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In water, which of the following amines is the strongest base, i.e., has the lowest pKb value?

1. (CH3)2NH
2. CH3NH2
3. (CH3)3N
4. C6H5NH2

Correct answer: (CH3)2NH

Solution

In aqueous solution basic strength of methylamines is (CH3)2NH > CH3NH2 > (CH3)3N, reflecting the balance of +I effect, solvation of the cation and steric hindrance. Aniline is far weaker (lone pair delocalised into the ring). So (CH3)2NH has the lowest pKb.

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