ALDEHYDE

any of a class of organic compounds characterized by the presence of a carbonyl group, in which at least one of the two remaining bonds to the carbon atom is to a hydrogen atom. They have the general formula Chemical Compounds: Aldehydes and ketones. The carbonyl group is highly polar. That is to say, it exhibits an uneven distribution of electrons, which renders the oxygen electrically negative and the carbon electrically positive. This increases the chemical activities of aldehydes and affects their physical properties as well. Many aldehydes have characteristic, pleasant odours. They are prepared synthetically by a wide variety of methods. In principle, they are derived from alcohols by dehydrogenation (removal of hydrogen), from which process came the name aldehyde. Aldehydes undergo a wide variety of chemical reactions. They are easily oxidized to acids and reduced to alcohols. Many nucleophiles (chemical species that seek positively charged centres) interact with the carbonyl carbon atom. Such reactions form the basis for important synthetic methods. For 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. The resulting carbon ion is a nucleophile that causes many useful reactions. Aldehydes also undergo polymerization. The simpler aldehydes, such as formaldehyde and acetaldehyde, readily link together into chains containing tens of thousands of the molecule. The combination of these aldehydes with other types of molecules produces several familiar plastics. Bakelite, for example, is made from formaldehyde and phenol, and Formica is made from formaldehyde and urea. Many aldehydes are manufactured on a large scale. They are useful as solvents, polymer components, perfume ingredients, and intermediates in the production of plastics, dyes, and pharmaceuticals. Certain aldehydes are involved in physiological processes. Examples are retinal (vitamin A aldehyde), important in human vision, and pyridoxal phosphate, one of the forms of vitamin B6. Formaldehyde prevents deterioration of substances rich in protein and is thus used as a preservative. Glucose and other so-called reducing sugars are aldehydes, as are several natural and synthetic hormones.