A Dibromination Dibromance Part III

The protocols for the in situ formation of bromine from hydrobromic acid and hydrogen peroxide and subsequent reaction with an alkene vary somewhat in the literature. If the alkene can be dissolved in a water-miscible solvent such as ethanol, then the reaction can be done without a phase transfer compound. For example, trans-stilbene is dissolved in hot ethanol. Concentrated hydrobromic acid is added to the reaction mixture followed by 30% w/w aqueous hydrogen peroxide solution. I had never worked with concentrated aqueous HBr before. I noticed that concentrated (48% w/w) aqueous HBr produces much less noxious hydrohalide fumes than concentrated (38% w/w) aqueous HCl!

Generally, an excess of hydrogen peroxide is used to convert concentrated hydrobromic acid into bromine. The theoretical yield (in moles) of the bromine is greater than the number of moles of trans-stilbene starting material.

I decided to go with trans-stilbene as the alkene starting material instead of trans-cinnamic or a trans-cinnamate ester. The symmetrical nature of trans-stilbene tends to simplify the results a little. Also it is nice to have a commercial source of the major product available for comparison.

I noticed that in the original Tetrahedron article the authors used both hydrochloric and hydrobromic acids as halogen sources. In order to create bromochloro compounds the alkene in dioxane was mixed with 10 equivalents of HCl before the addition of 1.5 equivalents of hydrogen peroxide and 1.0 equivalents of HBr. The authors state that the chloride ion attacks the intermediate bromonium ion to give the bromochloro products. They also reported that hydrogen peroxide would convert HCl to diatomic chlorine which would perform the dichlorination of the alkene. Therefore, I thought that a good way to study this reaction would be to vary the hydrohalide substrates and look at the formation of products (dibromo, dichloro, and bromochloro) by gas chromatography.


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