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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:

Group Chemical class Formula Prefix Suffix
Amine Amine R-NH2 amino- -amine
Carboxyl Carboxylic acid R-C(=O)OH hydrocarboxy- -oic acid
Hydroxyl Alcohol R-OH hydroxy- -ol
Carbonyl Aldehyde R-C(=O)H -al
Ketone R-C(=O)-R' keto- -one
Ester Ester R-C(=O)O-R' named according to the parent alcohol and acid, respectively: alkyl alkanoate
Ether Ether R-O-R' named according to the parent alcohols, respectively: alkylalkylether
Amide 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.