SYNTHESIS OF BENZYL METHYL KETONE

• Preparation of thorium carbonate-pumice catalyst. 

• Dissolve 294 g (0.5 mol) of thorium nitrate hexahydrate in the minimum of water (c. 450 ml) and add slowly a solution of 106 g (1 mol) of anhydrous sodium carbonate in 400 ml of water with stirring. 

• Allow the thorium carbonate to settle, decant as much as possible of the mother-liquor and wash the sediment once by decantation with 500 ml of water. 

• Make the resulting moist solid into a thick paste with distilled water and stir in pumice (4-8 mesh) until most of the suspension appears to be absorbed. 

• Dry the impregnated pumice in quantities of 200 g by heating in a large evaporating dish upon an electric hotplate and stirring constantly with a glass rod. 

• Stop the heating when the pumice particles no longer cling together. 

• Sieve the resulting pumice, 250 g of a white powder (consisting largely of thorium carbonate but containing some oxide) are recovered and can be used for impregnating more pumice. 

• The total weight of pumice catalyst thus prepared is about 1400g; the exact weight will depend upon the grade of pumice used.

Benzyl methyl ketone

• Fill the Pyrex combustion tube with catalyst but use a temperature of 400-450 °C for the conversion of thorium carbonate into the corresponding oxide; 6-12 hours are usually required for complete conversion and a slow stream of nitrogen should be maintained through the combustion tube. 

• Place a solution of 170g (1.25 mol) of pure phenylacetic acid (m.p. 77 °C) in 225 g (3.75 mol) of glacial acetic acid in the funnel, and adjust its rate of flow into the catalyst tube to 1 drop every 2 or 3 seconds. 

• Also pass a slow stream of nitrogen (1 bubble per second) through the apparatus in order to keep the gases in motion; the rate of flow may be estimated by passing the inert gas through a concentrated sulphuric acid wash-bottle or 'bubbler' before it enters the furnace. 

• When all the acid mixture has passed through the catalyst tube, separate the lower aqueous layer of the product and treat the organic layer with 10-20 per cent sodium hydroxide solution until the washings are alkaline to litmus and then twice with water. 

• Extract the aqueous layer twice with 50 ml portions of ether, wash the extracts successively with sodium hydroxide solution (until alkaline) and water, and add the resulting ether solution to the main product. 

• Dry with magnesium sulphate, remove the ether on a rotary evaporator and distil the residue under reduced pressure preferably through a fractionating column.

• Collect the benzyl methyl ketone at 102-102.5 °C/20mmHg; the yield is 85 g (51%). The residue in the flask is dibenzyl ketone; it may be purified by transferring to a smaller flask and redistilling (b.p. 200 °C/21 mmHg; m.p. 34-35 °C). The i.r. absorption for the carbon yl group appears at 1700 cm" '. The p.m.r. (CDC1 3 , TMS), shows signals at 5 2.02 (s,3H, Me), 3.58 (s,2H,CH 2 ) and 7.20 (s, 5H, Ph).

Cognate preparations: Benzyl ethyl ketone

• Use 204 g (1.5 mol) of phenylacetic acid (m.p. 77 °C) and 333 g (335.5 ml, 4.5 mol) of propanoic acid (b.p. 139-141 °C), but omit the extraction with ether when working up the distillate. 

• Distil the dried product from a 500-ml round-bottomed flask through an efficient fractionating column. 

• Collect the diethyl ketone at 99.5-102.5 °C (160g), and when the temperature rises to 130°C (b.p. 103-130°C: 7g) transfer the residue to a 250-ml flask and distil fractionally under reduced pressure. 

• The benzyl ethyl ketone passes over mainly at 1 18-123 °C/22 mmHg (105 g, 47%); the residue of high boiling point (34 g) consists largely of dibenzyl ketone. 

• Pure benzyl ethyl ketone may be obtained by redistilling the fraction, b.p. 118-23°C/22mmHg, and collecting the fraction of b.p. 113-1 15 °C/1 7 mmHg.

Benzyl propyl ketone

• Use 204 g ( 1 .5 mol) of pure phenylacetic acid and 396 g (414ml, 4.5 mol) of butanoic acid (b.p. 161-164 °C). 

• Upon working up as for benzyl ethyl ketone, 180g of dipropyl ketone, b.p. 140-145 °C (mainly 143-145 °C), 108 g (45%) of crude benzyl propyl ketone, b.p. 240-260 °C, and 49 g of crude dibenzyl ketone (residue in flask) are obtained. 

• Redistil the fraction of b.p. 240-260 °C and collect the benzyl propyl ketone at 243-247 °C (the pure ketone boils at 244 °C).

4-Phenylbutan-2-one (methyl 2-phenylethyl ketone)

• Use 100g (0.66 mol) of hydrocinnamic acid (3-phenylpropanoic acid) (m.p. 49-50 °C) and 160g (2.66 mol) of glacial acetic acid. The yield of methyl 2-phenylethyl ketone, b.p. 230-235 °C, is 70g (71%) (the pure ketone boils at 234 °C).

l-Phenylpentan-3-one (ethyl 2-phenylethyl ketone)

• Use 100 g (0.66 mol) of pure hydrocinnamic acid and 200g (201.5ml, 2.7 mol) of pure propanoic acid. 

• Fractionation of the distillate yields 70 g of diethyl ketone (b.p. 100-102 °C), 72 g (67%) of ethyl 2-phenylethyl ketone (b.p. 245-249 °C; the pure ketone boils at 248 °C) and 18 g of crude l,5-diphenylpentan-3-one (high b.p. residue).

l-Phenylhexan-3-one (propyl 2-phenylethyl ketone)

• Use 100 g (0.66 mol) of pure hydrocinnamic acid and 235 g (245.5 ml, 2.66 mol) of pure butanoic acid. 

• Upon working up as for benzyl ethyl ketone the following yields are obtained; 98 g of dipropyl ketone, b.p. 140-144 °C; 65 g (55%) of propyl 2-phenylethyl ketone, b.p. 139-143 °C/1 7 mmHg; and 22g of crude 1,5-diphenylpentan-3-one (high b.p. residue). 

• The required ketone, upon redistillation, boils almost completely at 138-139 °C/16 mmHg.