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MHTCET 23 Apr 2025 Shift 2 Paper

Question : 83 of 150

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Identify correct decreasing order of basic strength of amines from following.

$\left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N} > \left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH} > \mathrm{CH}_{3} \mathrm{NH}_{2}$
$\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH} > \mathrm{CH}_{3} \mathrm{NH}_{2} > \left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N}$
$\left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH} > \left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N} > \mathrm{CH}_{3} \mathrm{NH}_{2}$
$\mathrm{CH}_{3} \mathrm{NH}_{2} > \left(\mathrm{CH}_{3}\right)_{2} \mathrm{NH} > \left(\mathrm{CH}_{3}\right)_{3} \mathrm{~N}$

Solution:

Concept:

In aqueous phase, basicity of aliphatic amines depends on inductive effect, solvation, and steric hindrance, giving order:

2^\circ > 1^\circ > 3^\circ

for methylamines.

Explanation:

For methyl-substituted amines, the aqueous basicity order is:

(\mathrm{CH}_3)_2\mathrm{NH} > \mathrm{CH}_3\mathrm{NH}_2 > (\mathrm{CH}_3)_3\mathrm{N}

This is because the

2^\circ

amine balances electron-donating methyl groups and effective solvation of the ammonium ion.

The

3^\circ

amine has maximum inductive effect but suffers from greater steric hindrance and reduced solvation, making it less basic than the

1^\circ

amine in water.

Therefore, the correct decreasing order is:

(\mathrm{CH}_3)_2\mathrm{NH} > \mathrm{CH}_3\mathrm{NH}_2 > (\mathrm{CH}_3)_3\mathrm{N}

Answer:

Option B.

(\mathrm{CH}_3)_2\mathrm{NH} > \mathrm{CH}_3\mathrm{NH}_2 > (\mathrm{CH}_3)_3\mathrm{N}

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