SYNTHESIS OF 4-NITROHEPTANE

 

CAUTION: The reactions involving hydrogen peroxide and pertrifluoroacetic acid should be carried out in a fume cupboard behind a safety screen. Adequate precautions should be observed in handling the hydrogen peroxide solution (1).

Dipropyl ketoxime (heptan-4-one oxime):

• Heat a mixture of 20g (0.18 mol) of heptan-4-one, 17g (0.25 mol) of hydroxylamine hydrochloride, 19.6g (20 ml, 0.25 mol) of pyridine and 1 50 ml of ethanol under reflux for 1 hour in a 500-ml round-bottomed flask. 

• Rearrange the condenser for downward distillation and remove the ethanol on the water bath. 

• Allow the residue in the flask to cool and add 150 ml of water. 

• Extract the oxime with three 50 ml portions of ether. Wash the combined extracts with water and dry over magnesium sulphate. 

• Remove the ether on the rotary evaporator and distil the residue at atmospheric pressure. 

• Collect the fraction having b.p. 192-195 °C; the yield is 18.2g (80%). The oxime shows infrared absorptions at 3300cm⁻¹ (O — H stretch) and 1655 cm⁻¹ (C=N stretch).

4-Nitroheprane:

• Prepare a solution of pertrifluoroacetic acid in acetonitrile as follows. 

• Place 50 ml of acetonitrile in a two-necked, 250-ml round-bottomed flask fitted with a dropping funnel and a reflux condenser. 

• Insert a plastic-covered magnetic stirrer follower bar and cool the flask in an ice bath sited on the stirrer unit. 

• To the cooled and stirred solution add 5.8 ml (0.2mol) of 85 per cent hydrogen peroxide (1) and then 39.0ml (58.1g, 0.24 mol) of trifluoroacetic anhydride. 

• Stir the solution for 5 minutes and then allow to warm to room temperature. 

• In a three-necked, 500-ml round-bottomed flask fitted with a sealed stirrer unit, dropping funnel and reflux condenser place 200 ml of acetonitrile, 47g (0.56 mol) of sodium hydrogen carbonate, 2g of urea and 12.9g (0.1 mol) of dipropyl ketoxime. 

• Heat the stirred suspension under reflux on the water bath and add dropwise over 90 minutes the prepared solution of pertrifluoroacetic acid. 

• When the addition is complete heat the mixture under reflux for 1 hour. 

• Pour the cooled reaction mixture into 600 ml of cold water and extract with four 100 ml portions of dichloromethane (note: the organic layer is the upper layer in the first extraction, but subsequently it is the lower layer). 

• Wash the combined extracts with three 100 ml portions of saturated sodium hydrogen carbonate solution and dry over magnesium sulphate. 

• Remove the solvent on the rotary evaporator and distil the residue under reduced pressure through a short fractionating column packed with glass helices. 

• Site the apparatus behind a safety screen. 4-Nitroheptane distils at 66-69 °C/2 mmHg; the yield is 8.4g (58%). 

• A small amount of dipropyl ketone is obtained as a forerun. 

• The infrared spectrum of the nitroalkane shows absorption at 1555 and 1385 cm⁻¹ attributable to the antisymmetric and symmetric stretching of the nitro group. 

• The purity of the product may be investigated by g.l.c. using a 10 per cent Silicone oil on Chromosorb W column held at 100 °C; the nitroalkane has a slightly shorter retention time than the oxime; the ketone is rapidly eluted from the column.

Notes to keep in mind:

1. Considerable care should be exercised when carrying out reactions with 85 per cent w/v hydrogen peroxide. Rubber gloves and a face mask should be worn and reactions carried out behind a safety screen. Plenty of water should be at hand to wash away any spillages; fire may result if the peroxide is spilled on to combustible material. Any spillage on the skin similarly should be washed with plenty of water. Care should be taken to avoid the formation of potentially explosive emulsions with organic materials. Spirit thermometers and not mercury thermometers should be used; grease on taps and joints should be kept to the absolute minimum; safety pipettes must be used for pipetting. Disposal of high test perioxide solution may be effected by diluting with a large excess of water. The peroxide used in these experiments was supplied gratis by Interox Chemicals Ltd.