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

revised by TN 12/6/12

Expt #8. NBS Bromination

Relevant textbook readings – Klein, Chapter 11

Overview – Cyclohexene will be reacted with N-bromosuccinimide (NBS) to form 3-bromocyclohexene. Benzoyl peroxide (BPO) will be used as a radical initiator and cyclohexane will be used as a solvent.

Mechanism - The mechanism of radical bromination using NBS is discussed minimally in the Klein textbook. NBS provides a low concentration of Br₂ through its reaction with HBr (eq 2). Br₂ then reacts with the substrate (RH) (cyclohexene) by a radical chain mechanism (eq 3-4) to form the brominated product (RBr) and HBr, which reacts immediately with NBS to form more Br₂ (eq 2). BPO functions as a radical initiator for the radical chain reaction through its thermal homolysis to form benzoyl radicals (eq 5). The overall equation amounts to an exchange of H and Br between the substrate and the NBS (eq 6).

Procedures

Running the reaction.

· Add 6 mL cyclohexene and 6 mL cyclohexane to a 25-mL round bottom flask equipped with a magnetic stir bar.

· Add 5 mmol NBS and 1 mmol BPO to the flask. Caution: BPO is potentially explosive and can be detonated by friction. Weigh it out on wax paper and do not use a metal spatula to remove it from the bottle. Also take care to not get any of it on the interior of the ground glass joint of the flask.

· Attach a reflux condenser and heat the solution at reflux with stirring for 1 h.

· Cool the flask on a cold water bath to near room temperature.

Work-up procedures.

· Collect the solid precipitate by vacuum filtration washing with 15 mL of hexanes. (Hint: use hexanes to rinse any crystals that remain in the flask into the funnel.) This solid consists of the succinimide byproduct of the reaction as well as unreacted NBS. Discard it in the provided waste container .

· Use 10 mL diethyl ether to wash the filtrate (and all residue) in the filter flask to a separatory funnel. (See section 11.2 in Mohrig for how to properly use a separatory funnel.)

· Wash the solution with 5 mL 10% NaHCO₃, keep the organic layer.

· Wash the organic layer with 5 mL H₂O.

· Wash the organic layer with 5 mL saturated NaCl(aq).

· Dry the organic liquid with Na₂SO₄.

· Use the mini-scale rotary evaporator to distill off the ether solvent.

· Weigh the crude product. (This is your "crude yield".)

· Carry out a column chromatography to purify the crude product. More details to be provided later.

Characterization of Product

Obtain the ¹H and ¹³C NMR spectra using CDCl₃ as the solvent.

Prepare a sample for GC-MS analysis by adding 2-3 drops of the NMR sample solution to approx 10 mL dichloromethane.

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.