Question

Consider the given anions.
When attached to a ${sp}^3$ hybridised carbon, their leaving group ability in nucleophilic substitution reaction decreases in the order:

• A. $I>II>III>IV$
• B. $I>II>IV>III$
• C. $IV>I>II>III$
• D. $IV>III>II>I$

Answer 1

The correct option is B $I>II>IV>III$

In case of anions attached to $s{p}^3$ carbon,

the order of leaving group ability $\propto$ stability of anions.

In case of stability resonance plays the most important role.

In compound (I), Resonance occurs with 3 oxygen atoms and $C{F}_3$ is electron withdrawing group. Thus, maximum stability of (-)ve charge.

In compound (II), Resonance occurs with 3 oxygen atoms but $-C_6{H}_6$ is electron releasing group. Thus, stability less than (I).

In compound(III), $-C_6{H}_6$ is an electron releasing group, which destabilizes (-)ve charge. Thus, minimum stability.

So, the order for leaving group ability is

$I>II>IV>III$