Question

Explain the following briefly. 

Allyl chloride is hydrolysed readily as compared to 1-chloropropane.

Answer 1

1. Structure and Resonance Stability: Allyl chloride (CH₂=CH–CH₂Cl) contains a chlorine atom attached to an allylic carbon, which is adjacent to a double bond. The allylic carbon can stabilize the carbocation intermediate formed during hydrolysis by resonance delocalization of the positive charge over the π-system of the double bond. 1-Chloropropane (CH₃–CH₂–CH₂Cl) lacks this resonance stabilization as the chlorine is attached to a saturated carbon. 
2. Mechanism of Hydrolysis: Hydrolysis of alkyl halides typically proceeds by an SN₁ or SN₂ mechanism. Allyl chloride tends to undergo SN₁ hydrolysis more readily due to the resonance stabilization of the allylic carbocation intermediate. 1-chloropropane mainly undergoes SN₂ hydrolysis, which is slower because it depends entirely on the nucleophile’s back-side attack without carbocation stabilization.
3. Resonance Effect: The resonance structures for allyl carbocation are:
   \[\ce{CH2 = CH - \overset{+}{CH2} ↔ \overset{+}{CH2} - CH = CH2}\]
   This resonance spreads out the positive charge, lowering activation energy and increasing hydrolysis rate.
4. Reaction Equations:
   Hydrolysis of allyl chloride:
   \[\mathrm{CH_2=CH–CH_2Cl} + \mathrm{H_2O} \rightarrow \mathrm{CH_2=CH–CH_2OH} + \mathrm{HCl}\]
   Hydrolysis of 1-chloropropane:
   \[\mathrm{CH_3–CH_2–CH_2Cl} + \mathrm{H_2O} \rightarrow \mathrm{CH_3–CH_2–CH_2OH} + \mathrm{HCl}\]