Naming carbonyl compounds and acids

All three families in this chapter contain the carbonyl group, C=O:

• An aldehyde has the C=O at the end of a chain, with at least one H on the carbonyl carbon: R–CHO. Example: ethanal, CH₃CHO.
• A ketone has the C=O inside a chain, with a carbon group on both sides: R–CO–R′. Example: propanone (acetone), CH₃COCH₃.
• A carboxylic acid has an –OH on the carbonyl carbon: R–COOH. Example: ethanoic acid, CH₃COOH.

Related compounds you should recognise: an acid chloride (acyl chloride) R–COCl, an amide R–CONH₂, an ester R–COOR′, and an acid anhydride (RCO)₂O.

IUPAC names:

• Aldehydes end in -al: methanal (HCHO), ethanal, propanal. The CHO carbon is always C-1.
• Ketones end in -one, numbered so the C=O gets the lowest number: butan-2-one, pentan-3-one.
• Acids end in -oic acid: ethanoic acid, propanoic acid. The COOH carbon is C-1.
• When the group is on a ring, add -carbaldehyde or -carboxylic acid to the ring's name: cyclohexanecarbaldehyde, cyclohexanecarboxylic acid.

Many of these compounds have well-known common names worth learning: formaldehyde (methanal), acetaldehyde (ethanal), acetone (propanone), benzaldehyde (C₆H₅CHO), acetophenone (C₆H₅COCH₃), salicylaldehyde (2-hydroxybenzaldehyde). In common names, the carbons next to the carbonyl are labelled with Greek letters: α for the first, β for the next.

The carbonyl carbon is sp² hybridised, so the group is flat, with bond angles of about 120°. Oxygen is more electronegative than carbon, so the C=O bond is polar: the carbon is slightly positive (δ⁺) and the oxygen slightly negative (δ⁻). That positive carbon is where most reactions begin.