Expt #5. NBS Bromination of Cyclohexene

Relevant textbook readings – Klein, Chapter 10.7

Overview – Cyclohexene will be reacted with N-bromosuccinimide (NBS) in cyclohexane solvent to form 3-bromocyclohexene. Benzoyl peroxide (BPO) will be used as a radical initiator.The obtained product will be characterized by proton and C-13 NMR spectroscopy.

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 (cyclohexene) and the NBS (eq 6).

Procedures

Running the reaction.

·         Add 1 mL cyclohexene and 1 mL cyclohexane to a 5-mL conical reaction vial (with standard taper ground glass joint). Add a magnetic spin vane.

·         Add 1 mmol NBS and 0.2 mmol BPO to the reaction vessel. 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 reaction vial.

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

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

Work-up procedures.

·        Collect the solid precipitate by vacuum filtration washing with 3 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 2 mL diethyl ether to wash the filtrate (and all residue) in the filter flask into a test tube.

·         Wash the solution with 1 mL 10% NaHCO₃, keep the organic layer.

·         Wash the organic layer with 1 mL H₂O.

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

·         Dry the organic liquid over Na₂SO₄.

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

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

·        Carry out column chromatography using silica gel and dichloromethane to purify the crude product. The procedure is basically the same as that used in experiment 4 for purification of the crude bromohydrin.

Characterization of Product

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

Assigned Questions

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.