Functional group

In organic chemistry functional groups are submolecular structural motifs, characterized by specific elemental composition and connectivity, that confer reactivity upon the molecule that contains them.

Common functional groups include:

Term	Formula	Prefix	Suffix
Amine	R-NH2	amino-	-amine
Carboxylic acid (or carboxy group)	R-C(=O)OH	hydrocarboxy-	-oic acid
Alcohol (hydroxy[?])	R-OH	hydroxy-	-ol
Aldehyde	R-C(=O)H		-al
Ketone	R-C(=O)-R'	keto-	-one
Ester	R-C(=O)O-R'	named according to the parent alcohol and acid, respectively: alkyl alkanoate
Ether	R-O-R'	named according to the parent alcohols, respectively: alkylalkylether
Amide	R-C(=O)N(-H)-R'	name according to the parent amine and acid, respectively: alkyl alkanamide
Azo	R-N=N-R'
Alkene	R-CH=CH-R'	convert the part substituting for alk in the name of the alkane into the alk of the word alkene: ethane/ethene, propane/propene, butane/butene, etc.	-ene
Alkyne	R-CC-R'	convert the part substituting for alk in the name of the alkane into the alk of the word alkyne: ethane/ethyne, propane/propyne, butane/butyne, etc.	-yne
Phosphodiester[?]	R-OP(=O)2O-R'
Phenyl	R-C6H5
Pyridyl[?]	R-C5H4N
Nitrile[?]	R-CN	alkyl nitrile
Isonitrile[?]	R-NC	alkyl isonitrile
Isocyanate	R-NCO	alkyl isocyanate
Isothiocyanate[?]	R-NCS	alkyl isothiocyanate
Methyl	R-CH3	methyl- (similarly for higher alkyl substituents: ethyl, propyl, butyl, etc.)
Thioether	R-S-R'

Combining the names of functional groups with the names of the parent alkanes generates a powerful systematic nomenclature for naming organic compounds.

The non-hydrogen atoms of functional groups are always associated with each by covalent bonds, as well as with the rest of the molecule. When the group of atoms is associated with the rest of the molecule primarily by ionic forces, the group is referred to more properly as a polyatomic ion or complex ion. And all of these are called radicals, by a meaning of the term radical that predates the free radical.