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What is the correct order of pKb values of the following amines?

(a) Methanamine > Ethanamine > Benzenamine

(b) Benzenamine > Ethanamine > Methanamine

(c) Ethanamine > Methanamine > Benzenamine

(d) Benzenamine > Methanamine > Ethanamine

The question was posed to me in an interview for internship.

This interesting question is from Amines Chemical Reactions topic in division Amines of Chemistry – Class 12

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Correct option is (d) Benzenamine > Methanamine > Ethanamine

Best explanation: The higher pKb value means lower basic strength. This means that benzenamine has the lower basic strength among methanamine and ethanamine. This is because of the electron withdrawing nature of the aryl group. Also in primary amines, the increase in size of alkyl group increases +I effect leading to high electron density on N atom and as a result higher basicity.

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