Question

Consider the following anions.
When attached to $s{p}^{3-}$ hybridized carbon, their leaving group ability in nucleophilic substitution reactions 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 $I>II>IV>III$

When attached to $s{p}^3$−hybridized carbon, their leaving group ability in nucleophilic substitution reactions decreases in the order $I>II>IV>III$

$I$ is best leaving group due to electron withdrawing $(-I)$ effect of trifluoromethyl group and delocalization of negative charge over $3O$ atoms.

$III$ is worst leaving group because negative charge is delocalized over $C$ atoms.

The stabilization effect of resonance for the anion in which negative charge is delocalized over $O$ atoms is more pronounced over the delocalization in which the negative charge is delocalized over $C$ atoms.

II is better leaving group than $IV$ because in $II$, the extent of delocalisation of negative charge is much more than the extent of delocalization in $IV$.