Continuous Synthesis and Separation of p-Bromobenzyl Bromide Using Atom-Efficient Bromination of p-Bromotoluene without Any Organic Effluent: Potential for Green Industrial Practice

Abstract

This work focuses on the bromination of p-bromotoluene (PBT) using different brominating agents such as liquid Br2, NaBr–NaBrO3, NaBr–NaBrO3–NaCl, NaBr–H2O2, and HBr–H2O2. NaBr–NaBrO3–NaCl is an eco-friendly brominating agent obtained from a bromine recovery plant. Both NaBr–NaBrO3 and NaBr–NaBrO3–NaCl were found to be nonhazardous and efficient brominating agents. Pure NaBr–NaBrO3 resulted in the best PBT conversion with 79.7% Br atom efficiency in water and 98.2% average Br atom efficiency using dichloroethane as a solvent. Dichloroethane is de facto no longer used in the US and Europe and is not eco-friendly; the process with water as a solvent is the best. The substrate to active bromine molar ratio of 3:1 was found to be sufficient to get the maximum selectivity of p-bromobenzyl bromide (PBBB). The low-temperature crystallization method was used for separation cum purification of the product. Unreacted PBT was recycled along with the dibromo byproduct obtained. The dibromo product, which was built up gradually in the reaction mixture over 10 successive batches, was converted back into PBBB/PBT through NaBH4 treatment of the mother liquor. This continuous process is highly sustainable and produces zero organic waste, making it potentially attractive toward green industrial implementation.

Bromide Oxidation: A Safe Strategy for Electrophilic Brominations

“…[13,71] This can then be used to brominate organics, either in an electrophilic reaction, [71][72] or, when adding a radical initiator such as AIBN or through irradiation, in a radical reaction. [71,73]…”

“…The low conversion was offset by recycling the starting and side products, resulting in an efficient and sustainable process. [71] Scheme 5. Side reactions occurring with sodium bromate brominations.…”

“…The acidic form, bromic acid, is unstable and will revert back to bromine when enough bromide is present to allow Equation (12) to proceed [13,71] . This can then be used to brominate organics, either in an electrophilic reaction, [71–72] or, when adding a radical initiator such as AIBN or through irradiation, in a radical reaction [71,73] …”

“…Br−S was used to brominate toluene into p ‐bromobenzyl bromide, obtaining better results compared to the same reaction with H 2 O 2 as an oxidant or just using Br 2 . The low conversion was offset by recycling the starting and side products, resulting in an efficient and sustainable process [71] …”