Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
108;~ï~7
MD 29131
This invention relates to a chlorination process,
and more particularly to a process for the preparation
of 2,3,5-trichloropyridine.
It is known that a variety of polychloropyridines
may be obtained by the vapour phase chlorination of
pyridine or substituted pyridines.
In the specification of U.K. Patent No. 1,Oil,906
there is described a process for the manufacture of
substituted pyridines containing one or more chlorine
atoms as substituents in the pyridine ring which
comprises interacting pyridine or a substituted
pyridine with chlorine in the vapour phase at
elevated temperature. The process described therein
is especially applicable to the vapour-phase chlorina-
tion of pyridine itself, in which case high yields
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of pentachloropyridine may be obtained; penta-
chloropyridine is also the product when 2-chloro-
pyridine is the starting material.
In the specification of United States Patent
No 3,420,833 the product of a vapour-phase chlorination
of pyridine is described as comprising 2,4,6-trichloro-
pyridine; 2,3,4,6-tetrachloropyridine; 2,3,6-trichloro-
pyridine and 2,3,5,6-tetrachloropyridine with minor
proportions of pentachloropyridine and 2,6-dichloro-
pyridine.
2,3,5-trichloropyridine does not appear to be -_-
produced in any significant proportion either by.the
chlorination of pyridine itself or by the chlorination
of monochloropyridines.
We have now found that 2,3,5-trichloropyridine
may be produced in good yield provided that the
starting material is a specific dichloropyridine,
namely 3,5-dichloropyridine.
Thus according to the present invention there
20 is provided a process for the production of 2,3,5-
trichloropyridine which comprises interacting 3,5-
dichloropyridine with chlorine in the vapour phase
at an elevated t.emperature.
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The chlorination process is preferably carried
out at a temperature in the range ~rom 300C to
460 C. A temperature in the range from 340 to
400 (for example 380C) is especially preferred.
It is preferred to preheat separately the feed of
chlorine and the feed of 3,5-dichloropyridine.
The reactants are preferably diluted. The diluent
may be inorganic, for example nitrogen and/or steam, or
may be organic.
When an organic diluent is used, this is preferably
a compound which is inert towards chlorine (for example
carbon tetrachloride, which is the diluent especially
preferred) or a compound such that any reaction with
chlorine yields a product which is inert to further
chlorination (for example chloroform, which may yield
carbon tetrachloride.
Whe~ a gaseous or volatile diluent is used the
chloropyridine starting material may be vapourised in
the stream of diluent vapour which serves as a carrier
gas; when a liquid diluent is used, the 3,5-dichloro-
pyridine starting material may be dissolved in the
liquid diluent and the resulting solution may then
be vaporised as a whole.
It is preferred to use at least 1 mole of chlorine
per mole of 3,5-dichloropyridine. It is especially
preferred to use at least 2 moles of chlorine (for
example from 5 to 15 moles of chlorine) per mole of
3,S-dichloropyridine.
When using a diluent which is reactive towards
chlorine, for example chloroform, an appropriate
additional amount of chlorine may be used to allow
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5.
for that consumed by reaction with the diluent.
Convenient residence times of the mixture in
the reaction zone are, for example, between 10
and 30 seconds, but higher or lower residence times
may also be used if desired.
2,3,5-trichloropyridine may be separated from
any other chloropyridines produced by methods
conventional in the art, for example fractional
distillation and fractional crystallisation.
2,3,5-trichloropyridine is useful as an
intermediate for the manufacture of compounds of
the formula (I)
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C}~--O~O~HCI-OR
(I)
wherein R is a hydrogen atom or an alkyl group of 1
to 4 carbon atoms. Such compounds, and their salts,
are useful as selective herbicides for the control of
grass weeds, as set forth for `example in Belgian
Patent 834 495.
2,3,5-trichloropyridine may be converted into a
compound of the formula (I) for example by reaction
with a phenol derivative of formula:
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M ~ -~ H~OR
wherein R2 is an alkyl group of 1 to 4 carbon atoms
and M is an alkali metal, for example sodium or
potassium to give a compound of formula (I) above.
The invention is illustrated by the following
Examples:
EXAMPLE 1
A solution of 3,5-dichloropyridine (lOg) in
chloroform (55 ml) was fed to a packed vaporiser
maintained at a temperature of 300C to 310C.
The issuing vapours were passed to a vertical glass
tubular reactor of 1 inch bore held at a temperature
of 380C where they were mlxed with chlorine fed
at a rate of 0.2 litre/minute (measured at 20C).
The residen:ce time was 22 seconds. The initial
reaction mixture contained 6.6 moles of chlorine per
mole of 3,5-dichloropyridine.
The gaseous reactor effluent was condensed and
collected in cooled carbon tetrachloride. The
resulting carbon tetrachloride solution was distilled
to remove carbon tetrachloride, and the residual solid
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8~197
recrystallised from acetic acid. The product thus
obtained was analysed by gas-liquid chromatography
and nuclear mass spectroscopy. 50% of the 3,5-
dichloro-pyridine fed was converted into 2,3,5-
trichloropyridine.
EXAMPLE 2
The general procedure was similar to that
described in Example 1. The 3,5-dichloropyridine
was dissolved in carbon tetrachloride and the
reactor was a glass tubular reactor of 3.8 inch
bore. The reaction conditions and the products
obtained are summarized in the Table. The
conversions shown are based upon 3,5-dichloropyridine
fed.
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