Red phosphorus in bromination

Red phosphorus in bromination of 7 butj rolactone 46, 22 Reduction, of 3/3 acetoxy 5 pregnene 20-one with lithium aluminum tn t butoxyhydnde, 46, 57 of 4-f butylcyclohexanone with lithium aluminum hydride aluminum chloride, 47,16... 

Alternatively, place the mixture of alcohol and red phosphorus in a 500 ml. three-necked flask fitted with a mechanical stirrer, dropping funnel and double surface condenser. Heat the phosphorus - alcohol mixture to about 250°, and add the bromine whilst stirring vigorously. Work up the reaction product as above. 

A. Preparation of a-bromoisobutyryl bromide. To a mixture of 250 g. (2.85 moles) of isobutyric acid and 35 g. (1.13 moles) of red phosphorus in a 1-1. three-necked flask, fitted by ground-glass joints to a dropping funnel, mechanical stirrer, and reflux condenser, is added, dropwise with stirring, 880 g. (5.5 moles) of bromine. After the addition is complete, the solution is warmed to 100° over a period of 6 hours. The unreacted bromine and hydrogen bromide are removed under reduced pressure (30 mm.). The a-bromoisobutyryl bromide is decanted from the phosphorous acid and fractionated through a short helices-packed column. After a considerable fore-cut, the main fraction, 493-540 g. (75-83%), is collected at 91-98° (100 mm.). 

During preparation of hydrogen bromide by addition of bromine to a suspension of red phosphorus in water, the latter must be freshly prepared to avoid the possibility of explosion. This is due to formation of peroxides in the suspension on standing and subsequent thermal decomposition [1], In the earlier description of such an explosion, action of bromine on boiling tetralin was preferred to generate hydrogen bromide [2], which is now available in cylinders. 

Preparation of Phosphorus Tribromide by Reacting Red Phosphorus with Bromine. Perform the experiment in a fume cupboard Assemble an apparatus as shown in Fig. 101. Put 4 g of dry red phosphorus into three-neck round-bottomed flask 1. Fill cooler 2 with... 

Niobium Pentabromide, NbBrs.—This is the only bromine compound of niobium hitherto prepared. It is obtained by the action of bromine vapour on the coarsely powdered metal,2 or on a mixture of niobium pentoxide and carbon in the absence of air.2 In the latter case some of the oxybromide is also formed. Niobium pentabromide is a fine, purple-red powder, very similar to red phosphorus in appearance. The fused substance forms garnet-red prisms. On being strongly heated it becomes yellow, and volatilises. It melts at about 150° C., and distils undecomposed in an inert atmosphere at about 270° C. It can be distilled unchanged in an atmosphere of nitrogen or carbon dioxide. It is very hygroscopic, hydrolyses rapidly in damp air, and is decomposed by water with a hissing noise and considerable evolution of heat into niobie add and hydrobromic add. It is soluble in absolute alcohol and in dry ethyl bromide. 

Promotion of an SN2 displacement mechanism, and hence greater regioselectivity, may be effected by the addition of liquid bromine to a warm suspension of purified red phosphorus in the appropriate alcohol. The reaction is of general application with primary alcohols (isobutyl alcohol to hexadecan-l-ol) the yields are over 90 per cent of the theoretical, but with secondary alcohols the yields are in the range 50-80 per cent (Expt 5.55). This method is to be preferred to the direct use (rather than the in situ generation) of phosphorus tribromide which is the more hazardous reagent. The outline mechanism may be represented thus ... 

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... 

Bromination of acid derivatives is usually carried out not on the acid itself but by converting it to an acyl bromide or chloride, which is not isolated but gives the a-bromoacyl halide via the enol. This used to be done in one step with red phosphorus and bromine, but a two-step process is usually preferred now, and the bromoester is usually made directly without isolating any of the intermediates. We can summarize the overall process like this. 

It is generally prepared by dropping bromine from a tap-funnel on to red phosphorus in a ilask.10 The initial reaction occurs with flashes of flame, and external cooling is desirable. Afterwards the bromine is diluted by the PBrs and the combination proceeds less vigorously. If the phosphorus is in excess a little remains dissolved in the tribromide, which must be fractionally distilled after the addition of a slight excess of bromine. The reaction can also be carried out in the presence of benzene. 

Preparation of Phosphorus Tribromide by Reacting Red Phosphorus with Bromine. Perform the experiment in a fume cupboardl) Assemble an apparatus as shown in Fig. 101. Put 4 g of dry red phosphorus into three-neck round-bottomed flask 1. Fill cooler 2 with water. Displace the air from the setup with a stream of dry carbon dioxide and close tube 4 feeding in the gas with a clamp. Pour dry bromine in dropwise from dropping funnel 3. What amount of bromine should be poured into the funnel Agitate the flask during the experiment. What deposits on the walls of the flask Stop the reaction when a small amount of the unreacted phosphorus remains. Remove cooler 2 and close the opening with a stopper. 

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. 

Hell-Volhard-Zelinsky reaction. e-Benzoylaminocaproic acid is converted into the a-bromo derivative by stirring a mixture of the acid with red phosphorus in a flask cooled in an ice bath, adding bromine dropwise, gradually increasing the temperature, and stirring on the steam bath until practical disappearance of bromine... 

The second reaction takes place much more energetically, espe-dally when a phosphorus halide which has been previously made is used. This is not always necessary, at least in introdudng bromine and iodine in many cases it is better to generate the phosphorus halides in the course of the reaction, by adding to the mixture of alcohol and red phosphorus either bromine from a dropping funnel, or as above, finely pulverised iodine. This reaction, as well as the first, is applicable to poly-add alcohols and substituted alcohols, e.g. ... 

The phosphorus tribromide boiled at 171-173 /ybo mm., 168-170V72S mm., and was prepared in 90-95 per cent yield by adding bromine to a stirred suspension of red phosphorus in carbon tetrachloride. A good fractionating column is necessary. Old red phosphorus containing acids of phosphorus gives a poorer yield. 

In the Hell-Volhard-Zelinsky procedure548-550 the anhydrous acid is treated with red phosphorus and bromine. The red phosphorus should be washed with water until free from acid and dried at 100°, and the bromine should be rendered anhydrous by shaking with concentrated sulfuric acid. The reaction can be performed with 1 equivalent of red phosphorus or with catalytic amounts (ca. 3%) and catalytic amounts of PC13 may be used instead or red phosphorus. If the free a-bromo acid is required, 1 mole of a short- or medium-chain fatty acid is treated with ca. 1.1 mole of bromine and the resulting a-bromo acid is distilled from the reaction mixture in a vacuum for higher fatty acids somewhat more than 2 moles of bromine are used and the crude a-bromo fatty acids are diluted with CC14 and decomposed with water. 

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