Question

Consider the following reactions:
$I.C_2{H}_5-I+{NH}_3⟶C_2{H}_5\stackrel{\oplus }{{NH}_3}+I^{\ominus }$
$II.C_2{H}_5-I+\stackrel{\cdot \cdot }{P}{H}_3⟶C_2{H}_5\stackrel{\oplus }{{PH}_3}+I^{\ominus }$
$III.C_2{H}_5-I+Et{O}^{\ominus }⟶C_2{H}_5-OEt+I^{\ominus }$
$IV.Me-\stackrel{\oplus }{S}-{Me}_2+\stackrel{\ominus }{O}H⟶Me-OH+{Me}_{2}S$
In which of the reactions does the rate of $S_{N}2$ reaction increase with an increase in solvent polarity?

• A. $\left(I\right)$
• B. $\left(II\right)$
• C. $\left(III\right)$
• D. $\left(IV\right)$

Answer 1

Answer: (C), (D)

The correct options are
C $\left(III\right)$
D $\left(IV\right)$

If the reactant and/or nucleophile are charged, increasing the polarity of the solvent stabilizes any charged ground state species and decreases the rate of ${SN}^2$ reaction with an increase in the polarity of the solvent.