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Which among angelic acid and tiglic acid (cis and trans isomers of 2-methylbut-2-enoic acid respectively) more acidic and why?

It is given in a book(A guidebook to mechanism in organic chemistry for the JEE by Peter Sykes and B. H. Kishan 6ed. p.3.29 Q.1)that the cis(Z-) form is more acidic but I think the trans(E-) form will be more acidic due to interaction of oxygen in conjugate base with hydrogen on C3

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  • $\begingroup$ I agree with you $\endgroup$ – Alchimista Oct 21 '17 at 20:08
  • $\begingroup$ user32369 I am a student who is just getting introduced to BH kishan s book.Is the book good or bad? $\endgroup$ – starunique2016 Nov 12 '17 at 11:39
  • $\begingroup$ @user113223 the book is pretty good. It is not your conventional guide; I use it more as a reference book and would suggest you do the same. The book would be better if it had a proper index. Whatever you do, don’t learn from it. There are better books for learning from for JEE. $\endgroup$ – Mr_Pea Dec 4 '17 at 18:18
  • $\begingroup$ can you suggest me a proper book for learning what organic chemistry actually is? $\endgroup$ – starunique2016 Dec 12 '17 at 7:37
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I have some thoughts of my own. However, I am not certain if they are correct.

The key to the question certainly cannot lie in the electronic factor because the cis- and trans- isomers do not differ in this aspect, except for the difference in the overall dipole moment. Thus, I believe we can only rationalise this with steric considerations. Let's first take a look at the chemical structures.

https://en.wikipedia.org/wiki/Tiglic_acid Fig. 1: Structure of tiglic acid

https://en.wikipedia.org/wiki/Tiglic_acid Fig. 2: Structure of angelic acid

The steric interaction I would like to focus on is the repulsion between the methyl groups and the negatively-charged oxygen atom of the carboxylate ion. Due to resonance, the negative charge would be delocalised over the two oxygen atoms of the carboxylate ion conjugate base. In the trans-isomer (angelic acid), the methyl group is located closer to one of the oxygen atoms (i.e. the oxygen atom of the carbonyl group). Thus, there would be a stronger repulsive interaction between the two. In the cis-isomer (tiglic acid), the methyl group is located further away from that oxygen atom, thus the intramolecular repulsion would be lessened, thus it is more stable.

Since a more stable conjugate base implies that the acid is stronger, the cis-isomer (tiglic acid) is more acidic than the trans-isomer (angelic acid). This argument is in full agreement with the information presented in your reference book.

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  • $\begingroup$ No, this doesn’t really make sense since an oxygen anion is not really any larger than a carbonyl oxygen. $\endgroup$ – Jan Oct 22 '17 at 13:51
  • $\begingroup$ @Jan Does the size of the anion matter? I didn't assume that in my answer did I? $\endgroup$ – Tan Yong Boon Oct 22 '17 at 13:56
  • $\begingroup$ @Jan All I assumed in my answer is that there is significant repulsion between the electron cloud of the methyl group and the negatively-charged oxygen atom of the carboxylate ion. $\endgroup$ – Tan Yong Boon Oct 22 '17 at 13:58
  • $\begingroup$ I asked my teacher and he seems to have a similar idea as you(repulsive dipole-dipole interaction in cis form leading to slight instability in the acid), although he did mention a couple of times that the difference will be extremely small. $\endgroup$ – Mr_Pea Dec 4 '17 at 18:21
  • $\begingroup$ @user32369 I agree that the difference should be small. $\endgroup$ – Tan Yong Boon Dec 4 '17 at 23:01

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