Phosphorus -bromide PBr

Phosphorus bromide (PBr) Br Pl(g) 451 Potassiim borate (KgBgOjg) BgKgOlgd) 307... 

Acid chlorid-ea are prepared from carboxylic acids by reaction with thionyl chloride (SOCI2 , as we saw in the previous section. Reaction of A carboxylic acid with phosphorus tribromide (PBr l yields the acid bromide. 

Preparation.—This substance cannot be obtained fromabromid as HCl is obtained from a chlorid. It is produced, along with phosphorous acid, by the action of HjO upon phosphorus tri-bromid PBr,-j-3HjO = HsPOa-1-3HBr or by the action of Br upon paraffin. 

Carboxylic acids (RCO2H) can be brominated at the a-position by reaction with bromine and phosphorus tribromide (PBrs) in the Hell-Volhard-Zelinsky reaction. The reaction, which proceeds via an acid bromide (RCOBr), leads to the substitution of an a-hydrogen atom by a bromine atom. 

In a similar fashion, the treatment of a 1° or 2° alcohol with phosphorus tribromide, PBrs, forms an alkyl bromide. 

Phosphorus tribromide has three bromine atoms present and each PBr, molecule can react with three alcohol molecules to form three molecules of alkyl bromide. 

Outline of Process.—Pure dry bromine was allowed to act on pure dry phosphorus in a vacuum the PBrs was distilled into receivers which were sealed and weighed, then decomposed by breaking under an ammoniacal solution of hydrogen peroxide. The solution was acidified with nitric acid and the bromine precipitated and weighed as silver bromide. 

PBrs (phosphorus tribromide) converts alcohols into alkyl bromides. 

The bromides of phosphorus, sulphur, selenium, arsenic, and similar compounds furnish hydrogen bromide when treated with water—e.g. PBrs+SH O—H3PO3 +3HBr. Hydrogen bromide is also obtained by the joint action of bromine, water, and phosphorus. ... 

The Mitsunobu reaction was discussed in Chapter 17, p. 000. Mitsunobu chemistry involves using a phosphorus atom to remove the OH group, after the style of PBrs as a reagent to make alkyl bromides from alcohols. 

Furtliermore, we could show that the reaction is not limited to perfluoroalkyl iodides. When methyl or ethyl halides were passed through a mixture of red phosphorus and copper at around 350 °C mainly phosphorus dihalides were produced. The phosphinous halide is formed in small amount along with some phosphines. With methyl bromide, e.g., the reaction product is composed of 80-90% CH3PBr2, 2-3% (Cf j-PBr and 7-14% (CH3)2PBr3. 

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