A SYNTHESIS OF BENZOIC ACID
Benzoic acid will be prepared using the haloform reaction between molecular chlorine and acetophenone
under basic conditions. The chlorine will be generated in situ from common household bleach (aqueous
sodium hypochlorite). The crude benzoic acid will be isolated by suction filtration after acidification of
the reaction mixture. The benzoic acid will be recrystallized, and its purity will be assessed by measuring
the melting point, and by IR spectroscopy.
Prelab
Prepare for the experiment by tabulating the physical properties; i.e., characteristics such as state, colour,
melting point/boiling point/density and solubility in water and organic solvents, as well as any hazardous
or toxic properties of the of all reactants, reagents, solvents, products and byproducts that will be
encountered during the procedure. Write the balanced reaction for the equilibrium between molecular
chlorine and hypochlorite ion in bleach (a basic solution).
Calculate the theoretical yield of benzoic acid, given that the concentration of hypochlorite ion in the
bleach solution is approximately 0.70 M. Show all of your calculations.
Review the mechanism of the haloform reaction so that you understand the role of each reactant used.
Acetophenone will be converted into benzoic acid; however, because bleach is strongly basic, the benzoic
acid formed will immediately react to form the soluble benzoate anion. The addition of sulfuric acid (the
workup step) both neutralizes the excess hydroxide ion from the bleach and protonates the benzoate
anion to form benzoic acid.
The aromatic ring of acetophenone is deactivated to electrophilic substitution by the acetyl group, and by
running the reaction at room temperature, any side reactions involving chlorination of the aromatic ring
will be minimized.
Procedure
All steps up to the isolation of the crude product must be done in the fume hood.
1. Place a magnetic stirring bar and 60 mL of the bleach solution into a 250 mL Erlenmeyer flask. Gently
clamp the flask in place after putting it onto a magnetic stirrer in the fume hood. Turn on the stirrer
and add 1.5 mL of acetophenone.
2. The temperature of the solution will gradually rise (record the maximum value) as the reaction
proceeds, and the yellow colour should gradually disappear. Allow the reaction to stir for a total of
30 minutes.