Phosphorus Pentachloride

Phosphorus pentachloride ($PCl_5$) is the best-known phosphorus pentahalide and an important chlorinating agent in the laboratory.

Preparation

It is obtained by treating white phosphorus with an excess of dry chlorine:

It can also be made using sulphuryl chloride ($SO_2Cl_2$) as the chlorinating agent:

Properties

$PCl_5$ is a yellowish-white solid. In moist air it hydrolyses in two stages — first to phosphorus oxychloride, and on further reaction with water, all the way to phosphoric acid:

On heating, $PCl_5$ sublimes, but stronger heating causes it to decompose back to the trichloride:

As with $PCl_3$, it converts organic compounds containing an $-OH$ group into chloro derivatives:

It also reacts with finely divided metals on heating, converting them to their chlorides while itself being reduced to $PCl_3$:

Because of these reactions, $PCl_5$ is widely used in the synthesis of organic chlorides, such as $C_2H_5Cl$ and $CH_3COCl$. …