Functional groups are atoms or groups of atoms within an organic compound that impart distinctive chemical properties.  

One of the important functional groups in organic chemistry is the carbonyl functional group, composed of a carbon atom double-bonded to oxygen.

Two closely related carbonyl-based organic compounds are aldehydes and ketones. In aldehydes, the carbonyl carbon is bonded to an alkyl group and a hydrogen atom. The exception is formaldehyde, in which the carbonyl carbon is bonded to two hydrogens.

Meanwhile, in ketones, the carbonyl carbon atom is bonded to two other carbon atoms within a hydrocarbon skeleton.

The carbonyl group forms a part of many larger functional groups, such as the carboxyl group of carboxylic acids, which is composed of a hydroxyl group bound to the carbonyl carbon.

Functional derivatives of carboxylic acids include esters, in which the hydrogen of the carboxyl group is replaced by an alkyl group; amides, in which the OH of the carboxyl group is replaced by an amine; and acyl chlorides, in which the OH is replaced by a chloride.

Multiple bonds are a feature of other functional groups, such as the nitrile or cyano functional group, with a carbon atom triple-bonded to nitrogen.

Benzene is an aromatic hydrocarbon that is often represented as six carbon atoms in a ring with alternating single and double bonds. Benzene minus one hydrogen serves as phenyl—a functional group. A hydroxyl group attached to the phenyl-based aromatic ring forms a phenol.

An amine is a non‑carbon-containing functional group defined by an electronegative nitrogen atom with a lone pair of electrons singly bonded in a hydrocarbon framework. They are classified as primary, secondary, and tertiary, depending on the number of carbon atoms bonded to nitrogen.

Likewise, a phosphate functional group has a phosphorus atom covalently bonded to four oxygen atoms. Up to three hydrocarbon chains can be attached to the oxygen atoms singly bonded to the phosphorus to obtain organophosphates.