Overview

Functional groups are group of atoms with specific chemical properties that occur within organic molecules and sometimes denoted as “R”. Functional groups are found along the carbon backbone of macromolecules can form chains or rings of carbon atoms. Functional groups can “functionalize” a compound by enabling it to adopt different physical and chemical properties.  

Types of common functional groups

The table below summarizes some of the major functional groups in organic chemistry. (The functional groups are highlighted in red)

Class of organic compounds	Skeletal Structures of Compounds and functional groups (red)	Name of Compounds
Alkene		1-hexene
Alkyne		2-butyne
Alkyl halide		Chloromethane
Alcohol		Methanol
Thiol		Ethanethiol
Ether		Diethyl ether (ethoxyethane)
Sulfide		Dimethyl sulfide  (methylthiomethane)
Epoxide		Ethylene oxide (oxirane)

This text is adapted from Openstax, Chemistry 2e, Section 20: Organic Chemistry.

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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.