Preparation of Dibenzyl Ketone and Phenylacetone

Recently there was described¹ the preparation of Dibenzyl Ketone by heating a mixture of acetic anhydride and phenylacetic acid. Additional experience with this synthesis has shown that the method as originally published is not given in sufficient detail to ensure infallible results. These details are supplied in the present paper. It has been found also that potassium acetate is a helpful ingredient in the reaction mixture. With it, the product seems to be more easily purified than otherwise. In addition to Dibenzyl Ketone it has been found that Phenylacetone is also a product of the reaction. These steps account for the changes which occur:

PhCH₂COOH + Ac₂O → PhCH₂COOAc + AcOH

PhCH₂COOAc → PhCH₂COCH₃ + CO₂

2 PhCH₂COOAc → (PhCH₂CO)₂O + Ac₂O

(PhCH₂CO)₂O → PhCH₂COCH₂Ph + CO₂

Phenylacetone and Dibenzyl Ketone evidently are pyrolytic products of phenylacetic acetic anhydride and phenylacetic anhydride, respectively. That dibenzyl ketone is formed in greater yields than Phenylacetone is evidence that reaction 3 has a greater reaction rate than reaction 2. Or, if reaction 3 is an equilibrium reaction the evidence would indicate that phenylacetic acetic anhydride disappears more rapidly through reactions 3 and 4 than through reaction 2. Indirectly this may indicate that reaction 4 is faster under the conditions obtained in the experiment, than reaction 2.

Experimental

Fifty grams of phenylacetic acid was weighed into a 250 mL three-necked flask. Then, 50 g of re-distilled acetic anhydride and 2.5 g. of fused, anhydrous potassium acetate were added. A thermometer was placed in the liquid and the mixture refluxed for two hours, during which time the thermometer in the liquid reached 140-150°C. A good fractionating column was inserted in place of the reflux condenser and the mixture distilled. The distillation was carried out very slowly so that the distillate was mainly acetic acid.

After forty-five minutes carbon dioxide evolved slowly. The rate increased to a maximum at seventy-five minutes, but carbon dioxide was still being evolved slowly when the distillation was stopped. A total of 4 liters of gas was collected which was pure carbon dioxide (over 99.3% CO₂). Heating the liquid above 200-205°C produced resinification with a decrease in the yield of ketones. The residue was placed in a 50 cc. Claisen flask. Acetic anhydride (5 mL) was used as a rinse. The mixture was distilled at 3 mmHg and the following fractions collected: 30-75°C, mostly acetic anhydride. At 150-160°C a 26 g fraction then residue. Redistillation of the 26 g fraction gave 8 g (16% yield) of Phenylacetone, bp 215-220°C and 16 g (32% yield) of Dibenzyl acetone, bp 317-320°C (mp 30°C). The Phenylacetone was identified as the phenylhydrazone, (mp 84-85°C).