Photochlorination of toluene – the thin line between intensification and selectivity. Part 2: selectivity
Photochemical reactions, such as the light-driven side-chain chlorination of various aromatic compounds, are among the cornerstones of the chemical industry. Therefore, reoptimising reaction conditions using state-of-the-art reactor designs and irradiation techniques is economically and ecologically of uttermost importance. However, reaction rate enhancement needs to be accompanied by a high selectivity for the desired target compound. In part 1 of this work the potential of intensifying the side-chain photochlorination of toluene by various reactor setups and irradiation techniques was reported. While the reaction was accelerated by up to a factor of ten, its selectivity towards the desired side-chain chlorinated products decreased under certain conditions. Unexpectedly, significant amounts of ring chlorinated species were detected. High level ab initio molecular dynamic simulations allowed – in agreement with the experimental data – assessment of the underlying reaction mechanism leading to the undesired by-product. Photoexcitation of certain toluene–chlorine complexes, formed under highly intensified reaction conditions, yields charge-separated biradical species. In consequence, the pronounced driving force of charge- and radical-recombination leads to the formation of ring chlorinated species as derived by quantum chemical simulations. By applying dynamic irradiation conditions, not only was the formation of ring-chlorinated products suppressed but the energy consumption could also be reduced significantly.