Does phenol undergo Friedel–Crafts reactions or does it react with Lewis acids like aniline does?
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3 Answers
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Like aniline, phenol too reacts to a very less extent during Friedel-Crafts reaction.
The reason being that the oxygen atom of phenol has lone pair of electrons which coordinate with Lewis acid.
In fact most substituents with lone pair would give poor yield.
The two pathways involved in the reaction with phenol reduce the overall yeild:
Phenols are examples of bidentate nucleophiles, meaning that they can react at two positions:
- on the aromatic ring giving an aryl ketone via C-acylation, a Friedel-Crafts reaction
or,
- on the phenolic oxygen giving an ester via O-acylation, an esterification
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1$\begingroup$ Does the coordination to the Lewis acid catalyst take place for all alkoxy (e.g. methoxy, ethoxy etc.) groups as well? Technically, due to the electron-donating effect of the alkyl group, wouldn't the extent of coordination to the catalyst increase for these groups? $\endgroup$ Commented Feb 19, 2018 at 16:04
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1$\begingroup$ You can perhaps answer this question as well while you are responding to my query: chemistry.stackexchange.com/questions/72184/… $\endgroup$ Commented Feb 19, 2018 at 16:05
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To add to @user223679's answer. Phenol can react via two pathways with acyl chlorides to give either esters, via O-acylation, or hydroxyarylketones, via C-acylation.
However, phenol esters also undergo a Fries rearrangement under Friedel-Crafts conditions to produce the C-alkylated, hydroxyarylketones. This reaction is promoted by having an excess of catalyst present, either a Lewis acid such as $\ce{AlCl3}$ or strong Brønsted acids such as $\ce{HF}$ and $\ce{TfOH}$.
This paper reports yields of >90% (up to 99%) O-acylated products when phenol, and various derivatives is reacted with acyl chlorides in 1% $\ce{TfOH-CH3CN}$ solutions.
In neat $\ce{TfOH}$, yields of >90% C-acylated products are reported for phenol, and many ortho- and para-substituted derivatives, but significantly lower yields of 40-50% C-acylation are reported for meta substituted derivatives.
Additionally, various phenol esters were heated in neat $\ce{TfOH}$ and yields of >90% of the C-acylated Fries rearrangement products were reported. The yields were similar for ortho- and para-substituted phenol esters but again significantly lower (30-60%) for meta-substituted reactants.
Overall this suggests that the ratio of O- to C-acylated products is strongly influenced by the concentration of the catalyst, with high concentrations favouring C-acylation and low concentrations favouring O-acylation.
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The lone pair of electrons on the oxygen in phenol coordinates strongly with the catalyst(AlCl3) and hence,reduces the activity of the catalyst. And once the coordination of the catalyst takes place, the lone pairs available on the oxygen atom of phenol are no longer available for resonance, and the oxygen atom, being electronegative, deactivates the ring to electrophilic substitution(remember that Friedel Crafts reaction requires an electrophile to attack the benzene ring).
The reduction of catalyst activity, combined with the deactivation of the benzene ring towards electrophilic substitution, gives poor yield of the expected products of Friedel Craft's reaction.
The way to circumvent this problem is to use Fries rearrangement
