Bromobenzene [Mann & Saunders, Practical Organic Chemistry, 3rd Ed]

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.

Required: Purified benzene 34ml, pyridine 0.5ml, bromine, 24ml.

The benzene used in this preparation should be reasonably free from
toluene: therefore use a sample of benzene supplied by dealers as
"crystallizable benzene," i.e., one which crystallizes readily when cooled
in ice-water. 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 25-30øC for one hour, occasionally shaking the contents
of the flask: finally raise the temperature of the bath to 65-70øC 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 150øC, and
collecting that of bp 150-160øC. Yield, 28-29g  (about 19ml). A small
quantity of crude p-dibromobenzene remains in the flask.

Carefully refractionate the liquid of bp 150-160øC, 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- 156øC if distilled directly, and of bp 155-156øC if a column is
used. Yield in either case, about 22g (14-15 ml). Bromobenzene is a
colourless liquid of bp 156øC, 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 89øC separate: recrystallise a second time if
necessary to obtain colourless crystals.

