Q.Assertion: Acylation of amines gives a monosubstituted product whereas alkylation of amines gives polysubstituted product.
Reason: Acyl group sterically hinders the approach of further acyl groups.

Concept: Steric hindrance and resonance effects in acylation vs. alkylation of amines.

Reasoning:

1. Alkylation of amines produces a more nucleophilic product (e.g., $RNH_2 \rightarrow R_2NH$), so further alkylation occurs readily, giving polysubstitution.
2. Acylation introduces an acyl group ($-COR$). The lone pair on nitrogen is delocalised into the carbonyl $\pi$-system, making the amide nitrogen much less nucleophilic than the starting amine. …

1. Understanding the assertion.

   When an amine reacts with an alkyl halide (alkylation), the product is itself a more nucleophilic amine, so it reacts again — leading to a mixture of mono-, di-, and tri-alkylated products. This is a classic problem in organic synthesis.

   In contrast, when an amine reacts with an acyl chloride or anhydride (acylation), the reaction stops cleanly at the monoacylated stage. The product is an amide, and further acylation does not occur under normal conditions. So the assertion is correct.

2. Why does acylation stop at one step?

   The acyl group ($\ce{-COR}$) is strongly electron-withdrawing by resonance. Once it attaches to the nitrogen, the lone pair on nitrogen is delocalised into the carbonyl $\pi$-system:

The resulting amide nitrogen’s lone pair is partially tied up in resonance with the carbonyl, making it much less nucleophilic than the original amine. In fact, amides are essentially non-basic and do not undergo further acylation under mild conditions.

3. Now examine the reason given. …

Concept: Steric Hindrance in Acylation vs. Alkylation of Amines

Method: Steric and Electronic Effect Analysis

Step 1 — Understand the Assertion

• Acylation of amines (e.g., with acid chlorides or anhydrides) gives monosubstituted amides.
• Alkylation of amines (e.g., with alkyl halides) gives polysubstituted products (mixtures of mono-, di-, and trialkylated amines).

Step 2 — Understand the Reason

The reason claims that the acyl group is sterically hindering, preventing further acylation.

Step 3 — Evaluate the Reason

• The acyl group ($\ce{-COR}$) does introduce some steric bulk, but the primary reason for monosubstitution in acylation is electronic:
  • The amide formed ($\ce{RCONHR'}$) has a resonance-stabilized $\ce{N}$ atom — the lone pair is delocalized into the carbonyl, making it much less nucleophilic than the original amine. …

Common Mistakes & How to Avoid Them

Mistake 1: Thinking the Reason is Correct

Many students assume that steric hindrance is the main reason why acylation stops at the monosubstituted stage.

Why this is wrong:

The actual reason is electronic deactivation — after the first acylation, the amide formed ($\ce{RCONHR}$) has a carbonyl group that withdraws electrons via resonance, making the nitrogen less nucleophilic. This prevents further acylation.

How to avoid:

Always check the electronic effect of the substituent. Acyl groups are electron-withdrawing (via resonance), not just bulky. Steric hindrance plays a minor role here.

Mistake 2: Confusing Alkylation with Acylation

Students often think both reactions behave similarly because both involve adding a group to the nitrogen.

Key difference to remember:

• Alkylation: Alkyl groups are electron-donating → makes nitrogen more nucleophilic → polysubstitution occurs.
• Acylation: Acyl groups are electron-withdrawing → makes nitrogen less nucleophilic → stops at monosubstitution.

How to avoid:

Memorise this contrast:

Alkyl = donor → polysubstitution

Acyl = acceptor → monosubstitution

Mistake 3: Assuming the Reason Explains the Assertion

Even if you think the reason is correct, you must check if it logically explains the assertion.

Why it fails here:

The assertion is correct (acylation gives monosubstituted product), but the reason given (steric hindrance) is not the correct explanation. The correct explanation is electronic deactivation.

How to avoid:

For assertion-reason questions, always ask:

"Does this reason directly cause the observed effect?"

If the mechanism is different, mark it as "reason not correct explanation."

Mistake 4: Overlooking the "Polysubstitution" in Alkylation …