Question

How does phosphorus trihalide react with alcohols?

Answer 1

Hint : The reaction will undergo $S{N^1}$ reaction, which favours the formation of nucleophile and attack from the opposite side where bulky ness is lesser. Alcohols react with liquid phosphorus $(III)$ chloride also called trichloride to yield chloro alkanes.

Complete Step By Step Answer:
Alcohols react with liquid phosphorus $(III)$ chloride also called trichloride to yield chloro alkanes.
Firstly we will take three moles of ethanol and one mole of phosphorus trichloride.
The reaction is as followed-
$ROH{\text{ and }}HCl$
$3C{H_2}C{H_2}C{H_2}OH + PC{l_3} \to 3C{H_3}C{H_2}C{H_2}Cl + {H_3}P{O_3}$
Alcohols react violently with solid phosphorus pentachloride at room temperature, producing clouds of hydrogen chloride gas. While it is not a good approach to make chloro alkanes, it is a good test for the presence of $OH$ groups. To show that a substance was an alcohol, you would first have to eliminate all the other groups that also react with phosphorus pentachloride. For example, carboxylic acid also reacts with it as does water.
$C{H_3}C{H_2}C{H_2}OH + PC{l_5} \to C{H_3}C{H_2}C{H_2}Cl + POC{l_3} + HCl$
Other reaction involving phosphorus halides –
Instead of using phosphorus tribromide or iodide, the alcohol is usually heated under reflux with a mixture of red phosphorus and either bromide or iodide. The phosphorus first reacts with the bromine or iodine to give the phosphorus trihalide.
\begin{array}{*{20}{c}} 2P(s) + 3B{r_2} \to 2PB{r_3} \\\ {2P(s) + 3{I_2} \to 2P{I_3}} \end{array}
These then react with the alcohol to give the corresponding halogen alkanes, which can be distilled off.
\begin{array}{*{20}{c}} {3C{H_3}C{H_2}C{H_2}OH + PB{r_3} \to 3C{H_3}C{H_2}C{H_2}Br + {H_3}P{O_3}} \\\ {3C{H_3}C{H_2}C{H_2}OH + P{I_3} \to 3C{H_3}C{H_2}C{H_2}I + {H_3}P{O_3}} \end{array} .

Note :
There are various methods through which we can obtain chloroalkanes. Reaction of $ROH{\text{ and }}HCl$ also gives us chloroalkane. It depends on the person and what way they want to derive their equation.