Aldehydes typically contain a carbonyl group bonded to which atom?

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Aldehydes are defined by a carbonyl group (C=O) with one hydrogen attached to the carbonyl carbon, giving the structure R-CHO. This means the carbonyl carbon is bonded to hydrogen on one side and to the rest of the molecule (an R group) on the other. This distinguishes aldehydes from ketones (which have two carbon substituents on the carbonyl carbon) and from carboxylic acids (where the carbonyl carbon is bonded to a hydroxyl group, -COOH). A two-hydroxyl arrangement would be a diol, not an aldehyde, and aldehydes cannot have the carbonyl carbon bonded only to alkyl groups with no hydrogen. Therefore, the hydrogen is the atom the carbonyl carbon is bonded to in aldehydes.