Q.
$KI$ in acetone, undergoes $S_N2$ reaction with each $P, Q, R$ and $S$. The rates of the reaction vary as
Solution:
PLAN Acetone is an aprotic solvent and can dissolve both the nucleophile and the substrate and thus $5^2$ reaction is favoured.
$S_N2 \xleftarrow{1^{\circ}\,\, 2^{\circ}\,\, 3^{\circ}}$ Alkyl halides
$\rightarrow S_N1$
S
$1^{\circ}$ alkyl halide but $(C-C l) \cdot B \cdot E$. is
decreased by electron withdrawing $\left[ C _{6} H _{5} CO \right]$
group, (a case of $I$ -effect) Thus, maximum rate in $S _{ N } 2$ reaction
Q
$2^{\circ}$ alkyl halide, rate is minimum
R
$CH_3- Cl$
$1^{\circ}$ alkyl halide
S
$1^{\circ}$ allylic halide but allylic $1^{\circ}$ carbocation is resonance stabilised in $S _{ N }$ Ireaction
Thus, reactivity order is $S > P > R > Q$
| S | |
$1^{\circ}$ alkyl halide but $(C-C l) \cdot B \cdot E$. is decreased by electron withdrawing $\left[ C _{6} H _{5} CO \right]$ group, (a case of $I$ -effect) Thus, maximum rate in $S _{ N } 2$ reaction |
| Q | |
$2^{\circ}$ alkyl halide, rate is minimum |
| R | $CH_3- Cl$ | $1^{\circ}$ alkyl halide |
| S | |
$1^{\circ}$ allylic halide but allylic $1^{\circ}$ carbocation is resonance stabilised in $S _{ N }$ Ireaction |