Question

Which of the following has the lowest $ pK_b $ value?

• A. $ \text{CH}_3\text{-NH}_2 $
• B. $ (\text{CH}_3\text{CH}_2)_2\text{NH} $
• C.

  

• D.

  

Hint:

To determine the base strength and $ pK_b $ value: 1. Aliphatic amines are generally stronger bases than aromatic amines. 2. Secondary amines are stronger than primary amines due to additional electron donation from alkyl groups. 3. Lower $ pK_b $ values indicate stronger bases.

Answer 1

The Correct Option is B

Step 1: Understand $ pK_b $ and Base Strength
The $ pK_b $ value is related to the base strength of a compound: $$ pK_b = -\log(K_b) $$ where $ K_b $ is the base dissociation constant. A lower $ pK_b $ value indicates a stronger base because it corresponds to a higher $ K_b $ value. Step 2: Analyze Each Compound
1) $ \text{CH}_3\text{-NH}_2 $ (Methylamine)
Methylamine is a primary amine.
Primary amines are moderately strong bases due to the electron-donating effect of the alkyl group, but they are not very strong. 2) $ (\text{CH}_3\text{CH}_2)_2\text{NH} $ (Diethylamine)
Diethylamine is a secondary amine.
Secondary amines are stronger bases than primary amines because the presence of two alkyl groups provides more electron donation, stabilizing the conjugate acid formed during protonation. 3) $ \text{C}_6\text{H}_5\text{-NH}_2 $ (Aniline)
Aniline is an aromatic amine.
The phenyl group ($ \text{C}_6\text{H}_5 $) is electron-withdrawing due to resonance effects, which reduces the basicity of the nitrogen atom. Therefore, aniline is a weaker base compared to aliphatic amines. 4) $ \text{C}_6\text{H}_5\text{-NH-CH}_3 $ (N-Methylaniline)
N-Methylaniline is an aromatic amine with a methyl group attached to the nitrogen.
While the methyl group donates electrons, the overall effect is still weaker than that of aliphatic amines due to the electron-withdrawing nature of the phenyl group. Step 3: Compare Base Strengths
From the analysis:
$ (\text{CH}_3\text{CH}_2)_2\text{NH} $ (Diethylamine) is the strongest base among the given options because it is a secondary amine with two electron-donating alkyl groups.
Stronger bases have lower $ pK_b $ values.
Step 4: Identify the Correct Option
Option (1): $ \text{CH}_3\text{-NH}_2 $
Incorrect — primary amine, weaker than diethylamine. Option (2): $ (\text{CH}_3\text{CH}_2)_2\text{NH} $
Correct — secondary amine, strongest base among the options. Option (3): $ \text{C}_6\text{H}_5\text{-NH}_2 $
Incorrect — aromatic amine, weaker due to electron-withdrawing phenyl group. Option (4): $ \text{C}_6\text{H}_5\text{-NH-CH}_3 $
Incorrect — aromatic amine, weaker than aliphatic amines.