How does phosphorus trihalide react with alcohols?

Phosphorus trihalides, such as phosphorus trichloride (PCl3) and phosphorus tribromide (PBr3), are commonly used reagents in organic chemistry for various reactions, including the conversion of alcohols to alkyl halides. This reaction is known as the "phosphoryl halide method" or the "Haloform reaction."

Here's how the reaction typically proceeds with phosphorus trichloride (PCl3) as an example:

Reaction Setup:

Alcohol: R-OH (where R is an alkyl group)
Reagent: Phosphorus trichloride (PCl3)
Product: Alkyl chloride (R-Cl)
Reaction Mechanism:
The reaction usually occurs in the presence of a Lewis base, such as pyridine (C5H5N), which helps in the activation of PCl3. The mechanism involves the following steps:

a. Activation:
PCl3 reacts with the Lewis base (pyridine) to form a complex, which facilitates the reaction.

b. Nucleophilic Attack:
The lone pair of electrons on the oxygen atom in the alcohol attacks the phosphorus atom in the PCl3 complex, leading to the formation of a pentavalent intermediate.

c. Chlorine Substitution:
In the presence of excess PCl3, one chlorine atom is substituted for the oxygen atom in the intermediate, resulting in the formation of an alkyl chloride and the regeneration of the pyridine-P complex.

Final Product:
The final product of the reaction is an alkyl chloride (R-Cl), and the pyridine-P complex is regenerated for further reactions.

Here's the general reaction for the conversion of an alcohol to an alkyl chloride using phosphorus trichloride (PCl3):

R-OH + PCl3 (in the presence of a Lewis base, e.g., pyridine) → R-Cl + H3PO3

Please note that this reaction can be used to convert primary and secondary alcohols into their respective alkyl chlorides or bromides, but it is not typically effective for tertiary alcohols due to steric hindrance. Additionally, phosphorus tribromide (PBr3) can be used in a similar manner to convert alcohols into alkyl bromides. Always perform these reactions in a well-ventilated laboratory setting and follow safety precautions, as these reagents can be hazardous.