Isomers and isomerism

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Isomers and isomerism

  1. 1. 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
  2. 2. Types of isomerism• Mainly isomerism is of two types – 1. Structural isomerism – 2. Stereoisomerism1-Structural isomers – Are compounds having same molecular formula but different structural formula
  3. 3. –These are of further 5 types • Chain isomerism • Position isomerism • Functional isomerism • Metamerism • Tautomersim
  4. 4. • 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
  5. 5. 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
  6. 6. 2-methylbutane and 2, 2-dimethylpropane Positional isomers• Positional isomers have same molecularformula but differ in the position of afunctional group on the carbon chain. Forexample
  7. 7. •1-bromobutane and 2-bromobutane
  8. 8. Functional isomerism• Functional isomers have same molecular formula but differ in functional groups. For example – Ethyl alcohol and Dimethyl ether
  9. 9. 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
  10. 10. 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
  11. 11. 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
  12. 12. • This restriction of rotation is responsible for geometric isomerism in alkenes
  13. 13. • 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
  14. 14. • 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
  15. 15. • 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.
  16. 16. 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

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