Why is halohydrin not formed In bromination of stilbene?

Stilbene can be dibrominated using $\ce{HBr}$ and hydrogen peroxide in ethanol. Common sense would suggest that the major product should be the bromo-ethoxy product due to reaction with bromonium ion followed by the solvent since it outnumbers the strong nucleophile. But, in practice this is not seen, or only seen in very small amounts. Why is this?




As stated in [reference 2](https://d.web.umkc.edu/drewa/chem321l/handouts/lab10x2additiontoalkenesfs2010.pdf), both *anti*- and *syn*-products are formed, so your intuition may be correct, and you may, partly, be proposing the *correct* mechanism for the dibromination of stilbene.




---


Reagents produce $\ce{Br2}$ *in-situ*:


$$
\ce{2HBr + H2O2 -> Br2 + 2H2O}
$$


This reaction mechanism may be favorable at lower concentrations of $\ce{HBr}$. However, according to several references (e.g., 1, [2](https://d.web.umkc.edu/drewa/chem321l/handouts/lab10x2additiontoalkenesfs2010.pdf)), an appreciable concentration of $\ce{HBr}$ is required for dibromination of stilbene.


Moreover, bromide ion, being negatively charged, is a better nucleophile than ethanol, being neutral. Thus, you might expect your product to react again to form ***syn***-dibromo product, due to anti ethanol addition followed by $\mathrm{S\_N}2$:


[![mechanism of syn dibromination of stilbene in ethanol bromine hydrogen peroxide](https://i.stack.imgur.com/TsWe6.png)](https://i.stack.imgur.com/TsWe6.png)


Both *anti*- and *syn*-products are formed, so your intuition may be correct; however, *anti*-product is more favorable at the commonly used concentrations.


[![green chemical dibromination of stilbene with ethanol and peroxide](https://i.stack.imgur.com/RkSAK.png)](https://i.stack.imgur.com/RkSAK.png)


Typically, 70% yield is obtained in the experiment.$^\text{1}$


### References


1. McKenzie, L. C., Huffman, L. M., and Hutchison, J. E. (2005). The Evolution of a Green Chemistry Laboratory Experiment: Greener Brominations of Stilbene. *J. Chem. Educ.*, *82*(*2*), 306-310 (DOI: <https://doi.org/10.1021/ed082p306>).
2. Kilway, K. V., Horn, J. D. V.(2006), X2 Addition to Alkenes: Bromination of trans-Stilbene. Available as [PDF](https://d.web.umkc.edu/drewa/chem321l/handouts/lab10x2additiontoalkenesfs2010.pdf).


 4