Amines

Amines

Question

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

  • (CH3)2NH

  • CH3NH2

  • (CH3)3N

  • C6H5NH2

Answer

A.

(CH3)2NH

Order of basic strength of aliphatic amine in aqueous solution is as follows (order of Kb)
left parenthesis CH subscript 3 right parenthesis subscript 2 straight N with minus on top straight H space greater than thin space CH subscript 3 straight N with minus on top straight H subscript 2 greater than space left parenthesis CH subscript 3 right parenthesis subscript 3 straight N with minus on top space greater than thin space straight C subscript 6 straight H subscript 5 straight N with minus on top straight H subscript 2
As space we space know space pK subscript straight b space equals space minus space log space straight K subscript straight b
So (CH3)2 NH will have smallest pKb value.
In the case of phenylamine, N is attached to sp2 hybridised carbon, hence it has highest pKb and least basic strength.

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