(Structural isomerism) Constitutional isomerism in accordance with IUPAC, is a form of isomerism in which molecules with the same molecular formula have atoms bonded together in different orders, as opposed to stereoisomerism. There are multiple synonyms for constitutional isomers.
Three categories of constitutional isomers are skeletal, positional, and functional isomers. Positional isomers are also called regioisomers. Tautomers are subcategory of functional isomers.
In skeletal isomerism, or chain isomerism, components of the (usually carbon) skeleton are distinctly re-ordered to create different structures. Pentane exists as three isomers: n-pentane (often called simply "pentane"), isopentane (methylbutane) and neopentane (dimethylpropane).
In position isomerism a functional group or other substituent changes position on a parent structure. In the table below, the hydroxyl group can occupy three different positions on an n-pentane chain forming three different compounds.
Many aromatic isomers exist because substituents can be positioned on different parts of the benzene ring. Only one isomer of phenol or hydroxybenzene exists but cresol or methylphenol has three isomers where the additional methyl group can be placed on three different positions on the ring. Xylenol has one hydroxyl group and two methyl groups and a total of 6 isomers exist.
Functional group isomerism
Functional isomers are structural isomers that have the same molecular formula (that is, the same number of atoms of the same elements), but the atoms are connected together in different ways so that the groupings are dissimilar. These groups of atoms are called functional groups, functionalities, or moieties. Another way to say this is that two compounds with the same molecular formula, but different functional groups, are functional isomers.
For example, cyclohexane and hex-1-ene both have the formula C6H12. These two are considered functional group isomers because cyclohexane is a cycloalkane and hex-1-ene is an alkene.
For two molecules to be functional isomers, they must contain key groups of atoms arranged in particular ways. Some of the best examples come from organic chemistry. C2H6O is a molecular formula. Depending on how the atoms are arranged, it can represent two different compounds dimethyl ether CH3-O-CH3 or ethanol CH3CH2-O-H. Dimethyl ether and ethanol are functional isomers. The first is an ether. The carbon chain-oxygen-carbon chain functionality is called an ether. The second is an alcohol. The carbon chain-oxygen-hydrogen functionality is called an alcohol.
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