Question

Arrange the following $CH_3CH_2CH_2Cl\, (I)$ , $CH_3CH_2-CHCl-CH_3\, (II)$ , $(CH_3)_2CHCH_2Cl \,(III)$ and $(CH_3)_3C?Cl \,(IV)$ in order of decreasing tendency towards $S_N2$ reactions

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

Answer 1

Answer: (A)

$I > III > II > IV$

Answer 2

In $S_{N^2}$ reactions, the attack of nucleophile occurs from the back side on the carbon atom carrying the halogen. Therefore, presence of bulky substituents on or near this carbon atom tends to hinder the approach of the nucleophile to the a-carbon due to steric hindrance and thus makes the reaction difficult to occur. Thus, the reactivity in $S_{N^2}$ reactions follows the following order
$CH_3CH_2CH_2Cl (I) > (CH_3)_2CHCH_2Cl (III)

CH_3 -CH_2-CHCl-CH_3 (II)
(CH_3)_3C-Cl(IV)$.