Synthesis of Bromobenzene
Introduction
If cold benzene is treated with bromine in the absence of sunlight, very little reaction occurs; if, however, a «halogen carrier», such as iron, iodine, pyridine, etc., is also present, a rapid reaction by substitution occurs, forming first bromobenzene, and then mainly p-dibromobenzene.
The reaction must be carried out in the absence of direct sunlight, since sunlight causes the bromine to add directly on to benzene, particularly if the latter is warm, to give benzene hexabromide.
Procedure
Required: Purified benzene 34ml, pyridine 0.5ml, bromine, 24ml.
The benzene used in this preparation should be reasonably free from toluene. It should preferably be dried over calcium chloride and, immediately before use, filtered through a fluted filter-paper. The pyridine should also preferably be dried over solid potassium hydroxide and redistilled. Place 0.5ml of the pyridine in a 250 ml round- or flat-bottomed flask and add 34ml (30g) of benzene. Fit the flask with a reflux water-condenser, and then place it in a cold water-bath. If the experiment is conducted in a fume-cupboard, the top of the condenser can be closed with a calcium chloride tube bent downwards and the evolved hydrogen bromide subsequently allowed to escape: if, however, the experiment is performed in the open laboratory, fit to the top of the condenser a glass delivery-tube which leads through a piece of rubber tubing to an inverted glass funnel, the rim of which dips just below the surface of some water in a beaker, so that the hydrogen bromide may be absorbed without risk of «sucking back». Ensure that the apparatus is not standing in the direct rays of the sun. Now pour 24ml (76g) of bromine (care in manipulation!) down the condenser, and at once replace the calcium chloride tube or the delivery-tube, as a vigorous reaction occurs when the bromine dissolves in the benzene. When the initial evolution of hydrogen bromide slackens, heat the water-bath to 25-30°C for one hour, occasionally shaking the contents of the flask: finally raise the temperature of the bath to 65-70°C for a further 45 minutes. Now transfer the dark-coloured liquid to a separating-funnel and shake with an excess of 10% aqueous sodium hydroxide solution. the heavy lower layer of crude bromobenzene becomes almost colourless at this stage. Run off the bromobenzene, shake it again with water to ensure absence of alkali, and then dry with calcium chloride for 20-30 minutes. Filter through a small fluted filter- paper directly into a 50ml distilling-flask fitted with an air-condenser. Now distill the crude bromobenzene slowly, rejecting the fraction boiling up to 150°C, and collecting that of bp 150-160°C. Yield, 28-29g (about 19ml). A small quantity of crude p-dibromobenzene remains in the flask.
Carefully refractionate the liquid of bp 150-160°C, either by direct distillation from a small distilling-flask or (preferably) by using a short fractionating column. Almost pure bromobenzene is thus obtained, usually of bp 154- 156°C if distilled directly, and of bp 155-156°C if a column is used. Yield in either case, about 22g (14-15 ml). Bromobenzene is a colourless liquid of bp 156°C, and d 1.50: it has a faint agreeable odour. The bromine atom, being directly joined to the benzene ring, is very inert, and the only common reactions in which it is split off from the ring are the Fittig reaction and the Grignard reaction.
The p-dibromobenzene formed as a by-product in the above reaction usually solidifies when the undistilled residue obtained in the first distillation is chilled. It may be isolated by adding about 10ml of methylated spirit and some activated charcoal to the flask, boiling for a few minutes, and filtering hot. On cooling the filtrate in ice-water, crystals of p-dibromobenzene, mp 89°C separate: recrystallise a second time if necessary to obtain colourless crystals.
Reference: Mann & Saunders, Practical Organic Chemistry, 3rd Ed
