Reaction of Alcohols with Phosphorus Halides

Reaction of Alcohols with Phosphorus Halides

– Reaction of Alcohols with Phosphorus Halides gives alkyl halides.

– Several phosphorus halides are useful for converting alcohols to alkyl halides.

– Phosphorus tribromide, phosphorus trichloride, and phosphorus pentachloride work well and are commercially available.

3 R-OH + PCl₃ → 3 R-Cl + P(OH)₃

3 R-OH + PBr₃ → 3 R-Br + P(OH)₃

         R-OH + PCl₅ → R-Cl + POCl₃ + HCl

– Phosphorus triiodide is not sufficiently stable to be stored, but it can be generated in situ (in the reaction mixture) by the reaction of phosphorus with iodine.

2 P + 3 I₂ ↔ 2 PI₃

6 R-OH + 2 P + 3 I₂ → 6 R-I + 2 P(OH)₃

– Phosphorus halides produce good yields of most primary and secondary alkyl halides, but none works well with tertiary alcohols.

– The two phosphorus halides used most often are PBr₃ and the phosphorus/iodine combination.

– Phosphorus tribromide is often the best reagent for converting a primary or secondary alcohol to the alkyl bromide, especially if the alcohol might rearrange in strong acid.

– A phosphorus and iodine combination is one of the best reagents for converting a primary or secondary alcohol to the alkyl iodide.

– For the synthesis of alkyl chlorides, thionyl chloride (discussed in the next section) generally gives better yields than PCl₃ or PCl₅ especially with tertiary alcohols.

– The following examples show the conversion of primary and secondary alcohols to bromides and iodides by treatment with PBr₃ and P/I2.

Mechanism of the Reaction with Phosphorus Trihalides

– The mechanism of the reaction of alcohols with phosphorus trihalides explains why rearrangements are uncommon and why phosphorus halides work poorly with tertiary alcohols.

– The mechanism is shown here using PBr₃ as the reagent; PCl₃ and PI₃ (generated from phosphorus and iodine) react in a similar manner

Mechanism: Reaction of Alcohols with PBr₃

Step 1: PBr₃ is a strong electrophile. An alcohol displaces bromide ion from PBr₃ to give an excellent leaving group.

Step 2: Bromide displaces the leaving group to give the alkyl bromide.

Example: Reaction of (R)-pentan-2-ol with PBr₃.

Step 1: Displacement of bromide and formation of a leaving group.

Step 2: Bromide displaces the leaving group to give (S)-2-bromopentane.

– Rearrangements are uncommon because no carbocation is involved, so there is no opportunity for rearrangement.

– This mechanism also explains the poor yields with tertiary alcohols.

– The final step S_N2 is an displacement where bromide attacks the back side of the alkyl group.

– This attack is hindered if the alkyl group is tertiary.

– In the case of a tertiary alcohol, an ionization to a carbocation is needed.

– This ionization is slow, and it invites side reactions.