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HOW MANY STRUCTRAL ISOMERS?

Structural isomerism, or constitutional isomerism is a form of isomerism in which molecules with the same molecular formula have 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.

Chain isomerism 
In chain isomerism, or skeletal 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 (2-methylbutane) and neopentane (dimethylpropane).

Position isomerism
In position isomerism a functional group or other substituent changes position on a parent structure. 

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 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.


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