Question

The order of decreasing ease of abstraction of Hydrogen atoms in the following molecule is

• A. $H_a > H_b > H_c$
• B. $H_a > H_c > H_b$
• C. $H_b > H_a > H_c$
• D. $H_c > H_b > H_a$

Answer 1

Answer: (B)

$H_a > H_c > H_b$

Answer 2

Abstraction of $\left( H _{a}\right)$ hydrogen atom results in the formation of $3^{\circ}$ allylic carbocation whereas abstraction of $H _{c}$ gives $2^{\circ}$ allylic carbocation (which is less stable as compared to former one). However, abstraction of $H _{b}$ gives only a $2^{\circ}$ carbocation which is less stable as compared to former two. More the stability of generated carbocation, higher is the tendency of abstraction of hydrogen. Thus, the order of decreasing ease of abstraction of hydrogen atoms from the given molecule is
$H _{ a }> H _{c}> H _{b}$