Question

Account for the acidity of L-ascorbic acid ($p{K}_a=4.21$). Which of the following is most acidic $H$? (Marked in the structures as $H^a,H^b,H^c$ and $H^d$)

• A. $H^a$
• B. $H^b$
• C. $H^c$
• D. $H^d$

Answer 1

The correct option is C $H^c$

The anion formed by the removal of enolic $H$ is stable because the negative charge is stabilized by resonance, i.e., by delocalisation of the negative charge to the $O$ of the $(C=O)$ through $(C=C)$.

Hence, option C is correct.