# Why is d3-acetic acid more acidic than acetic acid?

The following table is from Solomons, Fryhle and Snyder Organic Chemistry, chapter 3, An Introduction to Organic Reactions and Their Mechanisms: Acids and Bases:

Table 3.8 Isotopic substituents and inductive effect $$\small \begin{array}{ccc} \hline \text{Acid} & \ce{pK_a} & \text{Inductive effect of the substituent} \\ \hline \ce{CH3-COOH} & 4.75 & \text{+I} \\ \ce{CD3-COOH} & 4.60 & \text{+I} \\ \hline \end{array}$$ +I effect of deuterium is stronger than that of hydrogen. +I ($$\ce{-CD3>-CH3}$$)

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

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
These findings indicate that acetic acid-d3 is less acidic than acetic acid-h3. This would be consistent with the operation of a simple inductive effect where protium is more electronegative than deuterium (as expected). Hence, the more electron withdrawing $$\ce{CH3}$$ group is better able to stabilize the electron rich carboxylate anion in the the ionized acid as compared to the less electron withdrawing $$\ce{CD3}$$ group.