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Considering the basic strength of amines in an aqueous solution, which one has the smallest pK_b, value?

A) $ (CH_{3})_{2}NH$

B) $ CH_{3}NH_{2}$

C) $ (CH_{3})_{32}N$

D) $ C_{6}H_{5}NH_{2}$

Option A

This problem can be solved by using the concept of effect of steric hindrance, hydration and H-bonding in basic strength of amines.

Order of basic strength of aliphatic amine solution is as follows (order of K_b)

$ ( CH_{3})_{2}\ddot{N}H>CH_{3}\ddot{N}H_{2}>(CH_{3})_{3}\ddot{N}>C_{6}H_{5}\ddot{N}H_{2}$

as we know, pK_b =-log K_b. So, $ ( CH_{3})_{2}\ddot{N}H$ will have smallest pK_b value.

In case of phenylamine, N is attached to sp² hybridised carbon, hence it has highest pK_b and least basic strength.

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