Recently, I read two interesting articles that are available online for free. Both can probably be located using just about any online search engine using the keywords "bromination, arenes, sodium bromide, oxone."
Here are the citations:
Bull. Korean Chem. Soc. 2002, Vol. 23, No. 5, pp. 773-775
" " 2001, Vol. 22, No. 6, pp. 549-550
In former publications wherein arenes were halogenated using the oxone/K-halogen(KCl, KBr, or KI) system, equimolar amounts of arene, K-halogen, and oxone in methanol are used in the reaction. In the above publications, equimolar amounts of arene, Na-halogen, and oxone in aqueous methanol (1:1) are used in the reaction.
According to Synthetic Communications Vol. 32, no. 15, pp. 2313-2318, 2002, specifically on p. 2316, the reaction between an arene, KI, and oxone occurs in the following manner:
ArH + KBr + 2KHSO5-KHSO4-K2SO4 =
ArBr + KOH + K2S2O8-KHSO4-K2SO4 + H2O
2KHSO5-KHSO4-K2SO4 + KBr =
HOBr + KOH + K2S2O8-KHSO4-K2SO4
2HOOSO3K = 2OH + 2OSO3K
KBr + 2HO +2OSO3K = KOH +HOBr + K2S2O8
ArH + HOBr = ArBr + H2O
For those a little confused like I was at first, K2S2O8 is potassium persulfate (formed, I would guess from the combination at the oxygen poles of two bisulfate ions; is this right?). Like oxone, it has the reputation of being a powerful oxidant. Acidification of potassium persulfate solutions, according to the peracid heading of the Encylopedia of Chemical Technology, yields peroxymonosulfuric acid and hydrogen peroxide. Other references indicate that through heat and/or moisture, potassium persulfate "decomposes".
According to Synthetic Communications, 31(23), 36-27-3632 (2001), specifically on p. 3628, 8 mmol of acetophenone, 40 mmol of sodium bromide, and 16 mmol of oxone are reacted in 30 mL of aqueous methanol (1:1) under reflux and stirring for 20 hours to provide a-bromoacetophenone in 71% yield. Under the same conditions, propiophenoe yields 67% of the a-bromoketone. This is interesting in that "free halogen" under acidic conditions was used to halogenate the ketone. However, most commercial sources of oxone are composed of "buffer" mixtures (approximately 15% sodium carbonate, according to some information I found online). Since the above reaction requires acidic conditions (why, I'm not sure) the addition of acid (sulfuric or bisulfate) would probably be necessary to ensure that a low pH is maintained. In aqueous methanol mixtures, this would prove to be an easy matter; however, performing such an addition to methanol-only solutions might be a bit more complicated. This would be especially true in the iodination reaction of KI/arene in methanol solvent using, presumably, commercial buffered-oxone as the oxidant.
Unlike the typical oxone/sodium or potassium bromide reactions, free bromine is desired rather than the hypobromite. So my question is, what are the products of the following reaction and what is the final balanced equation?:
8 mmol acetophenone + 40 mmol of NaBr + 16 mmol of oxone
Some more questions:
1.) Is the NaBr in excess of the oxone? If so, by how much and why?
2.) What is the oxidation potential of oxone?
3.) If upon reduction, oxone forms potassium persulfate, then what is its oxidation potential?
4.) What is the final product of a COMPLETE oxone reduction? Potassium bisulfate?
5.) How is it that in the above reaction using acetophenone as a substrate (is that what it's called?) does free Br2 form?
FYI, methanol is typically used for these reactions because neither oxone (JACS Vol. 56, p. 2198) nor bromine (Brittish Patent 607,538; a-halogenation reaction of arylketones using Br2 in methanol solvent) reacts with it to any appreciable extent. According to Tetahedron Letters, Vol. 38, No. 16, pp. 2805-2808, 1997, specifically on p. 2806, "Reaction of KBr with KHSO5 gave rise to a bright yellow-orange solution that is decolorized instantly upon addition of any of the three pyrimidine nucleotides." Just a little info as to what oxone/Na or K halide reactions might look like before and after they are completed.
As for the reputed explosive tendancies of oxone solutions, they seem to be highly overrated. According to JACS, Vol. 59, pp. 552-555 (1937)--which is a good article detailing the reactions between peroxymonosulfuric acid and lower alcohols, BTW--when concentrated peroxymonosulfuric acid was reacted with the tertiery alcohol ethyldimethylcarbinol, an explosion ensued. This is quite a unique circumstance and does not seem to apply to typical oxone reactions in which the potassium acid salt is quite stable.
Anyone ever wonder why it is that in JOC Vol. 25, pp. 1901-1906 (1960), sulfuric acid is added slowly drop by drop to alcohol non-aqueous solutions of oxone to form the desired ketone or alkylacetate? Simply to hydrolize the potassium peroxymonsulfate salt into free peroxymonosulfuric acid, which is soluble in the alcohol solvent. Interesting.
BTW, the above JOC article has experimental details on how to synthesize benzylhalides from refluxing mixtures of sodium halide/oxone/and toluene. Yields are low though.
It is also interesting to note that in a recent Synthetic Communications article, more specifically in a 2002, Vol. 32 issue, the oxone/NaBr system is used to effectively oxidize benzyl alcohols to benzaldehydes in high yield. I'd give you the exact citation, but the article from which I got the reference only has it listed as "in press". Sorry.
I hope somebee can answer my questions about the products of the oxone reduction using 40 mmol of NaBr to 16 mmole of oxone. It's the least somebee can do after my putting so much work into this thread.
Thanks.