Question

Identify the weakest base among the following.

• A. Ammonia
• B. iso-propyl amine
• C. n-propyl amine
• D. Phenyl amine

Hint:

In aniline the lone pair on nitrogen is delocalised into the benzene ring.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept
The basic strength of an amine depends on how freely the lone pair on nitrogen can accept a proton.

Step 2: Alkyl amines
In n-propylamine and isopropylamine, the alkyl group pushes electrons (+I effect) towards nitrogen, so they are stronger bases than ammonia.

Step 3: Phenylamine
In phenylamine (aniline), the lone pair on nitrogen is delocalised into the benzene ring by resonance. It is much less available for protonation.

Step 4: Check with \(\text{pK}_{b}\)
\(\text{pK}_{b}\) of aniline is about 9.4, ammonia is 4.75 and propylamines are about 3.4. The larger value means a weaker base.

Final Answer:
Phenylamine is the weakest base. \[ \boxed{\text{(D) }\text{Phenyl amine}} \]