Phosphorus pentabromide bromine

Replacement of a hydrogen with bromine in the polyfluoroalkyl group of a ketone or acyl fluoride can be carried out with phosphorus pentabromide [i50] (equation 45)... 

Bromine reacts with phosphorus to form pbospborus tribromide, PBrs or phosphorus pentabromide, PBrs. The pentabromide forms in tbe presence of excess bromine. Bromine oxidizes hydrogen sulfide to sulfur ... 

Two products (128 and 129) were obtained when 1,6-naphthyridin-4-one (127) reacted with phosphorus pentabromide.143 It is presumed that 128 is the precursor of 129. The 4-bromo derivative was obtained when compound 127 was treated with phosphorus oxybromide. The 3-bromo-l,6-naphthyridin-4-one (130) was prepared in good yield from compound 127, and on reaction with phosphorus oxybromide the 3,4-dibromo-l,6-naphthyridine (131) could be obtained. The reaction of l,7-naphthyridin-8-one with phosphorus oxybromide is unique,144 since it appears to be the only example in this series, in which further bromination occurs with this reagent, affording 8-monobromo and 3,8-dibromo-l,7-naphthyridine. 

PBr3,Br2uq.)=20 3 Cals. P. Walden examined the electric conductivity of the pentabromide dissolved in liquid sulphur dioxide or arsenic trichloride.. W. A. Plot-nikofi observed that phosphorus pentabromide dissolved in bromine conducts an electric current, depositing phosphorus on the cathode, and combines with bromine to form pentabromide. W. Finkelstein measured the decomposition potential of the pentabromide in bromine, liquid sulphur dioxide, arsenic trichloride, and nitrobenzene. 

The classical route to chloropyrazines is by treatment of a hydroxy-pyrazine with phosphoryl chloride bromopyrazines are similarly prepared by using phosphoryl bromide, phosphorus tribromide, or a mixture of both. Thus, treatment of hydro xypyrazine with phosphoryl chloride gives chloropyrazine in 92% yield,147 and treatment of the hydroxy compound with a mixture of phosphoryl bromide and phosphorus tribromide gives bromopyrazine in 58% yield.266 The use of phosphorus pentahalides frequently leads to substitution products (Scheme 22) for example, when hydroxypyrazine is treated with a mixture of phosphoryl bromide and phosphorus pentabromide both monobromo- and 2,6-dibromopyrazines are formed.287,268 Bromina-tion of hydroxypyrazine with bromine in the presence of small... 

Phosphorus pentabromide can be prepared by the reaction of PBr3 with bromine ... 

Phosphorus Tribromide, PBr3, was first prepared by Balard 7 who added bromine drop by drop to phosphorus. The reaction is violent, but may be moderated by the use of red phosphorus, by carrying out the reaction in CS2 solution,8 and by introducing the bromine as vapour.9 On account of the formation of volatile phosphorus compounds it is advantageous to have an excess of bromine, which is easily removed by distillation, any phosphorus pentabromide also being decomposed during this operation. 

Phosphorus Pentabromide, PBr5.—In the presence of excess of bromine phosphorus can form not only the pentabromide 5 but also, according to the thermal diagram, other perbromides.6 It was discovered early 5 that the presence of some iodine greatly facilitates the combination, probably by the formation of IC13, thus —... 

The bromine may be added in a stream of nitrogen which also serves to remove the liberated hydrogen halide. In the bromination of pinacolone, aluminum amalgam or aluminum chloride is used as a catalyst, " Phosphorus pentabromide, N-btomosuccinimide, " and pyridine hydrobromide perbromide have been used as brominating agents. 

With few exceptions, the formation of all known trityl ethers of the aldohexoses and their derivatives involves the hydroxyl in the 6-position. For example, treatment of methyl 6-trityl-2,3,4-triacetyl-a-D-glucopyr-anoside (I) with phosphorus pentabromide replaces the tritoxyl group by bromine and leads to 2,3,4-triacetyl-6-bromo-6-desoxy-D-glucopyranosyl bromide (II). 

It is generally prepared by the action of bromine on acetic acid in presence of red phosphorus. Phosphorus pentabromide is formed first, and this reacts with the acetic acid, forming acetyl bromide, as follows ... 

Finely divided reagent can be prepared by slowly adding the theoretical amount of PBra to a cold, vigorously stirred solution of 240 g. of bromine in 600-1000 ml. of petroleum ether (b.p. 30-60°). The solvent was decanted and the phosphorus pentabromide washed several limes with fresh solvent. Removal of the hydrocarbon in a vacuum desiccator gave 628 g. of reagent. 

Discussion When a bromide is treated with concentrated sulphuric acid, hydrobromic acid and an acid sulphate are formed, as would be expected but then a further reaction takes place. Just as hydrochloric acid can be oxidized by a sufficiently strong oxidizing agent, as was shown in Experiment 23, so hydrobromic acid also may be oxidized. But this latter compound is more readily oxidized than hydrochloric acid and even sulphuric acid, which does not react with hydrochloric acid, serves to bring about this oxidation. Hence the hydrobromic acid expected from the double decomposition of a bromide and sulphuric acid will contain bromine and the reduction products of the sulphuric acid. Pure hydrobromic acid may be prepared by the substitution of phosphoric for sulphuric acid or by some entirely different method, such as the reaction between phosphorus pentabromide and water. 

Phosphorus pentachloride is commonly used for the preparation of acid chlorides,along with thionyl chloride. Phosphorus pentabromide has been used to brominate aromatic rings, as in the conversion of 149 to 150 in 42% yield,but more vigorous conditions are required. 

Tri-m-nitrotri-p-tolylstibine dinitrate, (C7H6.N02)3Sb(N03)2. —Tri-p-tolylstibine is slowly added to the calculated quantity of well-cooled fuming nitric acid and the solution poured into water. The dmitrate is a light, cream-coloured powder, melting at 182° C. after several recrystalhsations from alcohol. When heated -with bromine and phosphorus pentabromide in chloroform solution for four hours at 130° C., it yields 4-bromo-2-mtrotoluene. 

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