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

Hint: 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 \[{{H}_{3}}P{{O}_{4}}\].