In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction(s) regardless of the size of the molecule it is a part of. However, its relative reactivity can be modified by nearby functional groups.
The word moiety is often used synonymously to "functional group," but, according to the IUPAC definition, a moiety is a part of a molecule that may include functional groups as substructures. For example, an ester is divided into an alcohol moiety and an acyl moiety, but has an ester functional group. Also, it may be divided into carboxylate and alkyl moieties. Each moiety may carry any number of functional groups, for example methyl parahydroxybenzoate carries a phenol functional group in the acyl moiety.
Combining the names of functional groups with the names of the parent alkanes generates a powerful systematic nomenclature for naming organic compounds.
The atoms of functional groups are linked to each other and to 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 atom after the carbon that attaches to the functional group is called the alpha carbon; the second, beta carbon, the third, gamma carbon, etc. If there is another functional group at a carbon, it may be named with the Greek letter, e.g. the gamma-amine in gamma-aminobutanoic acid is on the third carbon of the carbon chain attached to the carboxylic acid group.
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