Chemistry 351 Laboratory - Spring Semester 2011 – Professor T. Nalli, Winona State University

Expt #1. NBS Bromination of Isopropylbenzene

Relevant textbook readings – Smith, Chapter 13

Overview – Isopropylbenzene (common name = cumene) will be reacted with N-bromosuccinimide (NBS) and benzoyl peroxide (BPO) and the structure of the product determined by NMR spectroscopy (eq 1). The mechanism of bromination using NBS is discussed at length in the Smith textbook. NBS provides a low concentration of Br₂ through its reaction with the HBr side product of the bromination reaction (eq 2). Br₂ reacts with the arene by a radical chain mechanism involving bromine atoms (Br^.) as key intermediates (eq 3-4). Benzoyl peroxide functions as a radical initiator for the radical chain reaction through its thermal homolysis to form benzoyl radicals (eq 5). The location of the bromine in the final product is determined by which hydrogen is abstracted by the bromine atom in eq 3. In this lab you will examine the relative reactivites of the hydrogens in cumene toward Br^. by determining the structure of the C₉H₁₁Br product.

Procedures

Running the reaction.

· Add 10 mmol cumene and 15 mL hexane to a 25-mL round bottom flask equipped with a magnetic stir bar.

· Add 10 mmol NBS and 2.5 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 40 min.

· Cool the flask to room temperature.

Work-up procedures.

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

· Transfer the filtrate to a separatory funnel and wash it with 15 mL 10% NaHCO₃, keep the organic layer.

· Wash the organic layer with 15 mL H₂O.

· Wash the organic layer with 15 mL saturated NaCl(aq).

· Dry the organic liquid with Na₂SO₄.

· Transfer to a 25 mL rb flask and use a small-scale rotary evaporator to distill off the hexane solvent. The instructor will demonstrate the use of the rotary evaporator.

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

Characterization of Product

Obtain the ¹H NMR and ¹³C NMR spectra using CDCl₃ as the solvent. Make sure to integrate the ¹H 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 in the 1H NMR, CDCl₃ in the ¹³C NMR). Label these on the spectra but do not discuss or present them in the lab report. There might also be peaks due to incompletely evaporated hexane and/or unreacted cumene. Do not be too concerned about these, the main purpose of the lab is to determine the structure of the main product not to obtain it in pure form.