Chemistry Tutorial: Basic Concepts of Organic Chemistry: Structural Isomerism

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Sturctural Isomerism

Componds having the same molecular formula but different structures:

Compounds having the same molecular formula but different arrangement of carbon chains are called chain or nuclear isomers. For example (i) pentane, isopentane and neopentane (ii) hexane, 2-methulpentane, 3-methylpentane, 2,3-dimethylbutane and 2,2-dimethulbutane etc
Compounds having the same structure of the carbon chain but differing in position of multiple bonds, functional groups or substituents are called position isomers. For example, (i) but-1-ene and but-2-ene (ii) prpan-1-ol and propan-2-ol (iii) o-,m- and p- xylenes etc.
Compounds having the same molecular formula but different functional groups are called functional isomers. For example, (i) alcohols and ethers (ii) aldehydes and ketones (iii) alkynes and dienes (iv) various amines (1^o , 2^o , 3^o ) (v) cyanides and isocyanides etc.
Compounds having the same molecular formula but different number of carbon atoms on either side of the functional group (O,S or NH) are caled metamers. For example, (i) diethyl ether and methyl n-propyl ether (or isopropyl methyl ether). 2-pentanone and 3-pentanone are position isomers as well as metamers.
Isomers obtained by 1,3-migration of a proton from one polyvalent atom to the other within the same molecule are called tautomers. If one tautomer contains a keto group and the other the enol group, such a tautomerism is called keto-enol tautomerism. For example, acetaldehyde and vinyl alcohol.
The percentage of enol form is negligible in simple aldehydes and ketones, ie, acetone, acetaldehyde etc. The percentage of enol form, however, increases, if the enol form is stabilized by intramolecular H-bonding. For example, the percentage of enol form in acetoacetic ester is 7% while that in acetylacetone is 76%.
Only those aldehydes and ketones which contain one or more - hydrogens show keto-enol tautomerism.
Primary and secondary nitroalkanes also show tautomerism but tertiary nitroalkanes and nitroarenes do not.
Compounds having the same molecular formula but possessing open chain and cyclic structures are called ring chain isomers. For example, propene and cyclopropane.