Question

The mechanism for the reaction of ether with the concentrated HI is as follows.


Which of the following statement(s) is/are true about above mechanism?
I The reaction takes place with $HBr$ or $HI$ because these reagents are sufficiently acidic
II Iodide is a good nucleophile. It attacks the least substituted carbon of oxonium ion formed in step I and displaces an alcohol molecule by $S_{N}2$ mechanism
III When primary or secondary alkyl groups are present, it is the lower alkyl group that forms alkyl iodide $(S_{N}2$ reaction) Correct choice is

• A. $I$ and $II$
• B. $II$ and $III$
• C. $I$ and $III$
• D. $I,II$ and $III$

Answer 1

Answer: (D)

The reaction of an ether with concentrated $HI$ starts with protonation of ether molecule.

The reaction takes place with $HBr$ or $HI$ because these reagents are sufficiently acidic.
Step II Iodide is a good nucleophile. It attacks the least substituted carbon of the oxonium ion formed in step $1$ and displaces an alcohol molecule by $S_{N}2$ mechanism. When primary or secondary alkyl groups are present, it is the lower alkyl group that forms alkyl iodide $(S_{N}2$ reaction).

When $HI$ is in excess and the reaction is carried out at high temperature, ethanol reacts with another molecule of $HI$ and is converted to ethyl iodide.