Question

Iodine reacts with alcohols to give alkyl iodine only in presence of phosphorus. Why?

Answer 1

Hint: Iodine in its natural form does not have the ability to react with alcohols. The phosphorus is needed to produce an intermediate compound, which can then react with the alcohol to produce the alkyl iodide.

Complete step by step answer:
Iodine needs to be in the form of iodide ion for it to react with alcohols. Thus, when it is present as an iodine molecule, the reaction will not occur.
However, when phosphorus is present, the iodine reacts with it to form a compound known as phosphorus triiodide. This compound is very reactive with alcohols, and thus, can produce the required alkyl iodide.
The first reaction which occurs is as follows:
$3{I_2} + P \to 2P{I_3}$
This triiodide of phosphorus then reacts with the alcohol to form the alkyl iodide:
$3R - OH + P{I_3} \to 3R - I + {H_3}P{O_3}$
Thus, without the presence of phosphorus, the intermediate will not get formed, and thus, the reaction will not occur.

Additional information:
Alkyl iodide molecules are generally polar, due to the high electronegativity of the iodine atom. Thus, this makes alkyl iodides a good choice for alkylation, since the iodide ion is a good leaving group. Methyl iodide is a toxic chemical used in the manufacture of pesticides, while other alkyl halides are generally used as organic solvents due to their polar nature.

Note: Phosphorus is not considered as a catalyst in this reaction, since it is actively consumed during the course of the reaction. The type of phosphorus used is red phosphorus, since the other form (white phosphorus) is highly reactive with air. Note that a major use of alkyl iodides is in the preparation of Grignard reagents.