via the Upjohn dihydroxylation or enantioselectively using Sharpless asymmetric dihydroxylation with enantiomeric excesses as high as 100%. Bromination of trans-stilbene produces predominantly meso-1,2-dibromo-1,2-diphenylethane (sometimes called meso-stilbene dibromide), in line with a mechanism involving a cyclic bromonium ion intermediate of a typical electrophilic bromine addition reaction; cis-stilbene yields a racemic mixture of the two enantiomers of 1,2-dibromo-1,2-diphenylethane in a non-polar solvent such as carbon tetrachloride, but the extent of production of the meso compound increases with solvent polarity, with a yield of 90% in nitromethane. The formation of small quantities of the two enantiomers of stilbene dibromide from the trans-isomer suggests that the bromonium ion intermediate exists in chemical equilibrium with a carbocation intermediate PhCHBr–C(H)Ph with a vacant p orbital vulnerable to nucleophilic attack from either face. The addition of bromide or tribromide salts restores much of the stereospecificity even in solvents with a dielectric constant above 35. Upon UV irradiation it converts to cis-stilbene, a classic example of a photochemical reaction involving trans-cis isomerization, and can undergo further reaction to form phenanthrene. Derivatives and uses Synthetic (E)-Stilbene itself is of little value, but it is a precursor to other derivatives used as dyes, optical brighteners, phosphors, and scintillators. Stilbene is one of the gain mediums used in dye lasers. Disodium 4,4'-dinitrostilbene-2,2'-disulfonate is prepared by the sulfonation of 4-nitrotoluene to form 4-nitrotoluene-2-sulfonic acid, which can then be oxidatively coupled using sodium hypochlorite to form the (E)-stilbene derivative in a process originally developed by Arthur George Green and André Wahl in the late nineteenth century. Improvements to the process with higher yields have been developed, using air oxidation in liquid ammonia. The product is useful as its reaction with aniline derivatives results in the formation of azo dyes. Commercially important dyes derived from this compound include Direct Red 76, Direct Brown 78, and Direct Orange 40. Natural stilbenes The stilbenoids are naturally occurring stilbene derivatives. Examples include resveratrol and its cousin, pterostilbene. The stilbestrols, which are structurally but not synthetically related to (E)-stilbene, exhibit estrogenic activity. bMembers of this group include diethylstilbestrol, fosfestrol, and dienestrol. Some such derivative are produced by condensation of coenzyme A derivatives of cinnamic acid or 4-hydroxycinnamic acid and the malonic acid. Appendix Table 1. Vapor pressures References External links (E)-Stilbene on ChemExper Luminescence Fluorescent dyes Phosphors and scintillators Laser gain media Stilbenoids Phenyl compounds