Carbonyl Compounds

Introduction to Carbonyl Compounds

Carbonyl compounds contain the carbonyl functional group (-C=O), which consists of a carbon atom double-bonded to an oxygen atom.
The group includes aldehydes (R-CHO), ketones (R-CO-R’), carboxylic acids (R-COOH), esters (R-COOR’), and amides (R-CONR2).

Carbonyl compounds have high polarity due to the carbonyl group, leading to relatively high boiling points compared to hydrocarbons of similar size.
The polarity also means they are generally soluble in water and other polar solvents.
They act as nucleophiles and electrophiles, which make them highly reactive and able to undergo a variety of chemical reactions such as addition, reduction, and condensation.

Carbonyl compounds typically absorb light in the UV region, but ones with extended systems of conjugated double bonds can absorb light in the visible region and therefore appear coloured.
This property is utilised in the creation of synthetic dyestuffs such as azo dyes and phthalocyanine dyes.

The Brady’s test, also known as 2,4-dinitrophenylhydrazine test, can be used to detect the presence of a carbonyl group in a compound. A positive test results in a yellow-orange precipitate.
Tollens’ reagent, a clear, colourless solution of silver nitrate in ammonia, can be used to distinguish between aldehydes and ketones. Aldehydes are oxidised, forming a silver mirror on the test tube, whereas ketones do not react.
The Fehling’s test is another method to differentiate between aldehydes and ketones. Aldehydes reduce the blue Fehling’s solution to a red-brown precipitate, while ketones give no reaction.

Carbonyl compounds are vital in the synthesis of many pharmaceuticals such as anti-inflammatory drugs, cholesterol-lowering statins, and β-lactam antibiotics.
They are also essential in the production of polymers such as polyesters and polyamides, as well as in the manufacture of adhesives, paints, and coatings.