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Wikipedia article on ortho effect suggests that o-toluic acid is more acidic than benzoic acid. This fact is rationalized by suggesting that steric effect causes COOH group to "twist" out of the plane and which increases it's acidity as it inhibits "cross-conjugation"that it was previously taking part in.

My instructor told me that cross-conjugation is two parallel conjugated sets of atom both terminating on a common atom. He says this parallel character of such conjugation makes conjugation on common atom less effective.

My question is -

  1. How accurate is it to point out cross conjugation as the reason for acidity of o-toluic acid ?

  2. How correct is the explanation of cross conjugation by my instructor?

Edit #1

There are some valid concern raised by Alchimista. It seems like I have not made my question clear to understand. I would rephrase it a bit ->

I am not advocating the usage of cross conjugation to explain acidity of o-toulic acid. But merely seeking clarification for its usage to explain acidity of o-toluic acid by the wikipedia article(linked here for convenience -https://en.wikipedia.org/wiki/Ortho_effect#General_explanation_for_first_ortho_effect). To rephrase the question, Wikipedia article states that cross conjugation "destabilizes" conjugate base of benzoic acid but never justifies the statement.

My main concern is not the justification for acidity of o-toluic acid as there are plenty of answers here on that(Link->https://chemistry.stackexchange.com/questions/7683/ortho-effect-in-substituted-aromatic-acids-and-bases). What I want to know is, if cross conjugation destabilizes benzoic acid as mentioned in the article and if yes then how?

user55119
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Lovan Khorde
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  • It seems the use of the term criss-conjugation is used differently in your description. Also in the example of o-toluic ac. is steric hindrance that prevents effective conjugation, and this should make the conjugated base less stable → the acid less strong. But perhaps I misunderstood. Also note that conjugation is merely topological and shouldn't be use as synonymous of delocalisation. – Alchimista Aug 17 '19 at 07:09
  • @Alchimista -> 1."Also in the example of o-toluic ac. is steric hindrance that prevents effective conjugation, and this should make the conjugated base less stable→ the acid less strong". I am not able to comprehend your statement, are you implying that o toulic is weaker than benzoic? pKa of o toulic is 3.91 while that of benzoic is 4.19. Sincere apology if I have incorrect understanding of your statement. English is not my first language. – Lovan Khorde Aug 17 '19 at 08:20
  • 2."conjugation is merely topological and shouldn't be use as synonymous of delocalisation" completely agreed. Sorry for misusing terminology. – Lovan Khorde Aug 17 '19 at 08:28
  • 3.3."It seems the use of the term criss-conjugation is used differently in your description" I assume it was a typo and you are referring to cross conjugation and not something I am unaware of (I believe this is reasonable as "o" is pretty close to "I" in keyboard if this is not the case, it would be very kind of you to post some relevant article). I am completely sure in my understanding of term cross conjugation. Part (1/2) – Lovan Khorde Aug 17 '19 at 08:32
  • I agree that probably I am not able to explain it properly as definition given in my post is a mixture of my own and some translated thoughts expressed by my instructor in another language. Part(2/2) – Lovan Khorde Aug 17 '19 at 08:33
  • The Wikipedia article that I referred to in my post -> https://en.wikipedia.org/wiki/Ortho_effect#General_explanation_for_first_ortho_effect – Lovan Khorde Aug 17 '19 at 09:00
  • Yes the only thing I overlooked is about the fact that substituted benzoic ac.s are stronger as it is the delocalisation within the conj.acid that is inhibited. But cross conjugation is not the context here. We are dealing with delocalisation M effects and steric hindrance it suffice to say that the ring and the carboxylic gr. are not in plane. – Alchimista Aug 17 '19 at 11:07
  • Look at this figure it might clarify the situation https://qph.fs.quoracdn.net/main-qimg-61d6d230e0462107958eefa8fa369677 – Alchimista Aug 17 '19 at 11:11
  • @Alchimista->1."But cross conjugation is not the context here." I am not advocating the usage of cross conjugation to explain acidity of o-toulic acid. But merely seeking clarification for its usage to explain acidity of o-toulic acid by the wikipedia article (linked again for convenience ->https://en.wikipedia.org/wiki/Ortho_effect#General_explanation_for_first_ortho_effect). To rephrase the question in the post, Wikipedia article(not me) states that cross conjugation is responsible for "destabilization" of benzoic acid but never justifies the statement ,I am asking if article is correct (P1) – Lovan Khorde Aug 17 '19 at 11:43
  • I do not attribute 'twisting' of COOH to cross conjug. and understand that it is consequence of steric interaction. My main concern in not the justification for acidity of o-toulic acid(which I believe you are assuming) as there are plenty of answers here on that. I am only concerned by the aforementioned Wikipedia article's claim that cross conjugation destablisies benzoic acid. I edit the post to make it clearer. :)
    • – Lovan Khorde Aug 17 '19 at 11:53
  • Lovan I get confused. Basically the reason is that the +M of the phenyl ring is inhibited or suppressed. I do not see why cross-conj. is mentioned in Wikipedia. It might be a further matter of analysis if the substituent would be a group with its own mesomeric effect but certainly is just adding an unnecessary word in o-toluic acid. – Alchimista Aug 17 '19 at 14:00