Structural isomer

In chemistry, a structural isomer (or constitutional isomer in the IUPAC nomenclature) of a compound is a compound that contains the same number and type of atoms, but with a different connectivity (i.e. arrangement of bonds) between them. The term metamer was formerly used for the same concept.

For example, butanol H₃C−(CH₂)₃−OH, methyl propyl ether H₃C−(CH₂)₂−O−CH₃, and diethyl ether (H₃CCH₂−)₂O have the same molecular formula C₄H₁₀O but are three distinct structural isomers.

The concept applies also to polyatomic ions with the same total charge. A classical example is the cyanate ion O=C=N⁻ and the fulminate ion C⁻≡N⁺−O⁻. It is also extended to ionic compounds, so that (for example) ammonium cyanate [NH₄]⁺[O=C=N]⁻ and urea (H₂N−)₂C=O are considered structural isomers, and so are methylammonium formate [H₃C−NH₃]⁺[HCO₂]⁻ and ammonium acetate [NH₄]⁺[H₃C−CO₂]⁻.

Structural isomerism is the most radical type of isomerism. It is opposed to stereoisomerism, in which the atoms and bonding scheme are the same, but only the relative spatial arrangement of the atoms is different. Examples of the latter are the enantiomers, whose molecules are mirror images of each other, and the cis and trans versions of 2-butene.

Among the structural isomers, one can distinguish several classes including skeletal isomers, positional isomers (or regioisomers), functional isomers, tautomers, and structural isotopomers.

A skeletal isomer of a compound is a structural isomer that differs from it in the atoms and bonds that are considered to comprise the "skeleton" of the molecule. For organic compounds, such as alkanes, that usually means the carbon atoms and the bonds between them.

For example, there are three skeletal isomers of pentane: n-pentane (often called simply "pentane"), isopentane (2-methylbutane) and neopentane (dimethylpropane).

If the skeleton is acyclic, as in the above example, one may use the term chain isomerism.