Lesson 1 of 13 · 6 min

Names and the carbonyl group

NCERT §8.1

The flavour store holds cinnamon oil, vanilla extract, a drum of acetone for cleaning and a tank of vinegar. The chemist tells Tara that every one of them carries the same C=O unit.

The story this chapter follows: Tara's month in the pickle-factory lab

Tara, a Class 12 student, spends a month in the quality lab of a pickle-and-sauce factory. Her first bench job is a vinegar check: 10.0 mL of vinegar needs 16.0 mL of 0.500 M NaOH, which puts it at 4.80 g of acetic acid per 100 mL. Over the month she meets the flavour aldehydes, the preservatives and the tests that tell a carbonyl compound's family.

The lesson in notes

In short

The carbonyl group is >C=O. In an aldehyde its carbon is joined to one carbon and one hydrogen (methanal has two H); in a ketone it is joined to two carbons. Adding an –OH to the carbonyl carbon gives a carboxylic acid; –NH₂ gives an amide and a halogen gives an acyl halide. Esters and anhydrides are derivatives of the acids.

Carbonyl compounds run through plant and animal life and give many natural smells: cinnamon owes its scent to cinnamaldehyde, vanilla beans to vanillin, and meadow sweet to salicylaldehyde. Acetone is made on a large scale as a solvent, and the family also goes into fabrics, plastics, perfumes, resins, paints and adhesives.

Common names of aldehydes come from the common name of the matching acid, with the '-ic acid' ending changed to 'aldehyde' (formic acid → formaldehyde). Greek letters α, β, γ… mark substituent positions, α being the carbon next to –CHO.

Common names of ketones name the two groups on the carbonyl (methyl n-propyl ketone); the simplest, dimethyl ketone, is acetone. Alkyl phenyl ketones take the acyl name plus 'phenone', as in acetophenone.

IUPAC names change the final '-e' of the alkane to '-al' for aldehydes and '-one' for ketones. The aldehyde carbon is C-1; a ketone chain is numbered from the end nearer the C=O, and in a cyclic ketone the carbonyl carbon is C-1.

When –CHO is attached to a ring the suffix 'carbaldehyde' follows the ring name, and the ring carbon bearing –CHO is C-1. The simplest aromatic aldehyde is benzenecarbaldehyde, also accepted by IUPAC as benzaldehyde.

Name pairs (common = IUPAC): formaldehyde = methanal; acetaldehyde = ethanal; isobutyraldehyde = 2-methylpropanal; valeraldehyde = pentanal; acrolein = prop-2-enal; methyl n-propyl ketone = pentan-2-one; diisopropyl ketone = 2,4-dimethylpentan-3-one; mesityl oxide = 4-methylpent-3-en-2-one.

The carbonyl carbon is sp² hybridised: three σ bonds lie in one plane about 120° apart, and the leftover p orbital overlaps a p orbital of oxygen to form the π bond, whose cloud lies above and below that plane. Oxygen also holds two lone pairs.

Oxygen is more electronegative than carbon, so the C=O bond is polar: the carbon is an electrophilic (Lewis acid) centre and the oxygen a nucleophilic (Lewis base) centre. A dipolar resonance form, C⁺–O⁻, explains the large dipole moments; carbonyl compounds are more polar than ethers.