Isomerism - Practice Questions & MCQ

Structural isomerism
Compounds having the same molecular formula but different structures (manners in which atoms are linked) are classified as structural isomers. Some typical examples of different types of structural isomerism are given below:

• Chain isomerism: When two or more compounds have similar molecular formula but different carbon skeletons, these are referred to as chain isomers and the phenomenon is termed as chain isomerism. For example, C₅H₁₂ represents three compounds as follows:
   
  • Position isomerism: When two or more compounds differ in the position of substituent atom or functional group on the carbon skeleton, they are called position isomers and this phenomenon is termed as position isomerism. For example, the molecular formula C₃H₈O represents two alcohols:
    
            
• Functional group isomerism: Two or more compounds having the same molecular formula but different functional groups are called functional isomers and this phenomenon is termed as functional group isomerism. For example, the molecular formula C₃H₆O represents an aldehyde and a ketone as follows:

• Metamerism: It arises due to different alkyl chains on either side of the functional group in the molecule. For example, C₄H₁₀O represents methoxypropane (CH₃OC₃H₇) and ethoxyethane (C₂H₅OC₂H₅).

It is the type of isomerism in which the compounds possessing same molecular formula differ in their properties due to the difference in their geomtery that is, due to the difference in the direction of attachment of same atoms or groups in their molecule. It is not shown by single bonded compounds like(C-C) due to free rotation.
Geometrical isomerism is shown by [>C=C<], [>C=N-], [-N=N-] and cyclo alkanes.

Geometrical isomerism in Alkenes and Cyclo Alkanes

• Case I 
  These compounds cannot exhibit geometrical isomerism
  
  
  These compounds cannot exhibit geometrical isomerism
  
• Case II
  
  
  
   

Difference between Cis and Trans Forms

Cis

• Cis is more reactive, but less stable form as the same species are on the same side, so steric repulsion increases reactivity and decreases stability.
• The dipole moment of cis is more.
• It has less melting point as same groups are on the same side.
• The boiling point of cis is more.
• The solubility, viscosity, and refractive index of cis is more. Trans

Trans

• It is more stable, but less reactive form as same species are on the opposite sides.
• The dipole moment is mostly zero due to symmetry in case of symmetrical alkenes.
  For example,
  
• In case of unsymmetrical alkenes, due to little unsymmetry there may be some dipole moment value as well
  
  For example, trans-2-pentene has some dipole moment value, but lesser than cis form.
• It has more melting point than cis.
• The boiling point of trans form is less.
• The solubility, viscosity, and refractive index are less than cis form.

Geometrical isomerism in cycloalkanes

 It cannot show geometrical isomerism as one carbon atom has two similar species 'A'.

Geometrical isomerism in Oximes and Azo compounds

• In oximes
  
   
• In azo compounds
  
  Here, the attached groups may also differ, that is, A and B.

• When Ends are Different Number of geometrical isomers = 2ⁿ  
  Here, n = number of double bonds
  For example, CH₃-CH=CH-CH=CH-Cl
  Here, n=2
  Number of geometrical isomers = 2ⁿ =2² = 4
• When Ends are Same
  (i) When n is an even number
  Number of Geometric Isomers = 2^n-1 + 2^{n / 2-1}
  For example, X-CH=CH-CH-CH=CH-CH=CH-CH=CH-X
  n=4
  Number of geometric isomers = 2^4-1 + 2^{4 / 2-1}
  =2³+2¹ = 10
  
  (ii) When n is an odd number
  Number of geometric isomers = 2^n-1 + 2^{{(n+1) / 2} - 1}
  For example, CH₃-CH=CH-CH=CH-CH=CH-CH₃
  Here, n=3
  Number of geometric isomers = 2^3-1 + 2^{{(3+1) / 2} - 1}
  =2² + 2¹ = 6