NEET – 2015

Hint: Primary alcohols react with HCl via $SN_2$ mechanism, which is slow and inefficient.

Reactions (I) and (IV) can yield alkyl halide due to the presence of anhydrous $ZnCl_2$ as it acts a catalyst. Reaction (III) is also possible because halogenation will take place by $SN_1$ mechanism and alcohol is tertiary resulting in stable carbocation.

Primary alcohols react with HCl via $SN_2$ mechanism, which is slow and inefficient due to:

• Poor leaving group (-OH is a bad leaving group).

• HCl alone cannot easily protonate the OH group enough to make it a good leaving group.

• No carbocation is formed (since primary carbocations are unstable).

🔹 Why ZnCl₂ is needed (Lucas reagent):

• ZnCl₂ is a Lewis acid that coordinates with the -OH group.

• Converts the -OH into a better leaving group by forming a complex like:
  
  R-OH···ZnCl₂ → R-O⁺-ZnCl₂⁻

• This allows Cl⁻ from HCl to displace the complex and form the alkyl halide.