Question

In a bromination of stilbene, why is this procedure considered to be “greener” than bromination methods utilizing elemental bromide or pyridinium tribromide?

Answer 1

Pyridinium tribromide provides for gradual release of bromine into the reaction medium because it is in rapid equilibrium with pyridinium hydrobromide and molecular bromine. An additional advantage of pyridinium tribromide is that it is an easily weighed solid, in contrast to liquid bromine. As the bromine is consumed in the reaction, more is produced by the pyridinium tribromide. Because the hazardous reagent (Br₂) is produced in situ, it no longer needs to be handled directly. Another benefit of this reaction is that a more benign solvent, ethanol, can be used instead of a chlorinated solvent.