Question

Rank the Hydrogen atoms ($H_a,H_b,H_c$) in the following molecules according to their acidic strengths?

• A. $a>b>c$
• B. $b>a>c$
• C. $b>c>a$
• D. $a>c>b$

Answer 1

The correct option is C $b>c>a$

We need to figure out the corresponding conjugate bases to compare the acidity. Upon removing $H_b$, the conjugate base is an allyl anion. Hence, it is the most stable carbanion. The conjugate base upon removing $H_a$ is an $s{p}^3$ configured carboanion while the conjugate base upon removing $H_c$ is an $s{p}^2$ configured carbanion. $s{p}^3$ has $25$% $s$ character & $s{p}^2$ has $33$%. Therefore, $s{p}^2$ carbanion is more acidic. Thus , the acidity order is, $H_b>H_c>H_a$