Tribromide

Vanadium oxide tribromide, VOBr3. Dark red deliquescent liquid formed by heating V2O3 plus Brj. 

The boron so obtained is an amorphous powder. It can be obtained in the crystalline state by reducing the vapour of boron tribromide with hydrogen, either in an electric arc or in contact with an electrically-heated tungsten filament ... 

The tribromide and triodide of both boron and aluminium can be made by the direct combination of the elements although better methods are known for each halide. The properties of each halide closely resemble that of the chloride. 

Both aluminium tribromide and triodide are dimeric in the solid state. As expected the solids dissolve in non-polar solvents without the break-up of these dimeric units. 

The pure tribromide and triodide are unknown but their ammoniates have been prepared by the action of the appropriate halogen on ammonia. The tribromide, NBr bNUj, is a purple solid which decomposes explosively above 200K. The iodide, Nl3. NH3, is a black explosive crystalline solid, readily hydrolysed by water. 

The hydrolysis of phosphorus tribromide or triiodide is used in the preparation of hydrogen bromide and hydrogen iodide respectively ... 

The conversion of an aliphatic carboxylic acid into the a-bromo- (or a-chloro ) acid by treatment with bromine (or chlorine) in the presence of a catal3rtic amount of phosphorus tribromide (or trichloride) or of red phosphorus is known as the Hell-Volhard-Zelinsky reaction. The procedure probably involves the intermediate formation of the acyl halide, since it is known that halogens react more rapidly with acyl haUdes than with the acids themselves ... 

Boron exists naturally as 19.78% lOB isotope and 80.22% IIB isotope. High-purity crystalline boron may be prepared by the vapor phase reduction of boron trichloride or tribromide with hydrogen on electrically heated filaments. The impure or amorphous, boron, a brownish-black powder, can be obtained by heating the trioxide with magnesium powder. 

To 200 ml of 48% hydrobromic acid was added 0.40 mol of phosphorus tribromide (note 1). The mixture was agitated vigorously, while the temperature was kept between 20 and 30 0 by cooling in a water-bath at 10-15 0. After about 1 h the lower layer had disappeared completely. The solution was cooled to 0°C, then 0.40 mol of ammonium bromide, 0.10 mol (note 2) of copper(I) bromide (commercial product),... 

Note 1. The concentration of the aqueous HBr solution is increased by conversion of phosphorus tribromide with water. If available, a corresponding amount of gaseous HBr can be introduced into the 48% solution at 0°C. 

The most suitable synthetic method for these products is the heterocyc-lization reaction of N-thioacyl derivatives of amino acids (202) with phosphorus tribromide (378, 442-450, 559, 560) or anhydrous trifluoroacetic acid (448, 449, 451, 452) (Scheme 103). Treatment of N-thioacyl amino acids with acetic anhydride leads directly to the thiazolylacetate without isolation of an intermediate thiazolinone (365. 452). 2-Alkoxy-derivatives of A-2-thiazoline-5-one, however, can be obtained without acetylation by this method (453, 454). 

Carbon disulfide readily reacts with a-aminonitriles giving 2-mercapto-5-aminothiazoles (213), (271, 293) which can be converted to 5-aminothiazoles unsubstituted in the 2-position (Scheme 110 and Table II-34a). If this reaction is carried out in the presence of benzyl chloride in phosphorus tribromide, a 2-S-substituted thiazole derivative (214) is obtained in quantitative yield (Scheme 111), with R = hydrogen or phenyl (68, 304). 

Substituted 5-hydroxythiazoles (267b), Rj = alkylmercapto, acyl-amino, and sec-amino, are prepared by cydization of N-thioacyl-amino acids (266) with phosphorus tribromide or acetic anhydride (Scheme 137) (317, 350). i en the cydization of 266, R2 = H, is carried out with acetic anhydride in the presence of benzaldehyde (317, 325) or ethylformate (317), the benzylidene (268), R2=Ph, Rj = SR or CH2Ph, or 4-ethoxymethylene (268), Rj = SR and R2 = OEt, derivative is obtained directly (Scheme 138). 

Hydroxyalkylthiazoles are also obtained by cyclization or from alkoxyalkyl-thiazoles by hydrolysis (36, 44, 45, 52, 55-57) and by lithium aluminium hydride reduction of the esters of thiazolecarboxylic acids (58-60) or of the thiazoleacetic adds. The Cannizzaro reaction of 4-thiazolealdehyde gives 4-(hydroxymethyl)-thiazole (53). The main reactions of hydroxyalkyl thiazoles are the synthesis of halogenated derivatives by the action of hydrobroraic acid (55, 61-63), thionyl chloride (44, 45, 63-66), phosphoryl chloride (52, 62, 67), phosphorus penta-chloride (58), tribromide (38, 68), esterification (58, 68-71), and elimination that leads to the alkenylthiazoles (49, 72). 

Thionyl chloride and phosphorus tribromide are specialized reagents used to bring about particular functional group transformations For this reason we won t present the mechanisms by which they convert alcohols to alkyl halides but instead will limit our selves to those mechanisms that have broad applicability and enhance our knowledge of fundamental principles In those instances you will find that a mechanistic understand mg IS of great help m organizing the reaction types of organic chemistry... 

Reaction of alcohols with phosphorus tribromide (Section 4 13) As an alternative to converting alco hols to alkyl bromides with hydrogen bromide the inorganic reagent phosphorus tribromide is some times used... 

Reaction with phosphorus trihalides (Section 4 13) Phosphorus trichloride and phosphorus tribromide convert alcohols to alkyl halides... 

This method of a bromination of carboxylic acids is called the Hell-Volhard-Zelinsky reaction This reaction is sometimes carried out by using a small amount of phosphorus instead of phosphorus trichloride Phosphorus reacts with bromine to yield phosphorus tribromide as the active catalyst under these conditions... 

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