Which of the following statements is//are correct ?

Options

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

Answer

A, B, C

Explanation

(a) The statement is self-explanatory.
(b) Protic solvent helps in the ionisation 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 decrease the reactivity of anion (which acts as nucleophile). By complexing cation, the crown ether leves a 'bare' anion (i.e, nucleophile) with increased reactivity. That is why they act a phase-transfer catalyst by transferring an ionic salf from an aqueous medium or from a solid into a weakly polar solvent.
(d) The statement is wrong, since `SN^1` reaction does not is proceed by the formation of carbocation which is produced by the ionisation of substrate `(RX)` by protic solvent.