Chemistry 351 Laboratory - Spring Semester 2012 – Professor T. Nalli, Winona State University

(revised by TN on 3/14/2012)

Expt #1. NBS Bromination of Di-p-tolyl Ether

Relevant textbook readings – Klein, Chapter 11

Overview – Di-p-tolyl ether (DTE) (IUPAC name = 4,4'-oxybis(methylbenzene)) will be reacted with two equivalents of N-bromosuccinimide (NBS) to form the novel symmetrical dibromoditolyl ether (DBDTE) (IUPAC name = 4,4'-oxybis(bromomethylbenzene)) (eq 1). Benzoyl peroxide (BPO) will be used as the radical initiator for this useful free-radical chain reaction.

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) (in our case, DTE is the substrate) 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 5 mmol DTE and 7.5 mL hexane to a 25-mL round bottom flask equipped with a magnetic stir bar.

· Add 11 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 60 min.

· Cool the flask to room temperature.

Work-up procedures.

· Collect the solid succinimide byproduct by vacuum filtration washing with 15 mL of petroleum ether. (Hint: use pet ether to rinse any crystals that remain in the flask into the funnel.)

· Use 10 mL diethyl ether to wash the filtrate (and all residue) in the filter flask to 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.

· Filter the dried organic layer through a short column of alumina into a pre-weighed, dry 25 mL r.b. flask.

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

· Weigh the crude product and calculate the yield and percent yield.

· Recrystallize the crude product from hexanes.

Characterization of Product

Obtain the ¹H NMR spectrum using CDCl₃ as the solvent. Make sure to integrate the proton NMR and normalize the integrations so as to show the number of Hs in each peak. Label all peaks in each NMR spectrum. Remember that there will be NMR solvent peaks (CHCl₃ and H₂O). Label these on the spectra but do not discuss or present them in the results in the lab report.

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.