The correct order of decreasing basic strength in aqueous solution of the following is :
Show Hint
- $\mathrm{(C_2H_5)_3N}$ > $\mathrm{C_2H_5NH_2}$ > $\mathrm{(C_2H_5)_2NH}$
- $\mathrm{(C_2H_5)_2NH}$ > $\mathrm{C_2H_5NH_2}$ > $\mathrm{(C_2H_5)_3N}$
- $\mathrm{C_2H_5NH_2}$ > $\mathrm{(C_2H_5)_2NH}$ > $\mathrm{(C_2H_5)_3N}$
- $\mathrm{(C_2H_5)_2NH}$ > $\mathrm{(C_2H_5)_3N}$ > $\mathrm{C_2H_5NH_2}$
The Correct Option is D
Solution and Explanation
Step 1: The two opposite effects
More ethyl groups push more electron density onto the nitrogen, and that makes the amine a stronger base. This effect favours the tertiary amine. But once the amine grabs a proton, water steadies it using N−H bonds, and a tertiary amine has fewer N−H bonds, so water steadies it less. This second effect works against the tertiary amine.
Step 2: The actual measured order
When both effects are put together for ethyl amines, experiments in water show the secondary amine is the strongest, then the tertiary, then the primary: \[ (C_2H_5)_2NH > (C_2H_5)_3N > C_2H_5NH_2 \]
Step 3: Match the option
This order, secondary then tertiary then primary, is exactly what option (D) shows.
Answer: Option (D), $\mathrm{(C_2H_5)_2NH}$ > $\mathrm{(C_2H_5)_3N}$ > $\mathrm{C_2H_5NH_2}$.
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(i) In decreasing order of the pKb values: C2H5NH2 , C6H5NHCH3 , (C2H5 )2NH and C6H5NH2
(ii) In increasing order of basic strength: C6H5NH2 , C6H5N(CH3 )2 , (C2H5 )2NH and CH3NH2
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(iii) Methanol to ethanoic acid
(iv) Ethanamine into methanamine
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(viii) Propanoic acid into ethanoic acid?
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