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Isomers and Isomerism
• In organic chemistry, we come across some compounds which have similar number of atoms but differ in physical and chemical properties and are called isomers and the phenomenon is called isomerism
• Isomers have same molecular formula but differ in arrangement of atoms within a molecule
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Types of isomerism• Mainly isomerism is of two types
– 1. Structural isomerism– 2. Stereoisomerism
1-Structural isomers– Are compounds having same molecular formula but
different structural formula
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–These are of further 5 types• Chain isomerism• Position isomerism• Functional isomerism• Metamerism• Tautomersim
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• Stereoisomers–Are compounds having same structural
formula but different in arrangement of atoms in space
–Stereoisomers are of two types• Geometrical or Cis-Trans isomerism• Optical isomerism
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Structural isomerism• Chain Isomerism
– Such compounds have the same molecular formula but differ in the order in which the carbon atoms are bonded to each other
– Examples are • n-butane and isobutane
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Positional isomers• Positional isomers have same molecular formula but differ in the position of a functional group on the carbon chain. For example
2-methylbutane and 2, 2-dimethylpropane
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•1-bromobutane and 2-bromobutane
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Functional isomerism• Functional isomers have same molecular
formula but differ in functional groups. For example– Ethyl alcohol and Dimethyl ether
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Metamerism• This type of isomerism is due to unequal
distribution of carbon atoms on either side of the functional group
• Such compounds are members of homologous series
• Example: Diethyl ether and Methyl propyl ether
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Tautomerism• This is a special type of functional isomerism
in which isomers are in dynamic equilibrium with each other
• For example: ethyl acetoacetate is an equilibrium mixture of 2 forms-93% keto form and 6% enol form
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Geometric isomerism
• In alkenes two carbons are linked through sigma bond (SP2 hybridized orbitals) and a pi bond due to overlap of p orbitals
• In such compounds these two carbons along with other 4 substituent remain in plane and are locked
• Because rotation will break the double bond
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• This restriction of rotation is responsible for geometric isomerism in alkenes
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• Consider the case of 2 butene. It exists in two special arrangements
• Cis 2 butene and Trans 2 butene
• Such compounds are referred as geometrical isomers
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• Cis isomer is one in which two similar groups are on the same side of a double bond
• Trans isomer is that in which two similar groups are on the opposite side of the double bond
• Conversion of such isomers into each other is only possible if heated at high temperature or absorb light. Energy around 62 Kcal/mole is needed to break pi bond
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• Trans isomers are more stable than the corresponding cis isomer
• This is because in trans bulky groups are on the opposite side, hence less repulsion or steric hindrance
• Geometrical isomers have different physical and chemical properties. These can be separated by conventional techniques like distillation and gas chromatography etc.
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Geometrical isomerism in cyclic compounds
• Geometrical isomerism is possible in cyclic compounds
• There should be restriction of rotation if two carbons are linked with a cyclic structure