Question

Which of the following statements is/are correct?

• A. Protic solvents solvate the nucleophile, lower enthalpy of nucleophiles, increase $\Delta G$, and decrease tha reaction rate of $SN ^2$ reaction
• B. $SN ^1$ reactions are favoured in protic solvents
• C. Crown ether increases the reactivity of nucleophile by solving its cation
• D. $SN ^2$ reactions are more favoured in protic solvents

Answer 1

Answer: (A), (B), (C)

a. The statement is self-explanatory
b. Protic solvent helps in the ionization of substrate (RX), an essential condition for $SN ^1$ reaction
c. Crown ethers strongly complex metallic cations of the salts in the interior (the 'hole') of the ring by forming ion-dipole bonds 18-crown-6, that strongly complex and trap $K ^{\oplus}$ as shown

In non-polar or weakly polar solvents, salt exists as ion pairs. Ion pairing decreases the reactivity of anion (which acts as nucleophile). By complexing cation, the crown ether leaves a 'bare' anion (i.e., nucleophle) with increased reactivity. That is why they act as a phase-transfer catalyst by transferring an ionic salt from an aqueous medium or from a solid into a weakly polar solvent $d$. The statement is wrong, since $SN ^2$ reaction does not is proceed by the formation of carbocation which is produced by the ionization of substrate (RX) by protic solvent