In ecology functional groups are collections of organisms based on morphological, physiological, behavioral, biochemical, or environmental responses or on trophic criteria.

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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.

The following is a list of common functional groups. In the formulas, the symbols R and R' denote any group of atoms.

Chemical class	Group	Formula	Prefix	Suffix
Alcohol	Hydroxyl	R−OH	hydroxy-	-ol
Alkanes	Methyl	R−CH3	methyl- (similarly for higher alkyl substituents: ethyl, propyl, butyl, etc.)
Alkenes	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
Alkynes	Alkyne	R−C≡C−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
Amide	Amide	R−C(=O)N(−H)−R'	name according to the parent amine and acid, respectively: alkyl alkanamide
Amines	primary Amine	R−NH2	amino-	-amine
	secondary Amine	R−N(−H)−R'	amino-	-amine
	tertiary Amine	R−N(−R')−R	amino-	-amine
Azo	Azo	R-N=N-R'
	Nitrile	R−C≡N	alkyl nitrile
	Pyridyl	R−C5H4N
Carboxylic acid	Carboxyl	R−C(=O)OH non-ionized R−C(=O)O− ionized	hydrocarboxy-	-oic acid
Carbonyl	Aldehyde	R−C(=O)H		-al
	Ketone	R−C(=O)−R'	keto-	-one
Imines	primary Imine	R−C(=NH)−R'	imino-	-imine
	secondary Imine	R−C(−H)=N−R'	imino-	-imine
Ether	Ether	R−O−R'	named according to the parent alcohols, respectively: alkylalkylether
Ester	Ester	R−C(=O)O−R'	named according to the parent alcohol and acid, respectively: alkyl alkanoate
Halogens	Halogen	F,Cl,Br,etc.
Isocyanates	Isocyanate	R−N=C=O	alkyl isocyanate
	Isothiocyanate	R−N=C=S	alkyl isothiocyanate
Phenyl	Phenyl	R−C6H5
Benzyl	Benzyl	R−CH2−C6H5	benzyl
	Phosphodiester	R−OP(=O)2O−R'
Thiol	sulfhydryl group	R−SH		-thiol
	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 other and with the rest of the molecule by covalent bonds. 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.

The first carbon after the carbon that attaches to the functional group is called the alpha carbon .