Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1069135
This invention relates to a process for the preparation of
3-bromobenzaldehyde by the bromination of benzaldehyde.
It is known from J.Org.Chem. 1958, 23, 1412-9 that bromine reacts -
with benzaldehyde at a molar ratio of bromine to benzaldehyde of 1.1:1, in
the presence of aluminium chloride and in the absence of a solvent, to give
3-bromobenzaldehyde in 52% yield. When a solvent is used, as described in
"Organic Preparations and Procedures Int." 1974, 6, 251-3, the yield of
3-bromobenzaldehyde is 75%. In these known procedures one mol of bromine
is consumed per mol of benzaldehyde and one mol of hydrogen bromide is
evolved per mol of 3-bromobenzaldehyde.
The process of the invention consumes less bromine per mol of
benzaldehyde and gives 3-bromobenzaldehyde in higher yield compared with the
known procedures.
Accordingly the present invention provides a process for the
preparation of 3-bromobenzaldehyde which comprises brominating benzaldehyde
at a temperature in the range of 0C to 50C in the presence of a chlorinated
hydrocarbon solvent and a Lewis acid bromination catalyst, the molar ratio
of bromination catalyst to benzaldehyde being at least 1:1, wherein the
bromination is carried out using a mixture of chlorine and bromine wherein
the molar ratio of chlorine to bromine is from 0.9:1 to 1:1.
The chlorine and bromine may be mixed before they are brought into
contact with the benzaldehyde, bromination catalyst and solvent. For example,
the two halogens may be mixed in the chlorinated hydrocarbon solvent, and the
resulting solution added to a mixture of benzaldehyde, bromination catalyst
and solvent. The mixture of chlorine and bromine may also be formed in-situ.
Thus, chlorine and bromine may be introduced simultaneously but separately
into a mixture of benzaldehyde, bromination catalyst and solvent. Alternative-
ly chlorine may be introduced, for example, over a period of 1 to 5 hours,
into a mixture of benzaldehyde, bromination catalyst, bromine and solvent.
The bromination catalyst is preerably a Lewis acid such as
-2-
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\ 1069~35
aluminium chloride or ferric chloride. The molar ratio of bromination
catalyst to benzaldehyde is preferably from 1:1 to 1.5:1, in particular from
1.2:1 to 1.35:1.
The molar ratio of bromine to benzaldehyde is preferably from
0.5:1 to 0.6:1, i.e. the stoichiometric amount or a small excess of bromine
is preferably used. The molar ratio of chlorine to bromine used is as
indicated above from 0.90:1 to 1:1, i.e. approximately equimolar amounts
of each halogen are used.
The chlorinated hydrocarbon solvent is preferably a chlorinated
(cyclo)alkane of up to 6 carbon atoms such as 1,2-dichloroethane, dichlorome-
thane, chloroform, carbon tetrachloride or chlorocyclohexene. 1,2-Dichloroe-
thane is particularly preferred. The starting concentration of benzaldehyde
in the solvent is preferably from 2 to 7 mol per litre, in particular 2 to 6
mol per litre.
The temperature of the bromination is preferably from 10 to 25C.
3-Bromobenzaldehyde is an intermediate for the preparation of
3-phenoxybenzyl alcohol derivatives which form insecticidally active esters
with certain carboxylic acids.
The invention is illustrated further in the following Examples.
EXAMPLE I
Chlorine (0.5 mol) was added to a solution of bromine (0.5 mol)
in 1,2-dichloroethane (20 ml) cooled in a solid C02/isopropanol bath and the
resulting solution was made up to 100 ml with more 1,2-dichloroethane. This
solution was added over a period of 2 hours 20 minutes to a mixture of
aluminium chloride (1.3 mol) and benzaldehyde (1.0 mol) in 1,2-dichloroethane
~200 ml) maintained at 23 to 25C. The mixture was then stirred for a
further one hour at this temperature. At this stage the conversion of benzalde-
hyde was 89% and the selectivity to 3-bromobenzaldehyde was 95 to 96% as shown
by GLC.
3~ The re~ction mixture was diluted with sufficient water to redissolve
the initial precipitate of aluminium hydroxide. The organic layer was then
separated, washed with water ~100 ml) and dried (MgS04). The solvent
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was then removed and the residue was fractionally
distilled to give 3-bromobenzalde}lyde, b.p. 80C
at 1.5 mm Hg.
EXAMPLE II
Benzaldehyde (1.3 mol) was added over a period
of 0.75 hour to a mixture of alum:inium chloride (1.43
mol) and 1,2-dichloroethane (500 ml) at 22C. Liquid
bromine (0.65 mol) was then added to the mixture at
15C. Gaseous chlorine was then passed into the mixture
at 15C over a period of two hours to give a chlorine
to bromine molar ratio of 0.95:1. The mixture was
then stirred for a further one hour at 15C. A solution
; of formic acid (0.0355 mol) in water (500 ml) was
added to the mixture to destroy excess bromine. The
organic phase was then washed first with water (500 ml)
containing sodium thiosulphate to promote phase separation,
and then with water (500 ml). The washings were combined
and extracted with 1,2-dichloroethane ~250 ml). The
two organic phases were then combined and analysed
by GLC. The results are shown in the table.
EXAMPLES III to IX
These Examples were carried out in a similar manner
to that o~ Example II. The reaction conditions and
the results are shown in the Table.
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