Editing Structural isomer (section)

{{Short description|Chemical compounds with the same atoms but arranged and connected differently}}
{{Textbook|date=August 2020}}

In [[chemistry]], a '''structural isomer''' (or '''constitutional isomer'''  in the [[IUPAC]] nomenclature<ref name="GoldBook1"/>) of a [[chemical compound|compound]] is a compound that contains the same number and type of atoms, but with a different connectivity (i.e. arrangement of [[chemical bond|bonds]]) between them.<ref name=bett2009/><ref name=mumb2018/> The term '''metamer''' was formerly used for the same concept.<ref name=bynum2014/>

For example, [[butanol]] {{chem2|H3C\s(CH2)3\sOH}}, [[methyl propyl ether]] {{chem2|H3C\s(CH2)2\sO\sCH3}}, and [[diethyl ether]] {{chem2|(H3CCH2\s)2O}} have the same [[molecular formula]] {{chem2|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 {{chem2|O\dC\dN-}} and the [[fulminate]] ion {{chem2|C-\tN+\sO-}}. It is also extended to ionic compounds, so that (for example) [[ammonium cyanate]]  {{chem2|[NH4]+[O\dC\dN]-}} and [[urea]] {{chem2|(H2N\s)2C\dO}} are considered structural isomers,<ref name=bynum2014>William F. Bynum, E. Janet Browne, Roy Porter (2014): [https://archive.org/details/dictionaryofhist0000unse_u7i5/page/218/mode/1up?q=metamerism ''Dictionary of the History of Science'']. page 218. {{isbn|9781400853410}}</ref> and so are [[methylammonium formate]] {{chem2|[H3C\sNH3]+[HCO2]-}} and [[ammonium acetate]] {{chem2|[NH4]+[H3C\sCO2]-}}.

Structural isomerism is the most radical type of [[isomer]]ism.  It is opposed to [[stereoisomer]]ism, in which the atoms and bonding scheme are the same, but only the relative spatial arrangement of the atoms is different.<ref name=clark2000/><ref name=poppe2016/> Examples of the latter are the [[enantiomer]]s, 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''', '''[[tautomer]]s''',{{r|hibb1987}} and '''structural isotopomers'''.{{r|GoldBook2}}