Red Phosphorus with Bromine

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 

Replace the dropping funnel with a dephlegmator provided with a thermometer and a Liebig condenser (see Fig. 20). Carefully heat the flask with the flame of a gas burner. What happens Collect the fraction boiling at 172-174 °C. 

Transfer the prepared phosphorus tribromide into a weighed drawn out test tube and seal it (wear eye protection . Weigh the ampoule together with the remaining parts of the test tube. Calculate the yield in per cent. 

Preparation of Phosphorus Iodide Pgl. To prepare phosphorus iodide, one must have thoroughly dried carbon disulphide (see Preparation of Phosphonium Iodide ) and purified white phosphorus (see Preparation of Phosphorus Tribromide ). (Perform the experiment in a fume cupboard .  

Assemble an apparatus as shown in Fig. 102. Lubricate the ground-glass joints with phosphoric acid. Fill the apparatus with dry carbon dioxide using offtake 2. Pour 10 ml of dry carbon disulphide from dropping funnel 3 into flask 1. 

---

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. 

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... 

Introduce several grains of red phosphorus with a spatula inta the fourth tube with bromine. Write the equation of the reaction. 

Since this is an acid-catalysed reaction it can be stopped at this stage. This valuable reaction, known as the Hell—Volhaid-Zelinsky (HVZ) reaction, is normally carried out by treating the acid with red phosphorus and bromine. PBr5 is thus made on the spot. 

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. 

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. 

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. 

Although most phosphorus chlorides and bromides are commercially available, it is sometimes necessary to prepare fresh reagents. In the case of phosphorus iodides, the reagents have poor shelf-lives (they are unstable and decompose under mild conditions) and are commonly prepared in situ, or immediately prior to use by reaction of red phosphorus with iodine. Phosphorus bromides can also be prepared this way. Reaction of 1,2,3-propanetriol with red phosphorus and bromine, for example, gave l,3-dibromo-2-propanol. 3 iodides are similarly prepared from red phosphorus and iodine, as in the conversion of cetyl alcohol (155) to cetyl iodide (156) in 85% yield. Using P and I2 is a common method for the conversion of aliphatic alcohols to aliphatic iodides. Another popular method is illustrated by treatment of 157 with triphenylphosphine, iodine, and imidazole,. In this case, taken from Hiemstra s synthesis of roseophilin, 67 the primary alcohol unit was converted to iodide 158 in 96% yield. 

Scheme 8.49. A variety of possibilities to account for the formation of a-bromoacetic acid (bromoethanoic acid [BrCH2C02H]) from acetic acid (ethanoic acid [CH3CO2H]) with red phosphorus and bromine (the Hell-Volhard-Zelinsky reaction).

For bromoalkanes and iodoalkanes we can make the phosphorus(III) halide, PBtj or PI3, in situ using red phosphorus and bromine or iodine. These are warmed with the alcohol. For example ... 

White and red phosphorus combine directly with chlorine, bromine and iodine, the red allotrope reacting in each case at a slightly higher temperature. The reactions are very vigorous and white phosphorus is spontaneously inflammable in chlorine at room temperature. Both chlorine and bromine first form a trihalide ... 

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. 

Dibromobutane (from 1 4-butanediol). Use 45 g. of redistilled 1 4-butanediol, 6-84 g. of purified red phosphorus and 80 g. (26 ml.) of bromine. Heat the glycol - phosphorus mixture to 100-150° and add the bromine slowly use the apparatus of Fig. Ill, 37, 1. Continue heating at 100-150° for 1 hour after all the bromine has been introduced. Allow to cool, dilute with water, add 100 ml. of ether, and remove the excess of red phosphorus by filtration. Separate the ethereal solution of the dibromide, wash it successively with 10 per cent, sodium thiosulphate solution and water, then dry over anhydrous potassium carbonate. Remove the ether on a water bath and distil the residue under diminished pressure. Collect the 1 4-dibromobutane at 83-84°/12 mm. the yield 3 73 g. 

Bromination of fatty acids in the a-position can be effected quite readily in the presence of phosphorus trichloride, red phosphorus or pyridine as catalysts or halogen carriers with acetic acid, the addition of acetic anhydride (to ensure the absence of water) improves the yield and facilitates the bromination. Examples are —... 

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 ... 

Ethyl a-bromopropionate. This preparation illustrates the facile bromination of an acid chloride (propionyl chloride) in the presence of red phosphorus, and the subsequent conversion of the bromoacid chloride into the ethyl ester by direct interaction with ethanol. 

Equip a 1 litre bolt-head flask with dropi)ing fuuncl and a double surface reflux condenser to the top of the latter attach a device (e.g.. Fig. II, 8, 1. c) for the absorption of the hydrogen bromide evolved. Place 100 g. (108 ml.) of dry iso-valeric acid (Section 111,80) and 12 g. of pmified red phosphorus (Section 11,50,5) in the flask. Add 255 g. (82 ml.) of dry bromine (Section 11,49,5) slowly through the dropping funnel at such a rate that little or no bromine is lost with the hydrogen bromide evolved the addition occupies 2-3 hours. Warm the reaction mixture on a water bath until the evolution of hydrogen bromide is complete and the colour of the bromine has disappeared. Pour off the liquid reaction product into a Claisen flask and distil mider the reduced pressure of a water pump. Collect the a-bromo-wo-valeryl bromide at 117-122°/25-30 mm. The yield is 150 g. 

Bromine (7 ml) is added dropwise to a mixture of white sand (14 g) and red phosphorus (3 g, dried at 165° under vacuum) moistened with 5 ml of deuterium oxide. The apparatus is fitted with an exit tube to allow the liberated deuteriobromic acid to pass through two U-tubes and into a receiving flask. The first trap contains glass beads and is cooled in an ice-salt slurry. The second contains glass beads and red phosphorus moistened with deuterium oxide. The deuterium bromide gas is collected in the appropriate solvent at ice bath temperature. A small amount of phosphorus pentoxide should be added to remove any deuterium oxide if anhydrous reagent is required. 

---