Bromination of Acetone

Category: 
Demonstration
Topics: 
Atomic Structure
Kinetics
Organic Chemistry

 

This classic ketone reaction demonstrates acid catalyzed electrophilic alpha substitution adjacent to the carbonyl group. When bromine water is added to a colorless solution of acetone in water, the solution turns the characteristic yellowish-orange color of elemental bromine. But the color disappears within a couple of minutes as the elemental bromine reacts to form bromoacetone and hydrobromic acid. 

 

Curriculum Notes 
This reaction is usually used in an organic chemistry class when studying characteristic reactions of the carbonyl functional group. If you compare the rate of this reaction with the rate of reaction of deuterated acetone, it is a great illustration of the kinetic isotope effect for a physical chemistry class. Allow about ten minutes to perform this demonstration.

One day of lead time is required for this demonstration.


Materials 

  • a stoppered 100 mL Florence flask containing 10 ml of acetone, 20 mL of 1 M HCl, and 50 mL of deionized water
  • a stoppered 25 mL Erlenmeyer flask containing saturated bromine water
  • a small pipette and pipette pump

Procedure 

  • transfer 1 mL of saturated bromine water into the Florence flask using the pipette
  • stopper both flasks
  • the yellowish orange color should disappear within a couple of minutes

Safety Precautions 

  • Saturated bromine water is corrosive and the bromine vapor that it emits is irritating if breathed. Minimize exposure by replacing the stopper as soon as possible.
  • The product of this reaction, bromoacetone is a strong lachrymator. Do not remove the stopper of the Florence flask after the addition of the bromine.

Prep. Notes 

  • To prepare the saturated bromine water, put a few milliliters of bromine into a small Erlenmeyer flask containing 100 mL or so of water, loosely stopper it, and let it stand overnight. Decant the amount that you need the next day. Do all of this under the hood as bromine is very corrosive and emits corrosive fumes.
  • Be sure to include the bromine spill kit (dry sand and 5% sodium thiosulfate solution) when you prep and deliver this demo.
  • The product of this reaction, bromoacetone is a strong lachrymator. Do not unstopper the reaction vessel until you are under a vent hood.
  • Use good secondary containment for both of the flasks when you are transporting them.