Reading a rather interesting preparation of anhydrous hydrogen bromide, that's going to mean I can fuck off the setup I have up at the moment, thats proving a pain in the arse (essentially a pressure-equalized addition funnel leading to a two-neck flask, containing sodium bromide, the addition funnel containing 98% sulfuric acid, then because I couldn't find my damn gas taps and was feeling shitty because of a stomach upset, I jerry-rigged a pair of gas taps by sticking rubber tubing onto the vacuum takeoff of some fritted buchner funnels and by the miracle that is duct tape, 'stoppered' the top of the funnels, one connected to a wash bottle, then to a length of copper pipe, bent into a ==U== shape, and stuffed with fine copper dust, held at a red heat by means of a torch, the type that hooks up to a big gas bottle and can run for a good long time.
The copper is to cope with the fact that sulfuric acid is oxidizing, especially at high concentrations. An argon tank was hooked up to the HBr generating flask after the sulfuric was added, by means of the addition funnel, and some vacuum tubing, just to push the gases through the system. The oxidizing nature of H2SO4 means there was a fair lot of bromine generated as well as the desired hydrogen bromide. I was preparing a solution of anhydrous HBr in anhydrous acetic acid, at a 30% concentration of HBr the GAA (glacial acetic acid) as solvent, the hot copper scrubs the bromine, whilst the HBr passes through into an erlenmeyer filled with the desired quantity of anhydrous GAA, the flask standing on a digital scale, in order to register the quantity of hydrogen bromide absorbed.)
Quite a pain in the backside, maintaining the heating, watching the connections to the pipe.
I need to find those sodding gas taps.
This time, I'm going to prepare an azeotropic solution of hydrogen bromide in H2O, using in this case, phosphoric acid, since H3PO4 is not oxidizing and will liberate HBr without the bromine, eliminating the need for the red hot copper to scrub bromine, the Br2 was obvious with sulfuric acid, the entire gas generation flask turned orange-red, the rusty color of bromine, the liquid halogen which sits in between gaseous green chlorine, with its sharp, 'clinical' sort of smell, and dark, solid silvery iodine, which smells rather pleasant, much less sharp compared to chlorine.
Bromine on the other hand smells fucking vile. Sharp, penetrating and rather than the cleanly sort of odor, if unpleasant, of chlorine (I've never smelled fluorine gas, so can't comment on that one, and I don't want to either since it evolves lethal hydrofluoric acid, and is such a powerful oxidizing agent that a very dilute stream of fluorine gas will cause a brick to burst into intense, searing flaring flames), Br2 smells....sickening. Even through my gas mask, I need to replace the filters, a little got through, enough to detect the odor, and it smells...well 'bromos' in greek, is where the name comes from, the ancient greek word means 'stench' and I find it quite applicable, for the odor was rotten, stifling, like hydrogen sulfide (rotten eggs) only with sharp razorblades stuck in it, if that makes sense. Putrid.
The phosphoric acid version, involves making a conc. aqueous solution of hydrogen bromide, again using sodium bromide (I'll scrap the first version, and just add excess sulfuric acid and some peroxide to make damn sure its all oxidized to bromine, then distill off the Br2 and save it for future use, I had been meaning to prepare some elemental bromine anyway actually, already had a flask full of NaBr solution, although the method was to be passing chlorine gas through it, the more reactive halogen replacing the less reactive, and reacting as follows: 2NaBr+Cl2-->Br2+2NaCl)
Anyhow, it'll be phosphoric acid, aqueous NaBr, to produce sodium phosphate and HBr, then treated with triphenylphosphine, forming triphenylphosphonium bromide, extracted into either chloroform or dichloromethane, the product dried over anhydrous magnesium or sodium sulfate then the solvent (3x150ml extractions) pooled and stripped in vacuo, collecting the DCM or chloroform for recycling, followed by washing with ethyl acetate to remove any residual triphenylphosphine. Reported yield is near quantitative (97-98%) of Ph3P(+ charge on the phosphorus atom)HBr
The result, triphenylphosphonium bromide is heat-sensitive, although yields are variable, 60-90% if simply pyrolyzed. Reportedly adding the Ph3PBr to xylene and bringing it up to reflux decomposes the triphenylphosphonium bromide to triphenylphosphine and hydrogen bromide, allowing recycling of the TPP, distillation off of the xylene for recycle, and anhydrous, highly pure hydrogen bromide gas essentially 'on tap'. I figure I'll prepare a full mole of triphenylphosphonium bromide tomorrow, so I've got HBr as good as in a pressurized gas tank, only no renting a tank, no corrosion of regulators, just storage in anhydrous conditions, and I can use it whenever I want HBr.
And, I see a side use for it. Halogenating alcohols, to give useful alkyl halides. I figure, the intermediate of the Appel reaction, a reaction for chlorinating primary/secondary alcohols via SN2 nucleophilic substitution, there is a reaction between carbon tetrachloride and triphenylphosphine to form an intermediate triphenylphosponium chloride. Thats just changing the halogen from the intermediate reactive halogenating agent from Cl to Br. Probably even more useful actually, because bromine is a better leaving group than chlorine, so the typical use, as essentially a chemical 'stepping stone' for sticking other things on an alcohol, it'll give better yields.
Will have to experiment some with a similar preparation of hydriodic acid via decomposition of phosphorus triiodide made by mixing red phosphorus with elemental iodine (yes, just like making THAT substance) with water, decomposing the PI3 to HI, then I'll filter off the regenerated red phosphorus and dry it, then stick it in a container for storage.
I have to say, I'm getting to really love phosphorus chemistry, its so versatile. Triphenylphosphine, red phosphorus, white phosphorus, that toxic, violent , garlic-smelling little glow-in-the-dark tiger of an element, phosphorus trichloride, pentachloride, tribromide, pentabromide, PI3, P2I4...oh I am going to be busy as hell for a while and filling my lab logbooks with interesting and useful preparations for ages, not to mention making reagents of great versatility that are difficult and expensive for a private individual to buy, I can buy the likes of PCl3, POCl3, PCl5, PBr3 etc. but even I need to be resourceful in doing so.
But, I do have a kilo of triphenylphosphine, and 4x0.5kg tubs of red phossy. Especially with the way I like to recycle, that'll last me a while.
