# Is acetic acid or d3-acetic acid more acidic?

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.

• 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? – Martin - マーチン Sep 24 '18 at 16:57
• Organic chemistry by solomons and fryhles – 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.
• 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. – Yusuf Hasan Sep 25 '18 at 0:23
• 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 – Yusuf Hasan Sep 25 '18 at 0:31