Electrophilic Anti Addition of Bromine to 2-Methylbut-2-ene with the N-Methylpyrrolidin-2-one Hydrotribromide Complex

Abstract

Despite the fact that bromine is very toxic by inhalation, highly corrosive, and may cause serious burns, its electrophilic addition reaction continued to be featured in the second-year laboratory course at Paris XI University. It had customarily been performed by the addition of a solution of bromine in chloroform to olefinic compounds such as 2methylbut-2-ene (1). The laboratory exercise also required two liquid purification techniques: liquid‐liquid extraction followed by two kinds of distillation. Furthermore, preparation of the chloroform solution of bromine (8%) required manipulation of large quantities of bromine, and even the use of the resulting solution could not prevent bromine fumes. Moreover, the addition reaction of bromine to 2methylbut-2-ene is an exothermic process, inducing evaporation of olefinic compound and therefore decreasing the overall reaction yield. For safety considerations, we proposed that bromine no longer be used and be replaced in the student laboratory by another brominating agent. In our university, molecular bromine has been substituted by the N-methylpyrrolidin-2-one hydrotribromide complex (MPHT; ref 2 ) whose crystalline data have been recently published (3). This orange crystalline complex has been prepared by an experienced chemist in large quantities, following a slightly modified version of Daniels’ procedure (ref 2; Scheme I). The MPHT complex is a stable solid that can be stored several months at room temperature; after a six-month storage test at room temperature no decrease in the free bromine titer could be detected. MPHT is not corrosive, not necrosing, less toxic, and easier to handle for students than molecular bromine. This complex smoothly liberates bromine in organic solvents according to Scheme II. The addition reaction was performed at room temperature in dichloromethane using 2-methylbut-2-ene. Discoloration of the resulting orange solution could be observed by students as long as MPHT is consumed. N-Methylpyrrolidin-2-one hydrobromide and N-methylpyrrolidin-2-one are byproducts of the reaction and are removed by washing the organic layer with water. Over a one-year period, the majority of the 230 students successfully completed this experiment with overall yields ranging from 35 to 80%.