A CONTRIBUTION FROM THE CHEMICAL LABORATORY OF NORTHWESTERN UNIVERSITY 
AND THE RESEARCH LABORATORY OF THE UNIVERSAL OIL PRODUCTS COMPANY

Preparation of Dibenzyl Ketone and Phenylacetone 
By Charles D. Hurd and Charles L. Thomas

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.

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 Part 

Fifty grams of phenylacetic acid was weighed into a 250 cc. 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 degrees. 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. The following table gives a typical 
distillation.

Time min. 	Vapour Temp. 	Liquid Temp. 

0 		119 			150
45 		121 			154
57 		122 			160
70 		122.5 		165
74 		120 			171
78 		118 			182
80 		115 			193
81 		114 			199
83 		113 			204 

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 produced resinification 
with a decrease in the yield of ketones. The residue was placed in a 
50 cc. Claisen flask. Acetic anhydride (5 cc.) was used as a rinse. 
The mixture was distilled at 3 mm. and the following fractions 
collected: 30-75 degrees, mostly acetic anhydride. At 150-160 degrees 
a 26 gram fraction then residue. Redistillation of the 26 gram fraction 
gave 8 g. (16% yield) of Phenylacetone, b.p. 215-220 and 16 g. (32% 
yield) of Dibenzyl acetone , b.p. 317-320 (m.p. 30 deg.). The 
Phenylacetone was identified as the phenylhydrazone,2 (m.p. 84-85 deg). 


Summary

A detailed technique is given for producing Dibenzyl Ketone from phenyl-
acetic acid and acetic anhydride. Phenylacetone is formed concurrently.

Evanston, Illinois
Riverside, Illinois RECEIVED MAY 8

(1) Hurd, Christ, and Thomas, JOC 55, 2589 (1939)
(2) Beckh Ber., 31, 3163 (1898) lists. the m. p. at 85
(3) Kistler, Swann and Appel, Ind. Eng. Chem., 388 (1938)
