Q.Allyl chloride is hydrolysed more readily than n-propyl chloride. Why?

The key idea is that the carbocation intermediate formed during hydrolysis is stabilised by resonance in the allyl case, but not in the n-propyl case.

1. Allyl chloride can hydrolyse via the $S_N1$ pathway: its C–Cl bond ionises to give an allyl carbocation, $\text{CH}_2=\text{CH}-\text{CH}_2^+$, which is resonance-stabilised because the positive charge is delocalised over two carbon atoms.
2. n-Propyl chloride cannot follow this path readily — ionisation would give a simple primary carbocation, $\text{CH}_3-\text{CH}_2-\text{CH}_2^+$, which has no resonance stabilisation and is far too unstable, so n-propyl chloride reacts only slowly (via the $S_N2$ route instead). …

The key to this question lies in the stability of the intermediate carbocation formed during hydrolysis. Both allyl chloride and n-propyl chloride are primary alkyl halides, so you might expect them to react similarly. But allyl chloride is special — the double bond right next to the chlorine atom changes everything.

Let’s walk through the reasoning step by step.

1. Identify the reaction type. Hydrolysis of an alkyl halide (with water or aqueous base) typically follows either an SN1 or SN2 mechanism. For a primary halide like n-propyl chloride, SN2 is the usual path. But allyl chloride is an exception: it can react via SN1 even though it’s primary, because the carbocation formed is unusually stable.

2. Draw the carbocation intermediates.

   • For n-propyl chloride, if it were to ionise, you’d get a primary carbocation: $\text{CH}_3\text{CH}_2\text{CH}_2^+$ This is highly unstable — no resonance stabilisation, only weak hyperconjugation from three C–H bonds.
   • For allyl chloride, ionisation gives the allyl carbocation: $\text{CH}_2=\text{CH}-\text{CH}_2^+$ Here, the positive charge is not stuck on one carbon. The $\pi$ electrons of the double bond can delocalise into the empty p orbital, spreading the charge over two carbons.

3. Resonance stabilisation is the game-changer.

   The allyl carbocation has two equivalent resonance structures:

   $\text{CH}_2=\text{CH}-\overset{+}{\text{C}}\text{H}_2 \longleftrightarrow \overset{+}{\text{C}}\text{H}_2-\text{CH}=\text{CH}_2$

   This delocalisation lowers the energy of the carbocation significantly. In contrast, the n-propyl carbocation has no such resonance — it’s just a high-energy, localised positive charge.

4. Connect stability to reaction rate. In an SN1 reaction, the rate-determining step is the formation of the carbocation. The more stable the carbocation, the lower the activation energy for that step, and the faster the reaction.
   • Allyl chloride forms a resonance-stabilised carbocation → low $E_a$ → fast hydrolysis. …

Concept: Resonance Stabilisation of Carbocation Intermediates in Nucleophilic Substitution

The key concept is that allyl chloride undergoes hydrolysis via an SN1 mechanism where the intermediate carbocation is stabilised by resonance, while n-propyl chloride cannot form such a stabilised carbocation.

Method: Resonance Stabilisation Analysis

Step 1 — Identify the reaction type

Both compounds undergo hydrolysis (nucleophilic substitution with water). Allyl chloride reacts predominantly via SN1 because the carbocation formed is stabilised. n-Propyl chloride reacts via SN2 (or slow SN1) due to a less stable carbocation.

Step 2 — Write the carbocation formed from each

• Allyl chloride: $\text{CH}_2=\text{CH}-\text{CH}_2\text{Cl} \xrightarrow{\text{loss of Cl}^-} \text{CH}_2=\text{CH}-\text{CH}_2^+$
• n-Propyl chloride: $\text{CH}_3-\text{CH}_2-\text{CH}_2\text{Cl} \xrightarrow{\text{loss of Cl}^-} \text{CH}_3-\text{CH}_2-\text{CH}_2^+$

Step 3 — Draw resonance structures for the allyl carbocation

The positive charge is delocalised over two carbon atoms:

This resonance stabilisation lowers the energy of the carbocation. …

Why Allyl Chloride Hydrolyses Faster Than n-Propyl Chloride

The Core Concept

The key lies in carbocation stability during the hydrolysis (nucleophilic substitution) reaction.

• Allyl chloride ($CH_2=CH-CH_2Cl$) forms an allyl carbocation ($CH_2=CH-CH_2^+$) which is stabilised by resonance — the positive charge is delocalised over two carbon atoms.
• n-Propyl chloride ($CH_3-CH_2-CH_2Cl$) forms a primary carbocation ($CH_3-CH_2-CH_2^+$) which has no resonance stabilisation.

Result: Allyl chloride undergoes $S_N1$ hydrolysis much faster.

Common Mistakes & How to Avoid Them

✗ Mistake 1: "Allyl chloride is more reactive because of the double bond's electron-withdrawing effect"

Why it's wrong: The double bond is electron-donating via resonance, not withdrawing. It stabilises the positive charge, not destabilises the leaving group.

✓ How to avoid: Always think: Does the double bond help stabilise the carbocation? If yes, the reaction is faster. Draw the resonance structures of the allyl carbocation to see the delocalisation.

✗ Mistake 2: "Both undergo $S_N2$, but allyl is faster because of the double bond"

Why it's wrong: Allyl chloride can undergo $S_N2$ (slightly faster than n-propyl due to $\pi$-orbital participation), but the major reason for its much higher reactivity is $S_N1$ via a resonance-stabilised carbocation. n-Propyl chloride cannot do $S_N1$ easily.

✓ How to avoid: Compare the mechanism:

• Allyl chloride: Favours $S_N1$ (tertiary-like stability due to resonance)
• n-Propyl chloride: Only $S_N2$ possible (primary carbocation too unstable)

✗ Mistake 3: "The leaving group (Cl) is more easily removed in allyl chloride"

Why it's wrong: The C–Cl bond strength is nearly identical in both. The difference is not in the leaving group ability but in the stability of the intermediate.

✓ How to avoid: Focus on the carbocation intermediate, not the starting bond. The rate-determining step in $S_N1$ is carbocation formation — stabilising the carbocation lowers the activation energy.

✗ Mistake 4: "Allyl group is electron-withdrawing by induction, so it makes the carbon more positive and easier to attack"

Why it's wrong: The allyl group is electron-donating by resonance (the $\pi$ bond donates electron density to the positive centre). Inductive effects are weak here; resonance dominates.

✓ How to avoid: Draw the resonance hybrid:

The positive charge is shared — this stabilises the carbocation, making formation easier.

✗ Mistake 5: "n-Propyl chloride also forms a stable carbocation because of hyperconjugation" …