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I know that Friedel-Crafts reaction (either alkylation or acylation) fails with phenol and aniline, due to formation of a coordinate bond between Lewis acid catalyst and the lone pair of the respective functional group.

But I have read that even benzoic acid fails to undergo this reaction. I couldn't think of a reason for this. Can anyone explain why is it so?

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    $\begingroup$ Still the same reason. $\endgroup$
    – Mithoron
    Commented Jan 25, 2016 at 12:37
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    $\begingroup$ I personally performed Friedle-Crafts acilation on 4-hydroxy-[2,2]-paraciclphane (a kind of phenol) using TiCl4 and AcCl (or TiCl4 and (COCl)2). TiCl4, however, is required in slight excess to equimole amount in this reaction. $\endgroup$
    – permeakra
    Commented Jan 25, 2016 at 13:04
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    $\begingroup$ @Mithoron I did fell like that but is benzoic acid 'basic' enough to get coordinated? $\endgroup$
    – vamsi3
    Commented Jan 25, 2016 at 15:48
  • $\begingroup$ Is quite nucleophilic $\endgroup$
    – Mithoron
    Commented Jan 25, 2016 at 16:13
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    $\begingroup$ @Mithoron Through which 'O' atom does it get coordinated to the catalyst. $\endgroup$
    – vamsi3
    Commented Jan 25, 2016 at 16:32

2 Answers 2

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It could be that $\ce{-COOH}$ group is electron withdrawing hence there is an increased electron density in $\ce{-COOH}$ group due to -R resonating effect of electron from benzene ring.

Since friedel craft catalyst is a Lewis acid (electron deficient) the catalyst (anhydrous $\ce{AlCl3}$) reacts with electron rich COOH group of benzoic acid and gets bonded.

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    $\begingroup$ Hi, abhishek. Did you reach a conclusion in your answer? You have stated two possibilities - a deactivated ring due to $-R$, and a bond formed by $\ce{-COOH}$ with the Lewis acid - for FC reaction to not happen. Which one of these is your answer exactly? Thanks! $\endgroup$
    – Gaurang Tandon
    Commented Feb 16, 2018 at 5:45
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I think Friedel Craft reaction with Benzoic acid fails because the Benzene ring is deactivated. Here electrophile is Carbocation which is a weak electrophile. So the necessity for Friedel-Craft reaction is activated Benzene ring.

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  • $\begingroup$ I don't get it when you say "C+ is a weak electrophile". A carbocation? I don't think that's a weak electrophile. Anyway, do you mean that all mono-substituted benzene rings with deactivating groups (i.e., atleast as strongly deactivated as benzoic acid) fail to give Friedel- Crafts reactions. $\endgroup$
    – vamsi3
    Commented Jan 25, 2016 at 17:38
  • $\begingroup$ I think yes. And also I am doubtful whether carbocation is a strong electrophile. $\endgroup$
    – Aaditya Joshi
    Commented Jan 26, 2016 at 2:25
  • $\begingroup$ @AadityaJoshi Carbocations are pretty good electrophiles. The more important factor here is the coordination of the lewis acid to the carboxylic acid group. $\endgroup$
    – bon
    Commented Jan 26, 2016 at 15:47
  • $\begingroup$ @bon Yes, now I think it is so but through which of the two oxygen atoms does benzoic acid coordinate to lewis acid? $\endgroup$
    – vamsi3
    Commented Jan 27, 2016 at 8:18
  • $\begingroup$ @user104014 Probably the carbonyl oxygen but I'm not entirely sure. $\endgroup$
    – bon
    Commented Jan 27, 2016 at 13:38

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