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Compare acidic strength of crotonic acid (1) and isocrotonic acid (2):

1: (2E)-but-2-enoic acid; 2: cis-2-butenoic acid

On comparing $\mathrm{p}K_\mathrm{a}$ values, cis-form $(4.44)$ turns out to be more acidic than the trans-form $(4.69)$.

Most of the explanations I found on internet say that cis-form will be more acidic because in trans-form the $\ce{CH3}$ opposite to it, provides electrons by inductive effect directly in its direction (the dipole moment is almost directly in its direction) making the conjugate base unstable.

However, I am not convinced with this explanation as there's also a $\ce{C=C}$ bond between methyl group and $\ce{COOH}$ so same amount of electron density reaches $\ce{COOH}$ group via the double bond in both cases.

What's the flaw in my reasoning? How to compare the acidic strength correctly?

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    $\begingroup$ One possibility: in the cis anion a weak hydrogen bond forms between the carboxylate ion and the methyl group. A molecular orbital model where the oxygen lone pair overlaps a C-H antibonding orbitals could allow that. $\endgroup$ Feb 3, 2022 at 14:17
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    $\begingroup$ 0.25? That's hardly any difference at all. $\endgroup$
    – Mithoron
    Feb 14, 2022 at 16:14
  • $\begingroup$ @Mithoron I have asked this question to seek correct reason for a fact whose reasons that I found online seem incorrect to me $\endgroup$
    – Acc2
    Feb 14, 2022 at 18:23
  • $\begingroup$ For a related discussion on the acidity of o-toluic acid look here. $\endgroup$
    – user55119
    Aug 4, 2022 at 19:07

2 Answers 2

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Firstly, I've found many different pKa values for isocrotonic acid. For example, this website claims the pKa of isocrotonic acid to be $4.9$, so I'm not really sure which is more acidic.

If you look at the conjugate base of isocrotonic acid, due to the methyl group on the same side the conjugate base of isocrotonic acid could be less solvated compared to the conjugate base of crotonic acid, which has no such steric hindrance. This would mean that crotonic acid is more acidic.

I'm not really sure whether this is correct or not since I can't really predict how much of a role the methyl group will have in reducing the solvation of the conjugate base of isocrotonic acid, but this is the only factor which comes to my mind.

(I know this is more of a comment, but I have posted an answer so that more experienced users can talk about the correctness of this possibility and vote accordingly)

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    $\begingroup$ That makes sense logically, but the problem would just be that it's speculative. In my opinion, a 0.25 pKa difference can't be explained convincingly using the crude models of organic chemistry. $\endgroup$ Mar 7, 2022 at 17:15
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If one draws in all the hydrogens and rotates the position of the OH group in the carboxylic acid group we can see some steric hindrance between the methyl hydrogen and the acidic carboxyl hydrogen with cis isocrotonic acid.

This is one case where building a "full model" with the black, red, and white little balls and plastic connectors may be much more explanatory than trying to understand the "skeleton" organic chemistry notations.

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