SYNTHESIS OF 3-BROMOCYCLOHEXENE

CAUTION: Since many simple unsubstituted allylic compounds are powerful irritants by inhalation and by skin contact, this preparation should be carried out in an efficient fume cupboard and the product treated with appropriate caution.

• In a 500-ml round-bottomed flask place 39.4 g (0.20 mol) of JV-bromo-succinimide, 49.2 g (0.60 mol) of redistilled cyclohexene, 150 ml of carbon tetrachloride and about 500 mg of benzoyl peroxide, previously dried by pressing between filter papers. 

• Attach a double surface reflux condenser and allow the mixture to stand at room temperature when after a short induction period the reaction begins. 

• The reaction mixture becomes warm and the heavy yellowish N-bromosuccinimide begins to be transformed into the light colourless succinimide which becomes suspended in the reaction mixture. 

• When the reaction has moderated somewhat, transfer the flask to a steam bath and heat under reflux until all the JV-bromosuccinimide has been converted to succinimide (about 1.5 hours) (1). 

• Cool the flask, filter under suction and wash the residue with a little carbon tetrachloride. 

• Distil the filtrate and washings on a boiling water bath from a flask fitted with a Claisen still-head to remove most of the carbon tetrachloride (b.p. 77 °C) and unreacted cyclohexene (b.p. 83 °C). 

• Then fractionally distil the residue (2) under reduced pressure (water pump). 

• Further quantities (about 50 g) of cyclohexene and carbon tetrachloride are obtained as a first fraction, followed by the main fraction (about 24 g) of b.p. 72-77 °C/32-35 mmHg, which is only about 50 per cent pure by g.l.c. analysis and which rapidly goes brown on standing. 

• Subsequent redistillation under reduced pressure and using a short Vigreux column (12 cm) gives 14.4 g (45%) of colourless mobile 3-bromocyclohexene of b.p. 66-67 °C/20 mmHg, which is 99 per cent pure by g.l.c. analysis (10% Silicone oil on Chromosorb W, 1.5-m column held at 85 °C, nitrogen flow rate 40 ml/minute, t R 10.4 minutes).

Notes to keep in mind:

1. The completion of the reaction may be confirmed by treating starch-iodide paper with a drop of the reaction solution. At the end of the reaction no coloration is observed.

2. The flask and still-head should preferably have at least 24/29 sized joints to prevent losses due to foaming.