SYNTHESIS FOR (-)-CAMPHOR

• Equip a 500-ml three-necked flask with a mechanical stirrer, a thermometer and a two-way adapter carrying a dropping funnel and condenser; arrange for the occasional cooling of the flask in an ice-water bath.

• Place a solution of 7.7 g (0.05 mol) of ( — )-borneol in 25 ml of ether in the flask and cool to °C. Precool 50 ml (0.033 mol) of chromic acid solution (1) to 0°C before placing 25 ml in the dropping funnel. 

• Add the cold chromic acid solution to the vigorously stirred borneol solution during 5 minutes, keeping the remainder cold in an ice-water bath. 

• Add the remaining chromic acid over a further 5 minutes. 

• Stir the mixture with cooling in the ice-water bath for a further 5 minutes and then transfer to a separating funnel. 

• Carefully remove the ether layer (2) and extract the aqueous solution with two 25 ml portions of ether. 

• Combine the ether extracts, and wash with 30 ml of 5 per cent sodium carbonate solution followed by four 25 ml portions of water. 

• Dry the ether solution over anhydrous sodium sulphate, filter and remove the ether on the rotary evaporator. 

• The yield of crude ( — )-camphor, m.p. 159-164°C (sealed tube), is 6.3 g (83%). It may be purified by sublimation at 80-90 °C/1 2 mmHg; after two sublimations 5.76 g (76%) of material is obtained, which melts at 173-175 °C (sealed tube).

Notes to keep in mind:

1. The chromic acid solution may be prepared as follows. Dissolve 100g (0.33 mol) of sodium dichromate dihydrate in 300 ml of water and slowly add 134g (73 ml, 1.34 mol) of concentrated sulphuric acid (98%, d 1.84). Cool the solution and dilute to 500 ml with water in a graduated flask.

2. Both the ether and aqueous solutions are almost black and it is difficult to see the interface; it is best judged by noting the change in flow of the solutions through the tap.