Question

The increasing order of basic strength of the following amines in aqueous solution is: I. $\mathrm{CH_3NH_2}$ II. $(\mathrm{CH_3})_2\mathrm{NH}$ III. $(\mathrm{CH_3})_3\mathrm{N}$ IV. $\mathrm{NH_3}$

• A. $\mathrm{NH_3 < (CH_3)_3N < CH_3NH_2 < (CH_3)_2NH}$
• B. $(\mathrm{CH_3})_3\mathrm{N < NH_3 < CH_3NH_2 < (CH_3)_2NH}$
• C. $\mathrm{NH_3 < CH_3NH_2 < (CH_3)_3N < (CH_3)_2NH}$
• D. $(\mathrm{CH_3})_2\mathrm{NH < (CH_3)_3N < CH_3NH_2 < NH_3}$

Hint:

For simple aliphatic amines in aqueous solution: \[ 2^\circ > 1^\circ > 3^\circ > NH_3 \] because both inductive effect and solvation effect must be considered.

Answer 1

The Correct Option is A

Concept: The basic character of amines depends upon the availability of the lone pair of electrons on nitrogen for donation to a proton. In aqueous solution, not only the electron-releasing inductive effect of alkyl groups but also the solvation of the conjugate acid plays a very important role.

Step 1: Consider the effect of alkyl groups. Methyl groups exhibit a positive inductive effect $(+I)$ and increase the electron density on nitrogen. Therefore, \[ (\mathrm{CH_3})_3\mathrm{N} > (\mathrm{CH_3})_2\mathrm{NH} > \mathrm{CH_3NH_2} > \mathrm{NH_3} \] would be expected if only the inductive effect were considered.

Step 2: Consider solvation in aqueous medium. The protonated amines are stabilized by hydrogen bonding with water molecules. The extent of solvation follows: \[ \mathrm{RNH_3^+} > \mathrm{R_2NH_2^+} > \mathrm{R_3NH^+} \] because tertiary ammonium ions are sterically hindered and are less effectively solvated.

Step 3: Combine both effects. In aqueous solution, secondary amines are the strongest bases because they receive sufficient electron donation from alkyl groups and are still reasonably solvated. Thus, \[ (\mathrm{CH_3})_2\mathrm{NH} > \mathrm{CH_3NH_2} > (\mathrm{CH_3})_3\mathrm{N} > \mathrm{NH_3} \] in decreasing order of basic strength.

Step 4: Write increasing order. \[ \boxed{ \mathrm{NH_3} < (\mathrm{CH_3})_3\mathrm{N} < \mathrm{CH_3NH_2} < (\mathrm{CH_3})_2\mathrm{NH} } \] Hence, \[ \boxed{\text{Option (1)}} \]