JEE Main 2021 · Chemistry · Carbonyl Compounds, Carboxylic Acids and Amines · medium

Arrange the following in the correct order of basic strength in aqueous solution: NH3, C2H5NH2, (C2H5)2NH, (C2H5)3N

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Arrange the following in the correct order of basic strength in aqueous solution:

\mathrm{NH_3}

\mathrm{C_2H_5NH_2}

(\mathrm{C_2H_5})_2\mathrm{NH}

(\mathrm{C_2H_5})_3\mathrm{N}

A. $(\mathrm{C_2H_5})_3\mathrm{N}>(\mathrm{C_2H_5})_2\mathrm{NH}>\mathrm{C_2H_5NH_2}>\mathrm{NH_3}$
B. $(\mathrm{C_2H_5})_2\mathrm{NH}>(\mathrm{C_2H_5})_3\mathrm{N}>\mathrm{C_2H_5NH_2}>\mathrm{NH_3}$
C. $(\mathrm{C_2H_5})_2\mathrm{NH}>\mathrm{C_2H_5NH_2}>(\mathrm{C_2H_5})_3\mathrm{N}>\mathrm{NH_3}$
D. $\mathrm{C_2H_5NH_2}>(\mathrm{C_2H_5})_2\mathrm{NH}>(\mathrm{C_2H_5})_3\mathrm{N}>\mathrm{NH_3}$

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Answer: (B) $(\mathrm{C_2H_5})_2\mathrm{NH}>(\mathrm{C_2H_5})_3\mathrm{N}>\mathrm{C_2H_5NH_2}>\mathrm{NH_3}$

In water, basicity depends on the +I effect of alkyl groups (which stabilises the ammonium ion) and on solvation of the ammonium cation through hydrogen bonding. Each added ethyl group increases electron density on nitrogen, so

\mathrm{NH_3}

is the weakest. Going from secondary to tertiary, the cation

R_3NH^+

has only one N–H available for hydrogen bonding and the bulky groups hinder solvation, so the extra +I effect is outweighed. For ethyl groups the observed order is

\mathrm{R_2NH}>\mathrm{R_3N}>\mathrm{RNH_2}>\mathrm{NH_3}

(pKb about 3.0, 3.2, 3.3, 4.7).

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