The major product $ X $ in the following given reaction is

In $ S_{N} 2 $ substitution reaction of the type

$ R-\mathrm{Br}+\mathrm{Cl}^{-} \xrightarrow[\mathrm{BMF}]{ } R-\mathrm{Cl}+\mathrm{Br}^{-} $

Which one of the following has highest relative rate?

The major product of the reaction between $ \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{ONa} $ and $ \left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCl} $ in ethanol is

X and Y are