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.

  • $\begingroup$ 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

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.

  • $\begingroup$ 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

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.