Question

${{H}_{3}}P{{O}_{3}}$, formed in the preparation of alkyl halide by $PC{{l}_{3}}$ and alcohol, is called as:
A) Phosphorus acid
B) Phenyl chloride
C) Phosphoryl chloride
D) None of these

Answer 1

Alkyl halides can be easily prepared by the reaction of alcohols with liquid phosphorus chloride ($PC{{l}_{3}}$). The products are alkyl chloride, and ${{H}_{3}}P{{O}_{3}}$or $P{{(OH)}_{3}}$. This reaction is called a halogenation reaction. It is a best example for substitution reaction.

Complete-step- by- step answer:
We can prepare alkyl halides in two methods by using phosphorus pentachloride and by using phosphorus trichloride ($PC{{l}_{3}}$).

Method-1: Alcohols react with solid phosphorus (V) chloride (phosphorus pentachloride, $PC{{l}_{5}}$) at room temperature, produces Phosphoryl chloride ($POC{{l}_{3}}$) and hydrochloric acid. It is not a good approach to prepare chloroalkanes, it is a good test to find the presence of -OH groups in alcohols.

$C{{H}_{3}}C{{H}_{2}}OH+PC{{l}_{5}}\xrightarrow{{}}C{{H}_{3}}C{{H}_{2}}Cl+POC{{l}_{3}}+HCl$

In the above reaction ethanol reacts with phosphorus pentachloride and forms ethyl chloride, phosphoryl chloride ($POC{{l}_{3}}$) and hydrochloric acid.

Method-2: Alcohols react with $PC{{l}_{3}}$(phosphorus (III) chloride) at certain temperature produces chloroalkanes and phosphorus acid (${{H}_{3}}P{{O}_{3}}$or $P{{(OH)}_{3}}$).

$3C{{H}_{3}}C{{H}_{2}}OH+PC{{l}_{3}}\xrightarrow{\Delta }3C{{H}_{3}}C{{H}_{2}}Cl+P{{(OH)}_{3}}$

In the above reaction three moles of ethyl alcohol reacts with phosphorus trichloride and forms three moles of ethyl chloride and ${{H}_{3}}P{{O}_{3}}$.
Halogenation of alcohols is a best example for a substitution reaction.
${{H}_{3}}P{{O}_{3}}$ is formed as a by-product during the reaction of ethyl alcohol with phosphorus trichloride.
Generally ${{H}_{3}}P{{O}_{3}}$ is called Phosphorous acid.

So, the correct option is A.

Note: Don’t be confused between ${{H}_{3}}P{{O}_{3}}$and ${{H}_{3}}P{{O}_{4}}$. Both are not the same. ${{H}_{3}}P{{O}_{4}}$ is called as Phosphoric acid and ${{H}_{3}}P{{O}_{3}}$ is called as Phosphorus acid. ${{H}_{3}}P{{O}_{3}}$ is more acidic than ${{H}_{3}}P{{O}_{4}}$.