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:

• Ketones
• Carboxylic acids

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

There are three notable synthesis reactions of aldehydes which are:

• Reacting a primary alcohol with an oxidizing agent,
• Reacting an alkene with ozone will cause the bond to break if there is a vinylic hydrogen ,
• Reacting an ester with DIBAL-H can cause reduction, yielding an aldehyde.

The primary means of synthesis is the reaction of a primary alcohol and an acidified oxidizing agent such as such as pyridinium chloride or sodium dichromate (VI) . In terms of the primary alcohol this is a process known as oxidation. To yield an aldehyde the product must be separated from the reactiants as it forms using distillation apparatus. If the product is not separated as it forms and the experiment is performed with excess sodium dichromate (VI) the product will further oxidise to form a carboxylic acid. The equation is shown below with propan-1-ol being warmed with acidified sodium dichromate (VI) to form propanal.

CH₃CH₂CH₂OH —→ CH₃CH₂CHO

Common reactions

1. 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 (which is prepared by adding a drop of sodium hydroxide solution into silver(I) nitrate solution to give a precipitate of silver(I) oxide, and then add just enough dilute ammonia solution to redissolve the precipitate in aqueous ammoniato to produce [Ag(NH₃)₂]⁺ complex) will convert aldehydes to carboxylic acids without attacking carbon-carbon double bonds. See also oxidation of aldehyde
• Aldehydes can react with water (under acidic or basic conditions) to form hydrates, R-C(H)(OH)(OH). 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 hydrazine will reduce a C=O bond to CH₂ via the Wolff-Kishner reaction.

Nucleophilic addition

1. 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

• Methanal (Formaldehyde)
• Ethanal (Acetaldehyde)
• Propanal
• Butanal
• Pentanal
• Glucose

See also

• Ketone