Aldehydes are a class of highly reactive organic chemical compounds that contain a carbonyl group (in which a carbon atom is double-bound to an oxygen atom) and at least one hydrogen atom bound to the alpha carbon (the central carbon atom in the carbonyl group). The aldehydes are similar to the ketones, which also contain a carbonyl group. In the aldehydes, however, the carbonyl group is attached to the end of a chain of carbon atoms, which is not the case with the ketones. The word aldehyde is a combination of parts of the words alcohol and dehydrogenated, because the first aldehyde was prepared by removing two hydrogen atoms (dehydrogenation) from ethanol. Molecules that contain an aldehyde group can be converted to alcohols by the addition of two hydrogen atoms to the central carbon oxygen double bond (reduction). Organic acids are the result of the introduction of one oxygen atom to the carbonyl group (oxidation). Aldehydes are very easy to detect by smell. Some are very fragrant, and others have a smell resembling that of rotten fruit.
Formaldehyde is the simplest aldehyde. The central carbon atom in the carbonyl group is bound to two hydrogen atoms. Its chemical formula is H2C=O. Formaldehyde, discovered in Russia by A. M. Butlerov in 1859, is a gas in its pure state. It is either mixed with water and sold as Formalin solutions or as a solid polymer called paraformaldehyde. The rather small formaldehyde molecule is very reactive and has found applications in the manufacture of many organic chemicals such as dyes and medical drugs. Formaldehyde is also a good insecticide, and it is used to kill germs in warehouses and ships. It is probably most familiar to the general public in its application as a preservative. In biology laboratories, animals and organs are suspended in formaldehyde solutions, which are also used as embalming fluid to preserve dead bodies from decay.
Acetaldehyde is the name of the shortest carbon chain aldehyde. It has a central carbon atom that has a double bond to an oxygen atom (the carbonyl group), a single bond to a hydrogen atom, and a single bond to another carbon atom connected to three hydrogen atoms (methyl group). Its chemical formula is written as CH3CHO. Acetaldehyde is one of the oldest known aldehydes and was first made in 1774 by Carl Wilhelm Scheele. Its structure was not completely understood until 60 years later, when Justus von Liebig determined the constitution of acetaldehyde, described its preparation from ethanol, and gave the name of aldehydes to the chemical group.
The next larger aldehyde molecules have longer carbon atom chains with each carbon atom connected to two hydrogen atoms. This group of aldehydes is called aliphatic and has the general formula CH3(CH2)nCHO, where n=1-6. When n=1, the aldehyde formula is CH3CH2CHO and is named propionaldehyde; when n=2, it is CH3(CH2)2CHO or butyraldehyde. The aliphatic aldehydes have irritating smells. For example, the smell of butyraldehyde, in low concentrations, resembles that of rotten butter. These medium-length aldehyde molecules are used as intermediates in the manufacture of other chemicals such as acetone and ethyl acetate used in finger nail polish remover. They are also important in the production of plastics.
Fatty aldehydes contain long chains of carbon atoms connected to an aldehyde group. They have between eight and 13 carbon atoms in their molecular formula. The fatty aldehydes have a very pleasant odor, with a fruity or a floral aroma, and can be detected in very low concentrations. Because of these characteristics, the fatty aldehydes are used in the formulation of many perfumes. The aldehyde that contains eight carbon atoms in its molecular formula is called octyl aldehyde and smells like oranges. The next longer aldehyde molecule is nonyl aldehyde, with nine carbon atoms in its structure, and has the odor of roses. A very powerful smelling compound is the 10-carbon aldehyde (decyl aldehyde), which has a scent of orange peel and is present in small concentration in most perfumes. Citral, a more complicated 10-carbon aldehyde, has the odor of lemons. Lauryl aldehyde, the 12-carbon aldehyde, smells like lilacs or violets. Fatty aldehydes are also added to soaps and detergents to give them their "fresh lemon scent." The aromatic aldehydes have a benzene or phenyl ring connected to the aldehyde group. The aromatic aldehyde molecules have very complex structures but are probably the easiest to identify. Anisaldehyde smells like licorice. The odor of cinnamon found in various products is due to an aromatic aldehyde of complex structure named cinnamaldehyde. The aldehyde vanillin is a constituent in many vanilla-scented perfumes.
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