Question
Wed March 11, 2015 By: Rohit Virinchi

On what basis does the R-O Cleavage takes place when R-O-R reacts with HI ?
 eg-: CH3-CH2-CH2-O-CH3 + HI ---> CH3-CH2-CH2-OH + CH3I ,
Why is the product not CH3-CHI-CH3 + CH4 ? 

Expert Reply
Prachi Sawant
Thu March 12, 2015
The clevage of ethers by halogen acids occurs by the following mechanism:
step 1: Ethers being Lewis bases, undergo protonation to form oxonium ions. In this compound the oxonium ion formed is given below.
 
Step 2: Iodide ion is a good nucleophile. Due to steric hindrance, it attacks the smaller alkyl group of the oxonium ion formed in step 1 and displaces the alcohol molecule by SN2 mechanism as shown below:
In the given molecule, methyl group is smaller alkyl group, hence methyl iodide is formed.
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