Aldehyde Aldehyde Circus Maximus Category="Aldehydes"Category="Functional groups"An aldehyde is either a Functional group consisting of a Terminal carbonyl group, or a compound containing a terminal carbonyl group.

[image] (Where -R represents the carbon chain.)

Structure

The aldehyde functional group is a carbon atom bonded to a carbonyl group and a hydrogen atom.

α carbon & α hydrogen

An α (alpha) carbon is a carbon adjacent to a carbonyl group. An α hydrogen is a hydrogen atom bonded to the α carbon. The pKa of an α hydrogen is 20.

Carbonyl group

The other molecules containing carbonyl group are:

Nomenclature

Aldehydes are named by IUPAC nomenclature by changing the suffix -e of the parent Alkane to -al.

Aliphatic aldehydes are named as derivatives of their longest alkyl chain. Thus, HCHO is named as a derivative of methane, and CH₃CH₂CH₂CHO is named as a derivative of butane. The suffix -al replaces the -e of the alkane name. Thus, HCHO is named methanal, more commonly known as Formaldehyde, and CH₃CH₂CH₂CHO is named Butanal.

When a -CHO group is attached to a ring, the suffix -carbaldehyde is used. Thus, C₆H₁₁-CHO is known as cyclohexanecarbaldehyde. The name Benzaldehyde is used as the root for aldehydes derived from Benzene.

Physical properties

The carbonyl group is polar. Oxygen being more Electronegative, pulls the bond pair towards itself thus creating electron deficiency at the carbon atom.

Chemistry

Preparation

There are three notable methods for preparing aldehydes:

Reacting a primary alcohol with an Oxidizing agent,
Reacting an Alkene (if there is a vinylic hydrogen) with Ozone will cause the C=C bond to break yielding an aldehyde upon workup, in a process called Ozonolysis.
Reacting an Ester with DIBAL-H can cause reduction, yielding an aldehyde.

The primary means of synthesis is the oxidation of a primary alcohol. In the laboratory this may be achieved by heating the alcohol with a chromium(VI) reagent an acidified solution of Potassium dichromate, which is reduced to green Cr³⁺ during the reaction. Excess dichromate will further oxidise the aldehyde to form a Carboxylic acid, so either the aldehyde is distilled out as it forms (if volatile), or milder methods such as PCC oxidation, IBX acid, Dess-Martin periodinane or Swern oxidation are used. The equation is shown below with propan-1-ol being oxidised to form propanal.

: CH₃CH₂CH₂OH -→ CH₃CH₂CHO

A similar process reacting pentan-1-ol to form pentanal is illustrated below.

: [image]

Common reactions

aldehydes react with alcohols and an acid or base to a Hemiacetal, the hemiacetal and alcohol and more Acid catalyst further react to an Acetal and water

: Simple hemiacetals are usually unstable, although cyclic ones such as Glucose do exist. Acetals are stable, but these revert to the aldehyde in the presence of aqueous acid.

Treating aldehydes with oxidizing agents such as Potassium permanganate, Nitric acid, chromium(VI) oxide or acidified Potassium dichromate), will yield a Carboxylic acid.
Treating aldehydes 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) will convert aldehydes to carboxylic acids without attacking carbon-carbon double bonds. This reaction is also known as the silver mirror test. See also oxidation of aldehyde
Aldehydes can react with water (under acidic or basic conditions) to form hydrates, R-C(H)(OH)(OH), although these are only stable when strong electron withdrawing groups are present as in Chloral hydrate. The mechanism is identical to Hemiacetal formation.
Aldehydes can react with HCN to form cyanohydrins, R-C(H)(OH)(CN).
Treating an aldehyde with a Grignard reagent can yield an alcohol with a substituted group from the Grignard reagent.
Treating aldehydes with NH₃ derivatives (like NH₂OH, H₂NNH₂, 2,4-Dinitrophenylhydrazine) will result in the formation of an Oxime or Hydrazone. This is classed as an Addition-Elimination reaction or Addition-Condensation reaction. If the simple hydrazone (RCH=NHNH₂) of an aldehyde is heated with a base such as KOH, the terminal carbon is fully reduced via the Wolff-Kishner reaction to a methyl group. The Wolff-Kishner reaction may be performed as a one-pot reaction, giving the overall conversion RCH=O → RCH₃.
Reaction of aldehydes with reducing agents such as Magnesium gives diols in a Pinacol coupling reaction
The Wittig reaction takes aldehydes to alkenes and the Corey-Fuchs reaction takes aldehydes to alkynes both with a Triphenylphosphine reagent.

Nucleophilic addition

aldehyde + Nucleophile -→ Tetrahedral carbonyl addition compound
*aldehyde + Ammonia or primary amine -→ Tetrahedral carbonyl addition compound
**Tetrahedral carbonyl addition compound + acid (catalyst) -→ Imine + water

Keto-enol tautomerism

Equilibration of keto and enol tautomers is catalyzed by acid.

Oxidation & Reduction

Aldehydes are oxidized to carboxylic acids.
Aldehydes are reduced to primary alcohols.

Examples of Aldehydes

Etymology

The word seems to have arisen from "alcohol dehydrogenated". In old times aldehydes were sometimes named after the corresponding alcohols, for example "vinous aldehyde" for Acetaldehyde. (Latin vinum = "wine", the traditional source of Ethanol; compare Vinyl.)