Aldehydes and Ketones

An aldehyde and a ketone are both built on the carbonyl group, a carbon double-bonded to an oxygen, $C=O$. The carbonyl carbon is $sp^2$ hybridised and the group is flat, with the carbon, the oxygen and the two atoms bonded to the carbon lying in one plane at roughly $120^\circ$. One of the two bonds is a sigma bond and the other is a pi bond. Oxygen is more electronegative than carbon, so it drags the pi electrons towards itself and the group becomes polar, leaving the carbon with a partial positive charge and the oxygen with a partial negative charge, $C^{\delta+}=O^{\delta-}$. That single fact drives the whole chapter: the electron-poor carbon is where nucleophiles attack.

The two families are told apart by where the carbonyl sits. In an aldehyde it is on the terminal carbon and shares that carbon with at least one hydrogen, giving the general formula $RCHO$; formaldehyde is the extreme, with two hydrogens and no carbon group at all. In a ketone the carbonyl sits within the chain and shares its carbon with two carbon groups, $RCOR$. Both families answer to the same general formula for the saturated members, $C_nH_{2n}O$. Aromatic aldehydes carry the group on a ring, benzaldehyde being the standard, and acetophenone is the standard aromatic ketone, with the carbonyl bonded to both a ring and a methyl group.

Names come in two systems as usual. The common names are made from the carboxylic acid of the same carbon count by swapping the ending, giving formaldehyde, acetaldehyde, propionaldehyde and butyraldehyde. IUPAC replaces the final e of the parent alkane with al, so the same four are methanal, ethanal, propanal and butanal, and the carbonyl carbon is always C1, with the chain numbered from it. Aromatic aldehydes keep their trivial names and are not given IUPAC names. Ketones take the suffix one and the carbonyl carbon is numbered as low as possible, so 2-propanone is the familiar acetone and diisopropyl ketone is 2,4-dimethylpentan-3-one. Greek letters label the carbons beside the carbonyl, alpha being the first one out.

When a molecule carries more than one functional group, priority decides the naming. The aldehyde outranks the ketone, so a chain with both is named as an aldehyde and the ketone is demoted to the prefix oxo.