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Chapter 24: Phenols
Preparation of Phenols |
Reactions of Phenols |
Reactions of Aryl Ethers |
Spectroscopic Analysis |
Self Assessment |
Reactions of Aryl Ethers
Electrophilic Aromatic Substitution
Reaction type: Electrophilic
- Aryl ethers are
potentially very reactive towards electrophilic
- 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.
of Aryl Ethers by HI or HBr
Reaction type: Nucleophilic
- 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 SN1
or SN2 like.
Why don't phenols react with HBr to give aryl bromides ?
Could a diaryl ether be easily cleaved using this method ?
ACID CLEAVAGE OF ARYL ETHERS
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.
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.
Rearrangement of Aryl Allyl Ethers:
Reaction type : Electrocyclic
reaction or sigmatropic rearrangement
Caution: Do not confuse
this reaction with the Claisen condensation of esters.
allyl ethers undergo a thermal rearrangement to give ortho-allylphenols.
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.
how closely the mechanism of this reaction (table below) resembles that of
the Diels-Alder reaction:
MECHANISM OF CLAISEN
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....
Tautomerization of the ketone (actually a dienone) to the more stable
aromatic enol, the phenol.
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