Increasing environmental concerns are motivating efforts to develop and improve the efficiency of chemical reactions carried out in laboratories worldwide. One useful reaction that could be improved upon is the halogenation (chlorination or bromination) of organic and inorganic substrates. Even if a product does not actually contain a halogen, many synthetic processes involve the addition or substitution of a halogen to form useful precursors, such as pharmaceuticals. Practically speaking, in the case of chlorination, excess chlorine typically is used to ensure completion of the reaction. As a result, when the reaction is worked up, excess chlorine is frequently released into the environment, which leads to harmful health and environmental effects. As a greenhouse gas, chlorine and bromine atoms are substantially more efficient than CO₂. This has motivated the development of halogenation methods that do not involve direct treatment with molecular halogen, such as in the case of in situ generation of bromine or chlorine.
We have determined the stoichiometry of the chlorine generation reaction involving the reaction of sodium chlorate with aqueous hydrochloric acid. In addition, we have developed a strategy for physically separating the substrate from the chlorate/acid mixture, thereby allowing Cl2 gas to diffuse into the solution containing the substrate. We also have demonstrated that this method can be used to carry out solventless chlorination. In either approach, no purification steps are required, and the only side product is water.
Another approach for halogenating olefins is treatment with [Pt(tpy)X3]+ (X=Cl, Br). Although these complexes are relatively easily prepared, surprisingly little is known about their reactivity. A reaction of Br2 with an olefin in the presence of a nucleophilic solvent can lead to the solvent being incorporated into the products. When [Pt(tpy)X2]+ is allowed to react with olefins, the major product is the dibromoalkane. This is attributed to [Pt(tpy)X2]+ (X=Cl, Br) reacting with olefins via a different mechanistic pathway than found for the reaction of Br2 with olefins.