Aldehyde

An aldehyde is either a functional group consisting of a terminal carbonyl group, or a compound containing a terminal carbonyl group.

(Where -R represents the carbon chain.)

Structure

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

Nomenclature

Aldehydes are named by IUPAC nomenclature by changing the suffix -e of the parent alkane to -al. Aldehydes can be produced by oxidation of primary alcohols. In the laboratory this may be achieved by heating the alcohol in an acidified solution of potassium dichromate, which is reduced to green Cr³⁺ during the reaction, or by the so called "Swern oxidation" ((CO)₂Cl₂ + (Me)₂SO).

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, C₆H₅-CHO is known as benzenecarbaldehyde.

Physical properties

The carbonyl group is polar.

Reactions

Synthesis

Reacting a primary alcohol with some oxidizing agents, such as pyridinium chloride, yields an aldehyde.
Reacting an alkene with ozone will cause the bond to break if there is a vinylic hydrogen.
Reacting an ester with DIBAH can cause reduction, yielding an aldehyde.

Common reactions

aldehyde + alcohol + acid or base ---> hemiacetal
- hemiacetal + alcohol + acid catalyst ---> acetal + water

Treating aldehydes with oxidizing agents such as potassium permanganate, nitric acid, or chromium oxide, will yield a carboxylic acid.
Treating aldehydes with Tollens' reagent (Ag₂O in aqueous ammonia) will convert aldehydes to carboxylic acids without attacking carbon-carbon double bonds.
Aldehydes can react with water to form geminal diols.
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 hydrazine will reduce a C=O bond to CH₂ via the Wolff-Kishner reaction.

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