It is given that due to Steric Inhibition of Resonance (SIR), the acidity of ortho-substituted benzoic acid increases with respect to benzoic acid.

If this is the case then the conjugate base of the acid will not be stabilized by resonance with the benzene ring due to SIR Effect. So it means the ortho-substituted benzoic acid will not readily donate H+ and is less acidic.

Then kindly explain why is ortho-substituted benzoic acid is more acidic than benzoic acid.

  • $\begingroup$ which ortho substituent are you refering to? $\endgroup$ – Waylander Aug 1 '19 at 12:09
  • $\begingroup$ Think of a localized, in plane, benzoate conjugate base. The carbonyl can busy itself by resonance with the ring or resonate with the localized charge-bearing oxygen. The carbonyl is splitting its effort. But in the out-of-plane conformation (say, from o-toluic acid) the ring resonance is lost and the carbonyl can now focus on stabilizing the charge on oxygen. On this basis, the out-of-plane acid would be predicted to be more acidic.. $\endgroup$ – user55119 Aug 1 '19 at 12:37
  • $\begingroup$ @Waylander Ortho-substituents like alkyl group(methyl,ethyl,etc.), halogens(F,Cl,Br,I), etc., where the SIR effect depends on the size of the substituent. The substituent I referred does not include small ones like hydroxyl or amine groups. $\endgroup$ – Magnesium Aug 1 '19 at 13:20
  • $\begingroup$ @user55119 Ok I understood your point. But we know resonance is a stabilizing phenomenon and more the number of resonance structures, more must be the stability right. This one confuses me. Kindly guide me. $\endgroup$ – Magnesium Aug 1 '19 at 13:23
  • $\begingroup$ Ortho effect is discussed in the following link . chemistry.stackexchange.com/questions/7683/… $\endgroup$ – Chakravarthy Kalyan Aug 1 '19 at 13:39

Here resonance is destabilizing as resonance here would mean that two oxygens will have negative charge and that would destabilize the conjugate base.

  • 2
    $\begingroup$ While this bears some resemblance to an answer to the question, going into some additional explanation would make it a lot stronger (diagram, references, etc.) $\endgroup$ – jonsca Aug 11 '19 at 0:30

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