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ChemistryMediumClass 12

Reactions of Ethers — Cleavage with HI

Alcohols, Phenols & Ethers

8

JEE Qs

8%

Hard

60

min

Always identify the nature of the alkyl groups (primary, secondary, tertiary, aryl, benzyl, allyl) attached to the ether oxygen to correctly determine the reaction mechanism (SN1 vs SN2) and predict the products.

🧮 Key Formulas

R-O-R' + HI (cold) --> R-OH + R'-I (or R-I + R'-OH, depending on mechanism)
R-O-R' + excess HI + heat --> R-I + R'-I + H2O
Ar-O-R + HI --> Ar-OH + R-I

✅ Key Points for JEE

  • 1The mechanism (SN1 or SN2) of ether cleavage with HI depends on the nature of the alkyl groups attached to the oxygen.
  • 2For ethers with primary or secondary alkyl groups, cleavage proceeds via an SN2 mechanism, where the iodide ion attacks the less sterically hindered carbon of the protonated ether.
  • 3For ethers containing tertiary, benzylic, or allylic alkyl groups, cleavage occurs via an SN1 mechanism, forming a relatively stable carbocation which is then attacked by iodide.
  • 4Phenolic ethers (Ar-O-R) always cleave at the alkyl-oxygen bond (R-O), yielding phenol (Ar-OH) and alkyl iodide (R-I); the aryl-oxygen bond (Ar-O) does not cleave due to its partial double bond character.
  • 5When excess HI is used, especially under heating, any alcohol formed as an intermediate product will further react with HI to form the corresponding alkyl iodide, resulting in two alkyl iodides and water.

⚠️ Common Mistakes

  • ✕Incorrectly applying SN1/SN2 rules, leading to the wrong major product, especially for unsymmetrical ethers with mixed primary/tertiary or secondary/tertiary groups.
  • ✕Cleaving the aryl-oxygen bond in phenolic ethers (e.g., breaking Ar-O in anisole instead of Me-O), which is thermodynamically unfavorable.
  • ✕Failing to convert the intermediate alcohol to the corresponding alkyl iodide when excess HI and heating conditions are specified.
  • ✕Not considering carbocation rearrangements when an SN1 mechanism is involved, leading to an incorrect product structure.

NCERT Chapters

  • Class 12 Chemistry Ch 11: Alcohols, Phenols and Ethers