Aldehyde and Ketone Structure and Nomenclature

Core Idea

Aldehydes contain a carbonyl group (C=O) bonded to a hydrogen and an alkyl group; ketones have the carbonyl bonded to two alkyl groups. Both are classified under the carbonyl functional group. Aldehydes are named using the suffix -al; ketones use -one. The carbonyl carbon is always the highest priority in the parent chain, making aldehydes more oxidized and generally more reactive in nucleophilic addition.

Explainer

You already know from functional groups that the carbonyl group — a carbon double-bonded to oxygen (C=O) — is one of the most important structural motifs in organic chemistry. Aldehydes and ketones are the two simplest carbonyl-containing families, differing only in what is attached to the carbonyl carbon. In an aldehyde, the carbonyl carbon is bonded to at least one hydrogen (and one alkyl group or a second hydrogen in formaldehyde). In a ketone, the carbonyl carbon is bonded to two alkyl or aryl groups — no hydrogen directly on the carbonyl. This seemingly small structural difference has real chemical consequences: the hydrogen in aldehydes makes the carbonyl carbon more accessible to incoming nucleophiles and also means aldehydes can be further oxidized (to carboxylic acids), while ketones generally cannot.

Naming these compounds follows the IUPAC system you learned for alkanes, with modifications. For aldehydes, find the longest carbon chain that includes the carbonyl carbon, replace the -e ending of the parent alkane with -al, and number the chain so that the carbonyl carbon is always carbon 1 (no number is needed in the name since -al always means position 1). Methanal (formaldehyde), ethanal (acetaldehyde), and propanal are the simplest examples. For ketones, replace the -e ending with -one and number the chain to give the carbonyl carbon the lowest possible number. So pentan-2-one has a five-carbon chain with the carbonyl at position 2. When both an aldehyde and a ketone are present in the same molecule, the aldehyde takes naming priority because -al outranks -one in IUPAC conventions.

The carbonyl group's geometry and electronics set the stage for everything that follows in carbonyl chemistry. The carbon is sp² hybridized — trigonal planar with bond angles near 120°. Oxygen is more electronegative than carbon, so the C=O bond is strongly polarized: the oxygen carries a partial negative charge (δ⁻) and the carbon carries a partial positive charge (δ⁺). This makes the carbonyl carbon electrophilic — an inviting target for nucleophiles. In aldehydes, only one alkyl group flanks this electrophilic carbon, so there is less steric crowding and less electron donation compared to ketones, where two alkyl groups partially stabilize the positive character through induction. This is why aldehydes are generally more reactive toward nucleophilic addition than ketones.

Many important aldehydes and ketones also have widely used common names that predate IUPAC nomenclature. Formaldehyde (methanal), acetaldehyde (ethanal), acetone (propan-2-one), and benzaldehyde are names you will encounter constantly. Learning both naming systems is practical: IUPAC names are systematic and unambiguous, but common names dominate in laboratory conversation, reagent bottles, and biological chemistry. Recognizing the carbonyl group in either naming system is your entry point to the rich reaction chemistry of these compounds.