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Why is $\ce{CD3-COOH}$ more acidic than $\ce{CH3-COOH}$ despite being less electronegative? The book too said that $\ce{CD3} > \ce{CH3}$ in terms of + Inductive effect.

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  • $\begingroup$ Welcome to chemistry.se! Please include a citation for the book you are referring to. To what are you referring to when you say 'despite being less electronegative', the deuterium? $\endgroup$ – Martin - マーチン Sep 24 '18 at 16:57
  • $\begingroup$ Organic chemistry by solomons and fryhles $\endgroup$ – Akshay Mendon Sep 24 '18 at 22:01
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To compare acidity, you must have made the conjugate bases of these acids, that is, $\ce{CH3COO-}$ and $\ce{CD3COO-}$ . Now, the hyperconjugation effect is usually a more dominant electronic effect than the inductive effect, and as the $\ce{C-D}$ bond is stronger than $\ce{C-H}$ bond, so the reduced $\ce{+H}$ effect in the conjugate base of $\ce{CD3COOH}$ makes it more acidic.

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  • $\begingroup$ But there are zero alpha hydrogens on the carbon atom next to the oxygen anion so how is hyperconjugation possible ? $\endgroup$ – Akshay Mendon Sep 24 '18 at 22:00
  • $\begingroup$ There are α-hydrogens and α-deuteriums present with respect to the $\ce{COO-}$ group, so to see the hyperconjugative structures, break the $\ce{C-H}$ bond and shift the electron pair so that you get two negatively charged oxygens on one carbon, which can protonate and revert to the original carboxylate ion. Anyway, hyperconjugation is a theoretical effect, and it is not defined clearly for carbanions due to absence of vacant orbitals, so I would advise you not to set much store by the actual mechanism of hyperconjugation here, as the di-negative carbanaion formed was extremely unstable. $\endgroup$ – YUSUF HASAN Sep 25 '18 at 0:23
  • $\begingroup$ Oh okay i understand now ! $\endgroup$ – Akshay Mendon Sep 25 '18 at 0:26
  • $\begingroup$ Again, I would like to add here that since hyperconjugation is completely theoretical, so the only concrete thing we can say here is that by whichever path the $\ce{+H}$ maybe happening, it is clear that it will increase electron density on the $\ce{COO-}$ group, so please don't take these structures I just suggested as absolute $\endgroup$ – YUSUF HASAN Sep 25 '18 at 0:31
  • $\begingroup$ Yea i understood that reference $\endgroup$ – Akshay Mendon Sep 25 '18 at 0:32

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