in hydraulic fluids. Acetone (CH3COCH3), a liquid with a sweetish odor, is employed chiefly as a solvent in paints and lacquers for cellulose plastics. Possessed of important physiological properties, ketones occur in several sugars and in compounds for medicinal use, including natural and synthetic steroid hormones (q.v.). Molecules of the anti-inflammatory agent cortisone contain three ketone groups.