Note: Descriptions are shown in the official language in which they were submitted.
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This application is a division of application serial No.
179,044, filed August 17, 1973.
In U.S. Patent ~o. 3,737,469 issued June 5, 1973, there is
described and claimed a process for the simultaneous production of bromo-
styrene and methyl bromide which comprises reacting ~ -bromoethyl
bromobenzene or ~ -bromoethyl bromobenzene ("the substrate") and methanol
in a medium of certain molten inorganic salts.
A known process for the production of bromostyrene comprises
effecting a dehydration of the respective bromophenyl methyl carbinol, or
the respective bromophenyl ethyl alcohol. Another known process is the
dehydrobromination of either of the respective A - or ~ -haloethyl mono-
bromobenzene isomers.
This latter method is exemplified in British Patent No. 986,634.
The dehydrobromination of bromoethyl halobenzenes is effected by passing a
mixture of the reactants together with an excess of steam over granular
calcium sulfate catalyst. The use of calcium sulfate as catalyst is also
mentioned in U.S. Patent No. 2,485,524 in vapor-phase dehydrohalogenations
of substituted halo-benzenes. Other catalysts mentioned in the literature
for dehydrohalogenations are calcium chloride, calcium oxide, calcium
phosphate and various aluminas.
When applied to bromoethyl bromobenzene, all of these catalysts
exhibit the serious drawback of causing the formation and deposition on the
catalyst of tarry substances, most probably comprising a polymer formed from
the monomeric bromostyrene which is the primary product of the catalytic
dehydrohalogenation. The formation of the polymer cannot be avoided, even
~; when a large excess of an inert diluent, such as steam or nitrogen, is fed
together with the substrate into the reaction zone. The deposition of the
polymer on the catalyst reduces rapidly its activity and thus frequent
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catalyst regenerations are required. If the catalyst is discarded after
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relatively brief runs, the recharging with fresh catalyst is a compara-
tively expensive operation.
To eliminate the deposition of the tarry byproducts on the
fixed bed of the catalyst, a process is described in French patent No.
1,576,909, wherein the active catalyst as well as the reaction medium com-
prises certain mixtures of molten salts, and in particular mixtures
containing bivalent metal chlorides, such as copper chloride, in combina-
tion with other salts which decrease the melting point of the salt mixture.
While according to the above disclosure it is possible to
obtain, for example, high yields of vinyl chloride by the elimination of
hydrogen chloride from ethylene dichloride, the said process produces only
poor yields of monomers in the case of heavy, relatively non-volatile
substrates. In particular, when the substrates according to the present
invention are used, the conversion to the respective products is far from
complete.
In U.S. Patent No. 3,737,469 a process was described whereby
molten salts were used very efEectively as reaction medium for the pro-
duction of bromostyrene by passing through this medium the said substrate
together with a reactive diluent, such as an aliphatic alcohol, and in
particular methyl alcohol. The term "reactive diluent" designates a
suitable substance which acts as an acceptor - or scavenger - for the
hydrogen halide which is eliminated from the substrate during the reaction.
It is believed that the fast reaction of the "reactive diluent"
with the eliminated hydrogen halide has a pronounced beneficial effect on
the yield of the desired product for two reasons. It is know that elimina-
tion of hydrogen halide from the substrate is a reversible reaction. If
it is carried out in the presence of an acceptor for the hydrogen halide
that is being eliminated - the equilibrium is shifted in the direction of
the product, and the process can thereby be carried out at temperatures
that are substantially lower than in the absence of the acceptor. This has
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the effect of increasing the selectivity of the reaction with respect
to the desired product.
The enhanced selectivity of the reaction, which is achieved in
the presence of the "reactive diluent" is believed to be a consequence
also of the prompt removal of the hydrogen halide from the system, since
this is known to catalyze the polymerization of vinylic monomers to produce
oligomeric materials, which are highly undesirable.
Another advantage in use of "reactive diluents" is the rational
utilization of the bromi~e.
By reacting the hydrogen bromide, which is eliminated from the
bromoethyl - bromobenzene, with the "reactive diluent", useful and highly
valued by-products are directly obtained instead of hydrobromic acid, which
has a lower "bromine value".
As "reactive diluents" may be employed lower aliphatic alcohols
and the by-products are the respective alkyl halides. When methanol is used
in conjunction with the substrate, methyl bromide is obtained in quantita-
tive yield. Whell ethyl alcohol is used the resulting by-product is ethyl
bromide. These are widely used as soil sterilizors and fumigants, as well
as fire extinguishing agents.
While results according to U.S. Patent No. 3,737,469 are
generally satisfactory - the sald process has certain insufficiencies such
as for instance the need for special materials of construction to be used
for the reactors containing highly corrosive molten salts. Another drawback
of said process is in the high ratio of reactor-space to product produced
in unit time (low space-time yields).
A further drawback is the formation of small quantities of
saturated products such as ethyl bromobenzenes. These products have boiling
points very close to that of bromostyrene, and it is therefore very difficult
to separate them by fractional distillation. Furthermore, these products
are not polymerizable and even act as chain transfer agents, thus decreasing
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the molecular weight of the produced polymers. According to the present
invention a high purity monomer is obtained albeit the conversions are
lower than in the aforementioned patent.
The present invention relates to a process for the production
of either bromostyrene or dibromostyrene, together with an alkyl bromide,
by reacting bromoethyl bromobenzene or bromoethyl-dibromobenzene, respect-
ively, with a lower alkanol at an elevated temperature in the gaseous
phase and in the absence of alkali metal catalyst. The reaction is a
pyrolysis reaction, and is advantageously effected at a temperature in the
range of about 400 to about 550 C.
Bromostyrene is a reactive monomer which can be polymerized to
a hard, transparent plastic which has many uses. It can be copolymerized
with other monomers to result in copolymers which are fire retardant and
self-extinguishing. It is of particular interest as a reactive component
in unsaturated polyester compositions, to result in self-extinguishing
transparent plastic compositions, that do not become discolored upon pro-
longed exposure to weather conditions.
The term "bromostyrene" as used herein means para-bromo meta-
bromo, or ortho-bromo-styrene or a mixture of any of these. It will
henceforth be termed "the product".
Bromostyrene is a well-known monomer, being either a para-meta-,
or ortho-bromo derivative, or a mixture of these.
The advantage of the pyrolytic method over the catalytic method
described in the British Patent No. 986,634 is that in the pyrolytic method
the reaction tube used for pyrolysis can be easily cleaned from any tars
formed during the reaction by simply burning them by passing air through
the hot reactor.
Fuller details of the present invention are described in the-
following examples to which, however, it is not limited.
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Example 1~ Z3
300 g ~-bromoethyl-bromo-benzene were added through a motor
driven syringe into the reactor at a rate of 60 g/h. Methanol was added
at a rate of 14 g/h and nitrogen gas preheated to 300C at a rate of
2 l/h. The reactor was a glass spiral made of a 6 mm wide 3 m long glass
tube connected to a tube 30 mm wide and 25 cm long. The reactor was kept
at a temperature of 490-500 C. The liquid products were collected in a
trap submerged in an ice bath while the methylbromide was trapped in a
receptacleplaced in liquid nitrogen. The liquid products were then sub-
jected to fractional distillation in a vacuum to obtain 166 gr bromostyrene
and 45 gr unconverted ~ -bromo-bromoethyl benzene. The yield is 94%
based on reacted bromoethyl bromo-benzene, 90% yield of methyl bromide was
also obtained.
Example 2:
The experiment was carried out in the manner described in
Example 1, but without methanol. The conversion was only 74% and the yield
was only 44%. ~ substantial quantity of heavy products and tar-like
materials were obtained.
Example 3:
170g ~ -bromoethyl dibromobenzene (being mainly a mixture of
the ortho and para isomers) were driven into the reactor described in
Example 1 at a rate of 20 gr/h together with methanol at a rate of 4.4 grlh
and preheated nitrogen at a rate of 2 l/h. The products were distilled
in vacuum and consisted of 14 gr unconverted product and 108 gr. dibromo-
styrene, namely 91% yield.
