A fast and feasible synthesis for picryl bromide

摘要

A new and optimized procedure for the synthesis of picryl bromide is described. A previous procedure involved a two-step nitration of bromobenzene, using a nitration mixture consisting of concentrated sulfuric acid and white-fuming nitric acid for 17 and 12 hours at 140 °C. The crude product obtained in this way is then submitted to two subsequent crystal transformations from boiling etlianol, which makes this procedure very time-consuming. A second previous procedure used a nitration mixture consisting of concentrated sulfuric acid and potassium nitrate at 125 °C for 6 hours to achieve the direct one-step nitration of bromobenzene with yields of up to 60%. A third procedure used 1,3.5-trinitroanisole as the starting material, which was submitted to a two-step bromo-demethoxylation reaction, using KI/EtOH and PBr_3 as the key reagents to obtain the target compound. The key problem with this route of synthesis is the availability and price of the starting material. The newly disclosed procedure involves the direct one-step nitration of bromobenzene using a 5:1 mixed acid, consisting of oleum (30%) and white-fuming nitric acid (10 equivalents), as the nitration agent. The reaction mixture is stirred for 45 minutes at 0 °C and then heated to 130 °C using an aluminum heat block. It is kept at that temperature for 3 hours and afterward cooled to room-temperature. The mixture is then poured into ice-water and washed with large amounts of cold water to obtain up to 63% picryl bromide in pure form without crystal transformation or further purification. This method combines the use of cheap materials with a time-economic route of synthesis and less to no additional purification steps, which is an improvement compared to the state of the art methods.