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.

Automated summary

Photochemical reactions, such as light-driven side-chain chlorination of aromatic compounds, are essential in the chemical industry. Reoptimising reaction conditions using advanced reactor designs and irradiation techniques is crucial for economic and ecological reasons. However, enhancing reaction rate must be balanced with high selectivity for the desired target compound.