SYNTHESIS OF DIETHYL KETONE (Pentan-3-one)

• Preparation of manganese(n) carbonate-pumice catalyst. 

• Dissolve 70 g (0.35 mol) of manganese(n) chloride tetrahydrate in 100 ml of water and add a solution of 38 g (0.35 mol) of anhydrous sodium carbonate in 120 ml of water with mechanical stirring. 

• Filter the precipitated manganese(n) carbonate and wash well with distilled water. 

• Transfer the solid to a large evaporating basin and add sufficient water to form a thick paste. 

• Add sufficient pumice (4-8 mesh) with stirring so that most of the paste has been transferred to the pumice and heat cautiously (avoid local overheating) on an electric hotplate until the pumice lumps no longer cling together. If too much water is added to form the paste of manganese(n) carbonate initially, it will not adhere to the pumice satisfactorily.

Diethyl ketone

• Pack the catalyst into the Pyrex glass combustion tube of the pyrolysis apparatus and assemble the remaining components. 

• Displace the air in the apparatus with nitrogen, and while maintaining a continued gentle gas flow, heat the pumice for 8 hours at 360-400 °C in order to convert the manganese(n) carbonate into manganese(n) oxide. 

• If necessary the catalyst may be allowed to cool in a stream of nitrogen if the preparation needs to be interrupted at this point. 

• Place 740 g (746 ml, 10 mol) of redistilled propanoic acid, b.p. 139-141 °C, in the dropping funnel, and with the furnace at about 350 °C, add the acid to the catalyst dropwise (about 30 drops per minute); the stream of nitrogen is stopped when addition of acid is commenced. 

• The apparatus requires little attention and the addition of acid occupies 48-72 hours. 

• The distillate consists of two layers. Separate the lower aqueous layer, salt out the ketone with solid potassium carbonate and add it to the main ketonic layer. 

• Treat the combined ketone fractions with small quantities of anhydrous potassium carbonate (1) until effervescence ceases (this both removes the excess of acid and dries the ketone), filter and distil through a short fractionating column. 

• Collect the diethyl ketone at 101—103 °C. The yield is 252 g (29%). 

• An improved yield may be obtained by re-circulating the distillate over the catalyst, but in practice this is rarely worth while. It must be remembered that on each occasion that the catalyst is allowed to cool a slow stream of nitrogen must be passed through the apparatus to prevent the oxidation of the manganese(n) oxide catalyst. 

• Assign the p.m.r. absorptions which occur at S 1.00 (t, 3H) and 2.38 (q, 2H).

Notes to keep in mind:

1.  An alternative method of working up the distillate, which has its advantages when dealing with volatile ketones or when it is suspected that conversion into the ketone is incomplete, is to treat the combined fractions of ketones with sodium hydroxide pellets until the mixture is alkaline. Should solids separate, these may be dissolved by the addition of a little water. The ketone is then separated, dried over anhydrous potassium carbonate and fractionated.

Cognate preparations

• Heptan-4-one (dipropyl ketone). Use 880 g (920 ml, 10mol) of butanoic acid, b.p. 162-164 °C. The yield of ketone, b.p. 142-143 °C, is 285 g* (46%).

Pentan-2-one (methyl propyl ketone)

• Use 360 g (6 mol) of glacial acetic acid and 176g (184 ml, 2 mol) of butanoic acid. The yield of methyl propyl ketone, b.p. 102-104 °C, is 75 g (43%); 75 g of acetone, b.p. 56-57 °C,t are also obtained. Assign the l3 C-n.m.r. absorptions which occur at S 13.7, 17.5, 29.6, 45.6 and 208.3.

Hexan-3-one (ethyl propyl ketone) 

• Use 296 g (298 ml, 4 mol) of propanoic acid and 352 g (368 ml, 4 mol) of butanoic acid. The yield is 214 g (53%) of ethyl propyl ketone, b.p. 122-124 °C; the by-products are 98 g of diethyl ketone, b.p. 100-102 °C, and 66 g of dipropyl ketone, b.p. 144-146 °C.

Undecan-6-one (dipentyl ketone) 

• Use 400 g (428 ml, 3.45 mol) of hexanoic acid, b.p. 204-206 °C. The yield of ketone, b.p. 222-226 °C, is 225 g (76%). 100-102 °C, and 66 g of dipropyl ketone, b.p. 144-146 °C.