The correct sequence regarding base strength of aliphatic amines in aqueous solution is

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$R_{3} N > R_{2} NH > {RNH}_{2}$

$R_{2} NH > R_{3} N > {RNH}_{2}$

$R_{2} NH > {RNH}_{2} > R_{3} N$

${RNH}_{2} > R_{2} NH > R_{3} N$

answer is B.

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Detailed Solution

The correct sequence regarding base strength of aliphatic amines in aqueous solution is $R_{2} NH > {RNH}_{2} > R_{3} N$ .

The base strength of aliphatic amines is decided by the following two factors. Induction factor Increasing replacement of H by R (electron-releasing group) increases electron density on N, thus making it more basic.

Solvation factor Stabilization of conjugate acid through H-bonding with water. More the number of H atoms, more the stability and thus more basic the amine. Thus, induction and solvation operate in opposite directions. Induction dominates to make all three alkylamines more basic than ammonia. However, adding a third R as in $R_{3} N$ does not further enhance the basicity because now the opposing solvation effect assumes more importance.

Hence, option B is correct.

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