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:
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.
- aldehyde + nucleophile ---> tetrahedral carbonyl addition compound
- aldehyde + ammonia or primary amine ---> tetrahedral carbonyl addition compound
- tetrahedral carbonyl addition compound + acid (catalyst) ---> imine + water
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
See also
