Carboxylic acids, when heated to about 300°C in the presence of manganese (IV) oxide (MnO$_2$), form carbonyl compounds.

• Heating methanoic acid produces methanal (an aldehyde).
• Heating higher members of carboxylic acids produces ketones.

Example

HCOOH → HCHO (Methanal)

When calcium carboxylates are heated to about 400°C, carbonyl compounds are formed:

• Heating calcium methanoate produces methanal.
• Heating higher members of calcium carboxylates produces ketones.

Example

Ca(HCOO)2 → HCHO (Methanal)

Note

This method can result in an increase in the number of carbon atoms (from n to 2n–1) in the resulting compound.

Alkenes can react with acid and mercury sulphates to form carbonyl compounds.

Primary alcohols are oxidized to aldehydes using weak oxidizing agents to avoid further oxidation to carboxylic acids. This can be achieved by heating alcohols in the presence of a copper catalyst.

• Oxidation of secondary alcohols yields ketones.

Example

R–CH2OH → RCHO (Aldehyde)

Carboxylic acids can be reduced to aldehydes using reducing agents like lithium aluminum hydride (LiAlH$_4$) in limited amounts to prevent further reduction to alcohols.

Example

1. R–COOH → RCHO (Aldehyde)

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Acyl compounds react with hydrogen gas in the presence of a palladium catalyst poisoned with sulfur-containing compounds like BaSO$_4$. This process forms aldehydes.

The poisoning of the palladium catalyst ensures the reaction stops at the aldehyde stage and does not proceed to form alcohols.

Example

RCOCl + H2 → RCHO