18-18-9P - Organic Chemistry (Mcmurry)

Question

Why are HI and HBr more effective than HCl in cleaving ethers? (See Section 11-3).

Step-by-Step Solution

Answer

The displacement depends upon how much better the nucleophile is.  and  are better nucleophiles than . Thus, the cleavage of ether proceeds better when there is HI or HBr than with HCl.

Acid-induced cleavage of ether

In acid-induced cleavage of ether, ether is treated with strong acid. It generates oxonium ion, collapsing to form an alcohol and a tertiary carbocation which further forms an alkene.

Explanation for why HI and HBr are more effective than HCl in cleaving ethers

In acid-induced ether cleavage, HX protonates the oxygen atom of the ether, and halide then brings about a nucleophilic displacement. It forms alcohol and organic halide. The displacement depends upon how much better the nucleophile is. Since  and  are better nucleophiles than . Thus, the cleavage of ether proceeds better when there is HI or HBr than with HCl.

The mechanism of the acid-induced cleavage of ether

The general mechanism of the acid-induced cleavage is shown below,

               The mechanism of the reaction