Chapter 24: Phenols

Cleavage of Aryl Ethers by HI or HBr

Reaction type: Nucleophilic Substitution

Summary

• Like alkyl ethers, alkyl aryl ethers, R-O-Ar, are cleaved by the strong acids HI or HBr in a nucleophilic substitution reaction.
• The products are the phenol and the appropriate alkyl halide.
• Since phenols do not react with hydrogen halides to give aryl halides, there can be no further reaction (unlike alkyl ethers and simple alcohols)
• Protonation of the ethereal oxygen creates a good leaving group, a neutral phenol molecule.
• The halide ion, bromide or iodide are both good nucleophiles.
• Depending on the structure of the alkyl group, the reaction can be S_N1 or S_N2 like.

• Why don't phenols react with HBr to give aryl bromides ? ANSWER
• Could a diaryl ether be easily cleaved using this method ? ANSWER

Related reactions

• Cleavage of  Ethers by HI or HBr
• Reactions of Alcohols with Hydrogen Halides

MECHANISM FOR ACID CLEAVAGE OF ARYL ETHERS
Step 1: An acid/base reaction. Protonation of the ether oxygen to make a better leaving group. This step is very fast and reversible.  The lone pairs on the oxygen make it a Lewis base.
Step 2: The bromide ion functions as the nucleophile and attacks to displace the good leaving group, neutral phenol molecule, by cleaving the C-O bond. This results in the formation of an alkyl bromide and a phenol.

© Dr. Ian Hunt, Department of Chemistry