Chemistry 350 Laboratory - Fall Semester 2012 – Professor T. Nalli, Winona State University

Expt #8. NBS Bromination - Info and data for Tuesday lab section.

We will not do any further work on this reaction other than clean up. I ran NMR spectra of the crude product of one group and these are in the class storage folder. I also obtained GC/MS data and printouts are provided below. Please use these NMR and GC-MS data for your report.

GC-MS for Tuesday Crude Product.

Assigned Questions

1. Propose a mechanism for the reaction of NBS with HBr (eq 2) showing curved arrows. Hint: this is a two-step mechanism that starts with a proton transfer step.

2. Do the NMR spectra show evidence of the presence of benzoyl peroxide in the crude product? Identify peaks due to this compound.

3. The crude appears to contain a good amount of the desired product with a significant amount of unreacted 4-bromotoluene also present. Identify the peaks due to each compound and use peak integrations to calculate the percent of each in the mixture. (Do this both using the proton NMR and the GC-MS results provided.) What should have been done differently in order to get a more complete conversion of reactant to product.

4. The small singlet at 6.59 ppm in the proton NMR provides evidence for a slight amount of dibromination having occurred. Explain.