Question

The decreasing order of basicity of the following amines is

• A. $\mathrm{I}>\mathrm{II}>\mathrm{III}>\mathrm{IV}$
• B. $\mathrm{IV}>\mathrm{III}>\mathrm{I}>\mathrm{II}$
• C. $\mathrm{II}>\mathrm{I}>\mathrm{III}>\mathrm{IV}$
• D. $\mathrm{IV}>\mathrm{I}>\mathrm{II}>\mathrm{III}$

Answer 1

The correct option is B

$\mathrm{IV}>\mathrm{III}>\mathrm{I}>\mathrm{II}$

The explanation for the correct option:

Option (B): $\mathrm{IV}>\mathrm{III}>\mathrm{I}>\mathrm{II}$

1. Compound $\mathrm{I}$ has ${\mathrm{sp}}^2$ hybridized nitrogen, and resonance is caused by the lone pairs.
2. In the compound $\mathrm{II}$, the nitrogen is ${\mathrm{sp}}^2$ hybridized and the lone pair electron is effectively involved in a five-member ring with two carbon-carbon doubles to generate a conjugated system of pi electrons that increases stability or decreases basicity.
3. Nitrogen is ${\mathrm{sp}}^2$ hybridized in compound ($\mathrm{III}$), and as it is already stable, lone pairs are not implicated in resonance.
4. In the compound $\mathrm{IV}$ nitrogen is ${\mathrm{sp}}^3$ hybridized i.e. less "s" character more basic is the compound.
5. Hence, it is the correct option.

The explanation for the incorrect options:

Option (A): $\mathrm{I}>\mathrm{II}>\mathrm{III}>\mathrm{IV}$

1. In the compound $\mathrm{IV}$ nitrogen is ${\mathrm{sp}}^3$ hybridized i.e. less "s" character more basic is the compound.
2. Hence, it is an incorrect option.

Option (C): $\mathrm{II}>\mathrm{I}>\mathrm{III}>\mathrm{IV}$

1. In the compound $\mathrm{IV}$ nitrogen is ${\mathrm{sp}}^3$ hybridized i.e. less "s" character more basic is the compound.
2. Hence, it is an incorrect option.

Option (D): $\mathrm{IV}>\mathrm{I}>\mathrm{II}>\mathrm{III}$

1. In the compound$\mathrm{II}$, the nitrogen is ${\mathrm{sp}}^2$ hybridized and the lone pair electron is effectively involved in a five-member ring with two carbon-carbon doubles to generate a conjugated system of pi electrons that decreases its stability.
2. Hence, it is an incorrect option

Therefore, the decreasing order of basicity of the amines is $\mathrm{IV}>\mathrm{III}>\mathrm{I}>\mathrm{II}$