Electrophilic Aromatic Substitution

Summary

• Aryl ethers are potentially very reactive towards electrophilic aromatic substitution.
• This is because the alkoxy group, -OR,  is a strongly activating, ortho- / para- directing substituent (review)
• These reactions are essentially the same as those of the phenols (where R = H)
• Substitution typically occurs para to the alkoxy group unless the para position is blocked, then ortho substitution occurs.
• The strong activation often means that milder reaction conditions than those used for benzene itself can be used.
• Aryl ethers are so activated that polysubstitution can be a problem.

Related reactions

• Electrophilic Aromatic Substitutions reactions of Phenols
• Electrophilic Aromatic Substitutions reactions of Benzenes
• Electrophilic Aromatic Substitutions reactions of Aniline

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.

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

Related reactions

• Cleavage of  Ethers by HI or HBr
• Reactions of Alcohol 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.

Claisen Rearrangement of Aryl Allyl Ethers:

Summary

• Aryl allyl ethers undergo a thermal rearrangement to give ortho-allylphenols.
• This reaction is an intramolecular process.
• A sigmatropic rearrangement is a reaction is which a s bond migrates from one end of a p system to the other.
• Note how closely the mechanism of this reaction (table below) resembles that of the Diels-Alder reaction:

MECHANISM OF CLAISEN REARRANGEMENT
Step 1: An electrocyclic process. Push those electrons around the 6 membered ring.... Best appreciated by starting the electron flow by having the aromatic C=C attacking the allyl C=C and displacing an O leaving group....
Step 2: Tautomerization of the ketone (actually a dienone) to the more stable aromatic enol, the phenol.