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Aldehydes

Chemistry Grade 10 Difficulty 4/5

Beginning of the lesson

Structure, names and physical properties

Aldehydes contain a terminal carbonyl group in which the carbonyl carbon is bonded to hydrogen. The functional group is written R–C(=O)–H or –CHO. The carbonyl carbon belongs to the parent chain and is always carbon 1, so the suffix -al is used: methanal HCHO, ethanal CH3CHO and propanal CH3CH2CHO. Saturated open-chain aldehydes have the general formula CnH2nO.

The C=O bond is polar because oxygen attracts electron density more strongly than carbon. Aldehydes therefore have dipole–dipole attractions, but they cannot hydrogen-bond to one another as donors because they contain no O–H group. Small aldehydes can accept hydrogen bonds from water and are appreciably soluble; solubility falls as the hydrocarbon part grows.

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