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). Aldehydes are similar to 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 al cohol and dehyd rogenated, 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 adding 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 an odor resembling that of rotten fruit.
Formaldehyde, the simplest aldehyde, was discovered in Russia by A. M. Butlerov in 1859. A gas in its pure state, it has a central carbon atom in the carbonyl group bound to two hydrogen atoms and a chemical formula of H2 C = O. It is either mixed with water and methanol and sold as Formalin, or turned into a solid polymer called paraformaldehyde.
The rather small formaldehyde molecule is very reactive and has applications in the manufacture of many organic chemicals such as dyes and medical drugs. It’s also a good insecticide and germicide, but is probably most familiar as a preservative. In biology laboratories, animals and organs are suspended in formaldehyde solution, which is also used as an embalming fluid to preserve dead bodies from decay.
Acetaldehyde (CH3 CHO) is 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 (a methyl group). One of the oldest known aldehydes, it was first produced in 1774 by Carl Wilhelm Scheele, although its structure was not completely understood until 60 years later, when Justus von Liebig determined its constitution, described its preparation from ethanol, and gave the name “aldehydes” to the chemical group.
Aldehyde— A class of organic chemical compounds that contain a–CHO group.
Carbonyl group— A combination of a central carbon atom and an oxygen atom that have a double bond.
Dehydrogenation— The process of removing hydrogen atoms from a compound.
Methyl group— A terminal carbon atom connected to three hydrogen atoms.
Oxidation— The conversion of one chemical (compound) to another by the addition of oxygen atoms.
Reduction— The process by which an atom’s oxidation state is decreased, as it gains one or more electrons.
The next-larger aldehyde molecules have longer carbon atom chains with each carbon atom connected to two hydrogen atoms. These aliphatic aldehydes have the general formula CH3 (CH2)n CHO, where n = 1-6. When n = 1, the aldehyde formula is CH3 CH2 CHO and is named propionaldehyde; when n = 2, it is CH3 (CH2)2 CHO or butyraldehyde. 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 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 and have between eight and 13 carbon atoms in their molecular formula. They have very pleasant odors, with a fruity or a floral aroma, and can be detected in very low concentrations. Because of these characteristics, they 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, nonyl aldehyde— with nine carbon atoms in its structure—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.”
Aromatic aldehydes have a benzene or phenyl ring connected to the aldehyde group. These molecules have very complex structures but are probably the easiest to identify. Anisaldehyde smells like licorice. The cinnamon fragrance found in various products comes from a complex structure named cinnamalde-hyde. Vanillin is a constituent in many vanilla-scented perfumes.
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