Functional groups are atoms or groups of atoms within a molecule that exhibit characteristic chemical behavior. Molecules containing the same functional groups often display similar physical and chemical properties.

Functional groups serve as the basis for naming and classifying organic compounds. Accordingly, the hydrocarbon portions of the functionalized organic compounds are often abbreviated with the letter ‘R’.

Organic compounds with carbon–carbon multiple bonds as functional groups include alkenes and alkynes. Alkenes have double bonds, while alkynes have triple bonds. Though they replace hydrogens with carbon–carbon multiple bonds, these double and triple bonds contribute significantly towards chemical reactivity.

Many functional groups are connected to the alkyl carbon atom by a single bond to an electronegative atom. For example, haloalkanes, or alkyl halides, have an alkyl carbon bonded to one or more electronegative halogen atoms as the functional group.

Alcohols contain the hydroxyl or ‒OH functional group with the carbon bonded to the electronegative oxygen. Alcohols can be classified as primary, secondary, or tertiary, depending on the number of carbon atoms bonded to the carbon bearing the -OH group.

The sulfur analog of an alcohol is called a thiol, with an ‒SH functional group.

Some functional groups, such as ethers, are connected to more than one hydrocarbon chain through an electronegative oxygen atom.

Sulfides are structurally similar to ethers. However, the oxygen is replaced with a sulfur atom.

Cyclic ethers have an oxygen atom in a hydrocarbon ring. Epoxides are notable cyclic ethers, as they have an oxygen atom and two carbon atoms in a three-atom ring structure. The ring is similar to an equilateral triangle and is highly reactive.