Chemistry 351 - Principles of Organic Chemistry II
Spring Semester 2024, Winona State University, Dr. Thomas Nalli

Expt #1. NBS Bromination of trans-2-Hexene

References - (1) Greenwood, F.; Kellert, M.; Sedlak, J. Org. Syn. 1963, 4, 108
(2) N-Bromosuccinimide. https://en.wikipedia.org/wiki/N-Bromosuccinimide (accessed Nov. 2017).5.   

Relevant textbook readings – Karty, Chapter 27.1-4.

Overview – trans-2-Hexene will be reacted with N-bromosuccinimide using benzoyl peroxide as a radical initator. A molar excess of the hexene will be used to ensure the formation of primarily monobrominated products. Some literature references suggest that 4-bromo-2-hexene should be the only significant product. We will use GC-MS and NMR to test this assumption.

Mechanism - The mechanism of radical bromination using NBS is discussed in Chapter 27.4d in the Karty textbook. NBS provides a low concentration of Br2 through its reaction with HBr (eq 2). Br2 then reacts with the substrate (RH) (e.g., 2-hexene) 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 0.5 mL trans-2-hexene and 3.0 mL cyclohexane to a reaction tube equipped with a magnetic spin bar.
  •  Add 0.67 equiv recrystallized NBS and
  • 5 mg BPO to the tube. 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.
  • Attach a microscale distilling column using a rubber-connector-rod to serve as an air-cooled reflux condenser. Heat the solution on a sand bath at a gentle reflux with stirring for 45 min. (See diagram below, the wet pipe cleaner is optional.)
  • Remove from heat and add 5 mg additional BPO and then resume reflux for an additional 45 min.

  • Cool the reaction solution on a cold water bath to near room temperature.

Work-up procedures.

  • Use a small scale vacuum filtration to collect the organic liquid. The filtered solid should consist primarily of the succinimide by-product of the reaction.
  • Use this liquid to prepare your GC-MS sample by adding 1-2 drops of it to 5 mL of dichloromethane.
  • Submit your GC-MS sample in a labeled septum-capped GC-microvial.
  • Rotovap the rest of the liquid to obtain your crude product. Get the yield.
  • Obtain a C-13 NMR spectrum (in CDCl3 solvent).