Definitions

aldehyde

aldehyde

[al-duh-hahyd]
aldehyde [alcohol + New Lat. dehydrogenatus=dehydrogenated], any of a class of organic compounds that contain the carbonyl group, and in which the carbonyl group is bonded to at least one hydrogen; the general formula for an aldehyde is RCHO, where R is hydrogen or an alkyl or aryl group. Aldehydes are formed by partial oxidation of primary alcohols and form carboxylic acids when they are further oxidized. The common name for an aldehyde is often derived from the name of the acid it forms; the IUPAC name is usually derived from the name of the alcohol from which it is formed. Low molecular weight aldehydes, e.g., formaldehyde and acetaldehyde, have sharp, unpleasant odors; higher molecular weight aldehydes, e.g., benzaldehyde and furfural, have pleasant, often flowery, odors and are found in the essential oils of certain plants. Aldehydes are used for the manufacture of synthetic resins, e.g., bakelite, and for making dyestuffs, flavorings, perfumes, and other chemicals. Some are used as preservatives and disinfectants.

Any of a class of organic compounds that contain a carbonyl group (singlehorzbondCdoublehorzbondO; see functional group) in which the carbon atom is bonded to at least one hydrogen atom. Many have characteristic odours. Oxidation (see oxidation-reduction) of aldehydes yields acids; reduction produces alcohols. They participate in many chemical reactions and readily undergo polymerization into chains containing tens of thousands of the monomer molecule. The combination of aldehydes (e.g., formaldehyde) with other molecules results in several familiar plastics. Many aldehydes are large-scale industrial materials, useful as solvents, monomers, perfume ingredients, and intermediates. Many sugars are aldehydes, as are several natural and synthetic hormones and compounds such as retinal (a derivative of vitamin A, important in vision) and pyridoxal phosphate (a form of vitamin B6).

Learn more about aldehyde with a free trial on Britannica.com.

An aldehyde is an organic compound containing a terminal carbonyl group. This functional group, which consists of a carbon atom bonded to a hydrogen atom and double-bonded to an oxygen atom (chemical formula O=CH-), is called the aldehyde group. The aldehyde group is also called the formyl or methanoyl group.

The word aldehyde seems to have arisen from alcohol dehydrogenated. In the past, aldehydes were sometimes named after the corresponding alcohols, for example, vinous aldehyde for acetaldehyde. (Vinous is from Latin vinum = wine, the traditional source of ethanol; compare vinyl.)

The aldehyde group is polar. Oxygen, more electronegative than carbon, pulls the electrons in the carbon-oxygen bond towards itself, creating an electron deficiency at the carbon atom.

Owing to resonance stabilization of the conjugate base, an α-hydrogen in an aldehyde is more acidic than a hydrogen atom in an alkane, with a typical pKa of 17.

Nomenclature

IUPAC names for aldehydes

IUPAC prescribes the following nomenclature for aldehydes:

  1. Acyclic aliphatic aldehydes are named as derivatives of the longest carbon chain containing the aldehyde group. Thus, HCHO is named as a derivative of methane, and CH3CH2CH2CHO is named as a derivative of butane. The name is formed by changing the suffix -e of the parent alkane to -al, so that HCHO is named methanal, and CH3CH2CH2CHO is named butanal.
  2. In other cases, such as when a -CHO group is attached to a ring, the suffix -carbaldehyde may be used. Thus, C6H11CHO is known as cyclohexanecarbaldehyde. If the presence of another functional group demands the use of a suffix, the aldehyde group is named with the prefix formyl-. This prefix is preferred to methanoyl-.
  3. If the compound is a natural product or a carboxylic acid, the prefix oxo- may be used to indicate which carbon atom is part of the aldehyde group; for example, CHOCH2COOH is named 3-oxopropanoic acid.
  4. If replacing the aldehyde group with a carboxyl (-COOH) group would yield a carboxylic acid with a trivial name, the aldehyde may be named by replacing the suffix -ic acid or -oic acid in this trivial name by -aldehyde. For example:

*HCHO may be called formaldehyde.
*CH3CHO may be called acetaldehyde.
*C6H5CHO may be called benzaldehyde.

Other nomenclature

The carbon atom adjacent to a carbonyl group is called the α carbon. Carbon atoms further away from the group may be named β for the carbon atom bonded to the α carbon, γ for the next, and so on. Hydrogen atoms bonded to these carbon atoms are named likewise: an α hydrogen is a hydrogen atom bonded to the α carbon and so on.

A reaction that introduces an aldehyde group is known as a formylation reaction.

Chemistry

Synthesis

There are several methods for preparing aldehydes:

CH3CH2CH2OH —→ CH3CH2CHO
* Reacting an alkene (if there is a vinylic hydrogen) with ozone will form an ozonide (an unstable, explosive intermediate), which yields an aldehyde upon reduction with zinc and acid at reduced temperatures. This process is called ozonolysis.

Keto-enol tautomerism

Aldehydes can exist in either the keto or enol tautomers. Keto-enol tautomerism is catalyzed by either acid or base.

Common reactions

Reduction and oxidation

  • The aldehyde group can be reduced to the group -CH2OH, changing the aldehyde into a primary alcohol.
  • The aldehyde group can be oxidized to the group -COOH, yielding a carboxylic acid. Suitable oxidizing agents include potassium permanganate, nitric acid, chromium(VI) oxide, and acidified potassium dichromate.
  • The combination of manganese dioxide, acetic acid and methanol will convert the aldehyde to a methyl ester
  • Another oxidation reaction is the silver mirror test. In this test, an aldehyde is treated with Tollens' reagent, which is prepared by adding a drop of sodium hydroxide solution into silver nitrate solution to give a precipitate of silver(I) oxide, and then adding just enough dilute ammonia solution to redissolve the precipitate in aqueous ammonia to produce [Ag(NH3)2]+ complex. This reagent will convert aldehydes to carboxylic acids without attacking carbon-carbon double-bonds. The name silver mirror test arises because this reaction will produce a precipitate of silver whose presence can be used to test for the presence of an aldehyde.
  • If the aldehyde can not form an enolate (e.g. benzaldehyde), addition of strong base causes the Cannizzaro reaction to occur, producing a mixture of alcohol and carboxylic acid.

Nucleophilic addition reactions

In nucleophilic addition reactions a nucleophile can add to the carbon atom in the carbonyl group, yielding an addition compound in which this carbon atom has tetrahedral molecular geometry. Together with protonation of the oxygen atom in the carbonyl group (which can take place either before or after addition); this yields a product where the carbon atom in the carbonyl group is bonded to the nucleophile, a hydrogen atom, and a hydroxyl group.

In many cases, a water molecule is removed after the addition takes place; in this case, the reaction is classed as an addition-elimination or addition-condensation reaction.

There are various examples of nucleophilic addition reactions.

More complex reactions

Examples of aldehydes

Related compounds

Other kinds of organic compounds containing carbonyl groups include

References

Search another word or see aldehydeon Dictionary | Thesaurus |Spanish
Copyright © 2014 Dictionary.com, LLC. All rights reserved.
  • Please Login or Sign Up to use the Recent Searches feature
FAVORITES
RECENT

;