Solar Photothermochemical Reaction and Supercritical CO₂ Work up for a Fully Green Process of Preparation of Pure p‑Nitrobenzyl Bromide

It has been reported by us recently that p-nitrobenzyl bromide (PNBBr) can be synthesized from p-nitrotoluene (PNT) in high isolated yield with respect to available bromine in 2:1 Br^––BrO₃^– employed as brominating reagent. The reaction was conducted in ethylene dichloride (EDC) and the substrate was taken in excess to suppress dibromo impurity formation. The product was “cold crystallized” from the reaction mass and the mother liquor was recycled in the subsequent batch thereby eliminating organic discharge. The present work attempts to further advance the synthesis of this commercially important molecule employed in protection–deprotection strategies. Herein its successful synthesis employing neat substrate and solar radiation as the sole energy source to drive this photothermochemical reaction is reported. Further, 100% pure PNBBr could be isolated from the solid reaction mass in 87% yield by leaching out the excess substrate through supercritical CO₂ (Sc-CO₂) extraction. The reaction was therefore accomplished cleanly in all respects and with low carbon footprint.