Expt #4. Bromination of Acetophenone

Relevant textbook readings – Nichols - Organic Chemistry Lab Techniques, Chap 2.3 and Chap 3, especially 3.6A. Karty, Chap 10.7

Overview – In this experiment, we will react acetophenone (synonyms: methyl phenyl ketone, 1-phenyl-1-ethanone) with pyridinium tribromide and determine the product formed using proton NMR, IR, and GC/MS. Pyridinium tribromide is a safer substitute for Br₂ as a reagent for organic synthesis. You should be able to predict the product expected by reading the Karty chapter reference. Thin layer chromatography (TLC) will be used to check for reaction completion before proceeding with the workup procedures.

Procedures

Running the reaction

Add 10 mmol acetophenone to a 50-mL rbf equipped with a magnetic stirbar.  Dissolve the solid in 25 mL ethyl acetate and then add 11 mmol pyridinium tribromide. Attach a reflux condenser and heat at reflux for 15 min.

TLC

Check the reaction for completion using TLC (See ref 1 Chap 2.3E). You should make side by side spots on the TLC plate of acetophenone (use a solution made by dissolving approx 25 mg acetophenone in 5 mL DCM) and the reaction solution. After the solvent evaporates (this could take a while for the reaction solution - why?), develop the plate using a 1:1 hexane/ethyl acetate mixture.After the solvent front has advanced to within 1-2 cm of the top of the TLC plate remove the plate from the developing jar and quickly mark the solvent front with a pencil. After the solvent has evaporated visualize the plate under UV outlining all visible spots with a pencil. If acetophenone is still present in the reaction mixture then resume refluxing and use TLC to check again after another 15 min.

Work-up

Cool the solution to near room temperature and then pour it into 25 g of ice. After the ice melts transfer to a separatory funnel and extract with 10 mL of CH₂Cl₂. Repeat the extraction with two more 10-mL portions of CH₂Cl₂. Dry the combined organic layers over sodium sulfate and the decant into a dry pre-weighed 50-mL rbf for rotary evaporation.

Characterization of Product

Obtain a proton NMR spectrum of the product in CDCl₃. Prepare a sample for GC-MS analysis by adding 1 uL of the product to approx 10 mL pentane in a clean vial. Also obtain an IR. One group will also be asked to obtain a C-13 NMR of their product.