PBr3 Phosphorus bromide

Chemical Designations - Synonyms Phosphorus bromide Chemical Formula Pbr3. 

The method we shall use starts with bromine and allows it to react with red phosphorus and water. Phosphorus and bromine combine very easily to form PBr3, but the phosphorus bromide hydrolyzes completely and hydrogen bromide passes over to the absorption flask. 

PBr3 PHOSPHORUS TRI BROMIDE 56.758 1.0427E-01 -1.6430E-04 1.1542E-07 -2.9670E-11 298 1500 gas... 

In the following the reaction is outlined for an a-bromination. The reaction mechanism involves formation of the corresponding acyl bromide 3 by reaction of carboxylic acid 1 with phosphorus tribromide PBr3. The acyl bromide 3 is in equilibrium with the enol derivative 4, which further reacts with bromine to give the a -bromoacyl bromide 5 ... 

The bromides of phosphorus, sulphur, selenium, arsenic, and similar compounds furnish hydrogen bromide when treated with water—e.g. PBr3+3H20=H3P03 +3HBr. Hydrogen bromide is also obtained by the joint action of bromine, water, and phosphorus.0... 

Aluminum bromide AlBr3 is used as a catalyst and parallels AICI3 in this role. Strontium and magnesium bromides are used to a limited extent m phamiacentical applications. Ammonium bromide is nsed as a flame retardant in some paper and textile applications potassium bromide is used in photography. Phosphorus tribromidc PBr3 and silicon tetrabromide SiBi4 are nsed as intermediates and catalysts, notably in the production of phosphite esters. 

Other halides that are useful in converting alcohols to alkyl halides are PC15, PC13, PBr3, and Pl3, which are acid halides of phosphorus oxyacids. As with thionyl chloride, a weak base often is used to facilitate the reaction. The base acts to neutralize the acid formed, and also to generate bromide ion for SN reactions ... 

There is no ambiguity in the halogenation of acids as they can of course enolise on one side only. Reliable methods are bromination with PCI3 and bromine or red phosphorus and bromine. The acid is converted into the acyl chloride with PCI3 or the acid bromide by PBr3, formed in the reaction mixture from red phosphorus and bromine. Bromohexanoic acid 34 can be made in good yield if the reaction mixture is worked up with water.7... 

The reagents PBr3 and Pl3 can be used to convert alcohols to alkyl bromides and alkyl iodides in one step. The reactions are very similar to those described for thionyl chloride. First, the oxygen of the alcohol attacks the phosphorus, replacing a halogen... 

R—OH + SOCl2 — or PBr3 — R —Cl or R—Br SNlor Sm2 thionyl chloride, phosphorus tribromide, and phosphorus triiodide provide a good one-step procedure for conversion of an alcohol to an alkyl chloride, bromide, or iodide... 

Another way is to convert the OH group into a better leaving group by combination with an element that forms very strong bonds to oxygen. The most popular choices are phosphorus and sulfur. Making primary alkyl bromides with PBr3 usually works well. 

Acid chlorides are prepared from carboxylic acids by reaction with thionyl chloride (SOCN), as we saw in the previous section. Similar reaction of a carbox Tic acid with phosphorus tribromide (PBr3) yields the acid bromide. 

In a similar fashion, the treatment of a 1° or 2° alcohol with phosphorus tribromide, PBr3, forms an alkyl bromide. 

Hell-Volhard-Zelinskii reaction The reaction in which a carboxylic acid is treated with Br2 and PBr3 (often formed in situ from red phosphorus and bromine) to give the a-bromocarboxylic acid bromide. 

Carboxylic acids can be brominated at the a-position by reaction with bromine and phosphorus tribromide (PBr3) in the Hell-Volhard-Zelinsky reaction. The reaction, which proceeds via an acid bromide, leads to the substitution of an a-hydrogen atom by a bromine atom. 

In other cases warming is required to complete reaction. In this process, as well as when distilling the crude bromide from the reaction mixture, the temperature must be raised slowly so as to avoid formation of phosphorus hydrides which may inflame spontaneously. In the preparation of pentaerythritol tetrabromide, C(CH2Br)4, from pentaerythritol and PBr3 as described in Organic Syntheses935 the reaction is completed by heating in an oil-bath at 170-180° for 20 h. 

Carboxylic acid bromides are preparatively much less important than the chlorides. They can be obtained from the acids by using PBr3 or red phosphorus and bromine, or from the acid chlorides and dry HBr. 

A carboxylic acid can be converted into an acyl chloride by heating it either with thionyl chloride (SOCI2) or with phosphorus trichloride (PCI3). Acyl bromides can be synthesized by using phosphorus tribromide (PBr3). 

OR Bromination can also be brought about by phosphorus tribromide. The alcohol and PBr3 react to produce the bromide, CH3(CH2)eCHaBr. 

Examination indicates that the carbon skeleton remains intact in this synthesis. All that must be done is to alter the carboxylic acid to the alkyl bromide of the same carbon number. Now, if one had n-Ci1H23CH2OH (the corresponding alcohol), 1-bromododecane could be produced by reaction with the phosphorus trihalide PBr3. To obtain the alcohol, use the fact that lithium aluminum hydride, LiAli, is one of the few reagents that reduce a carboxylic acid to an alcohol of the same carbon length. Overall, then, the synthesis becomes ... 

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