Wondered about all the waste and huge amounts of chemicals used in the reported bromate oxidation of THF so SWIM decided to cut through some of the smoke and see if there wasn't a more efficient way to accomplish it. Thus, SWIM says,
Dissolve 60 grams NaBrO3 in 200 mls H2O with stirring in 2000 ml flask (high walls act as a kind of reflux mechanism. Or just set it up for reflux). Add 200 mls of a 27% solution of potassium hydrogen sulfate. Precipitation will occur, not to worry. Place the flask in a dish containing a few cups of crushed ice. This helps control the initial exothermic reaction and saves on bromine or solvent loss. Set it on the stirrer and commence stirring. Add 70 mls THF and either set up for reflux, no heat, or place a watch glass over the top, stoppering is not recommended (do say! need I mention that this is a product of trial and error people?). The next morning (16 hours) extract with 4 x 50 mls DCM. Bromine will come over with the DCM, not to fret. My feeling is that as long as you are getting bromine in the DCM washes (yellow color) you are also getting product, although yield does tend to drop off significantly after the 3rd wash. Combine the washes and remove the bromine as follows. Add about 30 mls of a 10% sodium sulfite solution and extract (about 1/20th as reported in the literature! And no bisulfate! why the hell did they include all that in the first place?). The yellow color of the bromine disappears and you have a relatively clean DCM solution of GBL, some residual THF (maybe), and traces of water and salt. Place in a warm (70oC) water bath with gentle breeze to drive off all solvents but GBL and decant or pipette from the crystalized salts (if any). Yield, approximately 20 mls water-white GBL, a little less if distilled.
Questions for you all: outside of balancing the bisulfate/bromate ratio and monitoring the kinetics, I don't believe this reaction has been optimized in the literature. What do you think the rate limiting factor happens to be? Extra THF was added and it is probable that some remained at the end of the reaction, though that's not certain- some evaporated during reaction, some was evaporated during workup and not a whole hell of a lot was converted. To carry the reaction past 16 hours caused a reduction in the yield of product according to the lit, but they were limited to equimolar quantities of THF which was eventually consumed leaving the bromine mechanism to work on the GBL, further oxidizing it to succinates and other compounds after a time. By having the THF in excess, the additional reactant should compete with the side reactions, ultimately resulting in higher yeilds of product, besides having more precursor. It did not. Let the reaction as described above continue longer? 24 hours? More? Keep adding THF incrementally, just as the bromine mechanism begins to kick in, say at 12-14 hours? What is the theoretical upper limit to yield on this reaction? Again, what is the rate limiting factor, or is there one? Is it truly an autocatalytic system? (I'm thinking bromate as the rlf, but once bromine is produced can you keep the reaction going through addition of H2O2?). I think some fine tuning of this reaction might make it an effective one indeed. Will keep you informed. Feedback appreciated.