The product of the reaction
$\mathrm{But}-2-\mathrm{ene} + \mathrm{HBr}$ $\stackrel{}{\to }$?

$\mathrm{A}+\mathrm{HBr}\to {\mathrm{C}}_5{\mathrm{H}}_{11}\mathrm{Br}$. 

$\mathrm{A}$ is most likely to be an _____.

The reaction is:

${\text{R-CH}}_{\text{2}}\text{-OH + HCl }\stackrel{}{\to }{\text{R-CH}}_{\text{2}}{\text{-Cl+H}}_{\text{2}}\text{O}$

For the completion of the reaction, ______ is used.

In the reaction;

$\mathrm{X}+{\mathrm{PCl}}_{\mathrm{5}}\mathrm{ }\stackrel{\mathrm{\Delta }}{\to }{\mathrm{CH}}_{\mathrm{3}}{\mathrm{CH}}_{\mathrm{2}}\mathrm{Cl}+{\mathrm{POCl}}_{\mathrm{3}}$, $\mathrm{X}$ is ______.