What are Aldehydes?

Aldehydes are organic compounds which have the functional group -CHO.

These carbonyl compounds consist of a central carbonyl carbon doubly bonded to an oxygen and single bonded to the R group (any alkyl group) and a hydrogen atom.

Preparation of Aldehydes

Acid chlorides are reduced to aldehydes with hydrogen in the presence of palladium catalyst spread on barium sulfate.

This reaction is known as Rosenmund reduction and is mostly used for the preparation of aromatic aldehydes. This reaction cannot be used for the preparation of ketones and formaldehyde.

Properties of Aldehydes

• The structure of aldehydes shows a sp² hybridized central carbon which is doubly connected to oxygen and has a single bond with hydrogen.
• Small aldehydes are quite soluble in water.
• Formaldehyde and acetaldehyde are great examples of this. These two aldehydes are quite important industrially.
• Aldehydes generally exhibit a tendency to undergo oligomerization or polymerization.
• The carbonyl centre of the aldehyde has an electron-withdrawing nature. Therefore, the aldehyde group can be considered somewhat polar.

Nomenclature of Aldehydes

• Acyclic and Aliphatic Aldehydes are named after their longest carbon chain along with the “-al” suffix. For example, CH₃CH₃CH₂CHO is called butanal since it has four carbons in the chain.
• When the aldehyde functional group is added to a ring, the “-carbaldehyde” suffix must be used. For example, C₆H₁₁CHO is called cyclohexanecarbaldehyde.
• In the case of natural compounds or carboxylic acids, “-oxo” is used as a prefix to highlight the carbon which is a part of the aldehyde functional group. For example, (CHO)-CH₂COOH is called 3-oxopropanoic acid.

Some common and IUPAC names for some aldehydes are tabulated below.

Formula	Common name	IUPAC name
HCHO	Formaldehyde	Methanal
CH3CHO	Acetaldehyde	Ethanal
CH3-CH(CH3)-CHO	Isobutyraldehyde	2-Methylpropanal
CH3-CH=CH-CHO	Crotonaldehyde	2-Butenal

Uses of Aldehydes

• Formaldehyde is used as a disinfectant and as a preservative for biological specimens.
• Aldehyde is used for silvering mirrors.
• Formaldehyde is used for the production of a variety of plastic and resins.
• Benzaldehyde is used in perfumery and in the dye industry