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P2P from phenylacetic acid and calcium acetate

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CXVIII.-The Preparation of Mixed Ketones by Heating the Mixed Calcium Salts of Organic Acids.
J. Chem. Soc. 81, 1185-1193 (1895)

By Ernest Bowman Ludlam M.Sc. (Vict.).

The method of heating a calcium salt in a sulphur vapour bath so as to form the corresponding ketone and of removing the ketone, as soon as formed, by a current of carbon dioxide (Young, Trans., 1981, 59, 623) gave such remarkably good results in the case of dibenzyl ketone that it seemed desirable to test its general applicability, and, in particular, to extend the method to the case of mixtures of calcium salts.

[...]

The acids first employed were acetic and phenylacetic. On heating mixtures of the calcium salts of these two acids, dimethyl, methyl benzyl, and dibenzyl ketones are obtained, and as they boil respectively at 56, 217, and 330, there is no difficulty in separating them by fractional distillation. Moreover, the mixed ketone contains the two radicles methyl and benzyl, which were sufficiently unlike to render the ketone a suitable one for further investigation.

Subsequently calcium phenylacetate was distilled with calcium propionate and with calcium butyrate respectively. The fractionation of the ketones obtained from these salts was again quite an easy operation.

Preparation of Methyl Benzyl Ketone.

The preparation was carried out with various proportions of the two salts, starting with a slight excess of calcium phenylacetate, then using molecular proportions of the two, and, later, increasing the proportion of acetate.

(1) In the first distillation, 20 grams of calcium acetate, previously well dried by heating in an air-bath to 140, were thoroughly mixed and ground in a mortar with 45 grams of dry calcium phenylacetate, the latter weight being 5 grams in excess of the calculated quantity required to form a molecular mixture.

The mixed salts were placed in a parting flask and heated to the temperature of boiling sulphur; a current of washed and dried carbon dioxide was passed through and maintained until the decomposition was complete. During the heating, there was just a little frothing, but although the mass became viscid and semi-transparent it did not become quite liquid. The ketone collected in the capillary tube in columns which were carried over into the receiver by the current of carbon dioxide. Towards the close of the distillation, the product became darker in colour and white fumes were formed, which condensed with difficulty, forming a dark coloured oil.

[...]

   From a practical point of view, the figures in Table IX show that the best yield of mixed ketone obtainable from a given weight of the more expensive salt, in this case calcium phenylacetate, is obtained by using a large excess of the cheaper salt.

Table IX.
Experiment Weight of
Calcium Acetate
Weight of
Calcium
Phenylacetate
Weight of
Mixed Ketone
Obtained
Weight of
Mixed Ketone
per 100 grams
of Calcium
Phenylacetate
1 20 45 18 40
2 20 40 17 42.5
3 25 35 13 37
4 30 30 15 50
5 30 30 15.5 52

The increase in the yield shown in experiment 5 as compared with that in experiment 1 is sufficiently marked. The intermediate stages are fairly well represented by the figures as displayed, with the exception of experiment 3, owing to the fact that in this experiment the yield of all three ketones was poor. There is little doubt that the average number of experiments would have yielded a value nearer 45 than 37 when the errors due to a single experiment were eliminated.

The yield of Phenyl-2-Propanone in Expt. 5 is 59%, based on phenylacetic acid.