Treatment of sec-butyl bromide with alcoholic $\mathrm{KOH}$ forms ______.

Which of the following haloalkanes is most reactive towards ${\mathrm{S}}_{\mathrm{N}}1$?

The order of ${\mathrm{S}}_{\mathrm{N}}1$ reactivity in aqueous acetic acid solution for the compounds is:

$1$.

$2$. ${\mathrm{H}}_3\mathrm{C}-{\mathrm{CH}}_2-{\mathrm{CH}}_2-\mathrm{Cl}$

$3$. ${\left({\mathrm{H}}_3\mathrm{C}\right)}_3\mathrm{C}-\mathrm{Cl}$ 

$\mathrm{R}-\mathrm{Cl}+\mathrm{AgCN}\to \mathrm{A}\stackrel{ \mathrm{Reduction}}{⟶}\mathrm{B}$

$\mathrm{A}$ and $\mathrm{B}$, respectively are

The major product $\left[\mathrm{X}\right]$ is: