Q39 / 120 · VITEEE 2006 · Chemistry

The sequence of basic strength of alky1 amines follows the order

  • Correct answer
    A
    $\mathrm{RNH}_{2} < \mathrm{R}_{2} \mathrm{NH}>\mathrm{R}_{3} \mathrm{~N}$
  • B
    $\mathrm{R}_{2} \mathrm{NH}_{2} < \mathrm{R}_{2} \mathrm{NH} < \mathrm{R}_{3} \mathrm{~N}$
  • C
    $\mathrm{R}_{2} \mathrm{NH} < \mathrm{RNH}_{2} < \mathrm{R}_{3} \mathrm{~N}$
  • D
    $\mathrm{RNH}_{2} < \mathrm{R}_{2} \mathrm{NH} < \mathrm{R}_{3} \mathrm{~N}$

Solution

It is expected that the basic nature of amines should be in order tertiary > secondary $>$ primary but the observed order in the case of lower members is found to be as secondary > Primary $>$ tetriary. This anomalous behaviour of tetriary amines is due to steric factors i.e crowding of alkyl groups cover nitrogen atom from all sides thus makes the approach and bonding by a proton relatively difficult which results the maximum steric strain in tetiary amines. The electrons are there but the path is blocked, resulting the reduction in basicity. Thus the correct order is $\mathrm{R}_{2} \mathrm{NH}>\mathrm{RNH}_{2}>\mathrm{R}_{3} \mathrm{~N}$

Hence, the correct answer is option: 'A'.

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