Note: Descriptions are shown in the official language in which they were submitted.
2 1 93û55
AGW 2445
Process for the manufacture of thioglycolic acid
Akzo Nobel nv, Arnhem
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Description:
The invention relates to a process for the manufacture of
thioglycolic acid from monochloroacetic acid and hydrogen
sulfide.
Thioglycolic acid, also known as mercaptoacetic acid, is a
chemical which has a variety of uses, primarily in the form
of its salts and esters. For instance, thioglycolates are
employed in cold-waving lotions in hair treatment, among
other things they are used in permanent deformation of
woolens; isooctyl thioglycolate is used for the formation
of PVC stabilizers which contain tin, thioglycolic acid es-
ters are also used as antioxidants in the rubber industry.
The manufacture of thioglycolic acid is primarily conducted
by the reaction of monochloroacetic acid or its salts with
hydrogen sulfides such as potassium or sodium hydrogen sul-
fide.
The reaction of the hydrogen sulfide such as sodium or am-
monium hydrogen sulfide with the monochloroacetic acid can
also be conducted by passing hydrogen sulfide into an aque-
ous solution of monochloroacetic acid and sodium or ammo-
nium hydroxide, as described for example in DE-AS 2 354
098.
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The disadvantage of processes of this kind is that not in-
considerable quantities of undesired by-products are pro-
duced and above all that, according to the following reac-
tion equation, sodium chloride or ammonium chloride, re-
spectively, is produced as a waste product which must be
disposed of.
Cl-CH2COONa + NaSH ----> HS-CH2COONa + NaCl (1)
Quite apart from that fact that here, too, not inconsider-
able quantities of by-products are formed, the process de-
scribed in DE-OS 2 711 867, in which this conversion takes
place under a high partial pressure of carbon dioxide, can-
not remedy the disadvantage of the formation of sodium
chloride either.
Processes have also been made known in which chloroacetic
acid is directly converted with hydrogen sulfide. For in-
stance, according to the SU patent specification 740.761
chloroacetic acid is converted with hydrogen sulfide at
temperatures of 520 to 620 C at atmospheric pressure.
Apart from the fact that there are disadvantages involved
in working at such high temperatures, the only 90 % yield
also leaves much to be desired. In addition, our own ex-
periments have shown that a considerable proportion of by-
products which can only be removed with difficulty is
formed, such as thiodiglycolic acid or dithiodiglycolic
acid.
The US patent specification 4 082 790 describes a process
for the manufacture of mercaptans in which an organic chlo-
ride or bromide is converted under autogenic pressure with
a mixture of hydrogen sulfide and ammonia or an amine. A
disadvantage of this process are the frequently very long
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reaction times which last from several hours to the magni-
tude of one day; in addition, working in a closed autoclave
is awkward.
From the list in column 2 of this patent specification it
can be gathered that a large number of substances contain-
ing chlorine or bromine can be converted. Apart from alkyl
chlorides such as methyl, ethyl chloride etc., halogenated
ethers, ketones etc., a series of halogenated carboxylic
acids including chloroacetic acid are also named.
A large number of primary amines with methylamine, buty-
lamine etc., secondary amines such as dimethylamine, dipro-
pylamine etc., and tertiary amines such as trimethylamine,
triethylamine or ethyldimethylamine are recommended aside
from ammonia as the base in the reaction. The manufacture
of thioglycolic acid from chloroacetic acid is not men-
tioned in the examples. What is also missing is any infor-
mation on the conditions under which this particular sub-
stance can advantageously be converted.
Although a whole series of processes by which thioglycolic
acid can be manufactured from chloroacetic acid is now
known, the need remains for an improved, economical process
for the manufacture of thioglycolic acid which takes the
interests of the environment into account.
The object of the invention is therefore to make available
a process which works with high selectivities, i.e. a proc-
ess in which no, or only small quantities of, by-products
are formed and in which no inorganic waste products such as
sodium chloride or potassium chloride are formed.
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A further object of the invention is to make available a
process for which short reaction times suffice and which is
particularly suitable for continuous execution.
This task is solved by a process for the manufacture of
thioglycolic acid by the conversion of monochloroacetic
acid with hydrogen sulfide under pressure in the presence
of tertiary amines in solution, whereby care is taken that
the partial pressure of hydrogen sulfide over the reaction
medium is at least 2 bar up to the end of the reaction, and
the thioglycolic acid formed as an ammonium salt is re-
leased by means of acid.
The partial pressure of hydrogen sulfide can be maintained
during the conversion in advantageous manner by linking the
reaction chamber with a source of hydrogen sulfide under
pressure.
The partial pressure of hydrogen sulfide is preferably over
2 bar and it is an advantage if it is between 10 and 20
bar.
The reaction can be conducted advantageously at room tem-
perature or slightly raised temperatures, i.e. at tempera-
tures between 15 and 40 C.
In a further especially favorable embodiment of the process
in accordance with the invention the conversion is con-
ducted from beginning to end under a constant partial pres-
sure of hydrogen sulfide of at least 2 bar.
In a further particularly favorable embodiment of the proc-
ess in accordance with the invention trimethylamine is em-
ployed as the tertiary alkylamine.
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It is beneficial for the conversion to be conducted in the
present of about 2 to 2.5 mol tertiary amine per mol
monochloroacetic acid. Most suitable for the conversion is
monochloroacetic acid dissolved in water. It is an advan-
tage to conduct the conversion in homogeneous aqueous solu-
tion, i.e. such that both the starting substances and the
substances formed remain dissolved in water. In this case
the conversion is beneficially conducted at 10 to 60 C,
particularly between 30 and 40C.
The conversion can also be conducted in an organic solvent
or in a mixture of organic solvents. Methyl t-butyl ether
and diisobutyl ketone may be named as examples of suitable
solvents.
To release the thioglycolic acid, the use of hydrochloric
acid is preferred.
The manufacture of thioglycolic acid can be performed in
the following manner. Monochloroacetic acid is dissolved in
water, and in addition, in a separate step, a solution of
trimethylamine and water is first saturated with hydrogen
sulfide and the desired partial pressure of hydrogen sul-
fide is set; the concentration of water can vary here over
a wide range. It is preferable to have a 15 to 60 % concen-
tration of water based on the total reaction mixture. Par-
ticularly beneficial are concentrations of water which al-
low the reaction to be conducted from beginning to end in a
homogeneous phase, i.e. in solution, that means a concen-
tration at which both the starting substances and the end
product formed are soluble. When trimethylamine is used,
for example, this is in the range of about 40 to 50 % by
weight of water.
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It is obvious that the concentration of water is also de-
pendent on the tertiary amine used and the reaction tem-
perature.
Therefore, by saturation under pressure a partial pressure
of hydrogen sulfide can be set which is so high that it is
at least 2 bar throughout the reaction time, without having
to pass additional fresh hydrogen sulfide into the reaction
chamber during the conversion.
The monochloroacetic acid dissolved in water is then me-
tered into the trimethylamine solution which has been satu-
rated with hydrogen sulfide and is under the desired par-
tial pressure of hydrogen sulfide. Naturally it is also
possible, however, to meter the aqueous trimethylamine so-
lution into a receiver of monochloroacetic acid under hy-
drogen sulfide.
In principle any tertiary amine can be used, whereby short-
chain tertiary alkylamines such as triethylamine are pre-
ferred.
Most particularly advantageous is the use of trimethy-
lamine, particularly on the grounds that the trimethylamine
hydrochloride formed in the conversion is a valuable sal-
able product.
The reaction temperature, too, can be varied over a wide
range and is preferably in the range of about 10 to 60 C,
although naturally, higher or lower temperatures are also
possible. The temperature range of 30 to 40 C is espe-
cially favorable.
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The ratio of trimethylamine to monochloroacetic acid ought
generally to be at least 1.5 : 1 mol.
However, it is preferable to employ quantities of reactants
according to the following reaction equation which are
stoichiometric or slightly hyperstoichiometric, i.e. the
ratio of tertiary amine to monochloroacetic acid is 2 : 1
to 2.5 : 1, and preferably 2.1 to 2.2 : 1.
~3
Cl-CH2-COOH+2(CH3)3N+H2S->HS-CH2-COO (CH3)3HN+(CH3)3N.HCl (1)
A further beneficial embodiment consists of making sure
that during the conversion the reaction vessel or the reac-
tion chamber respectively is linked to a source of hydrogen
sulfide at above atmospheric pressure. This can be effected
for example by linking a reaction vessel via a feed line
with a reservoir containing hydrogen sulfide at above at-
mospheric pressure. The hydrogen sulfide at above atmos-
pheric pressure can be fed in above the reaction medium,
especially above the reaction liquid, but it is also possi-
ble to feed in the hydrogen sulfide via a tube or a suit-
able feed line using a jet.
In an especially advantageous embodiment the above atmos-
pheric partial pressure of hydrogen sulfide is kept con-
stant throughout the reaction time.
Separation of the reaction ~roducts:
According to the above reaction equation the desired prod-
uct is present in the form of its trimethyl ammonium salt.
In order to-obtain the thioglycolic acid in the form of
free acid, after the reaction vessel has relaxed a corre-
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sponding quantity of hydrochloric acid is added to the re-
action mixture, causing the formation of free thioglycolic
acid and a second mol of trimethylamine hydrochloride. The
thioglycolic acid can be separated without difficulty by
extraction from the aqueous solution. Suitable extracting
agents are ethers, ketones, especially methyl tert-butyl
ether MTBE, t-Amyl methyl ether TAME, diisobutyl ketone. In
the next separation step the extracting agent is distilled
off and can thus be re-used for extraction. The final proc-
essing step consists of the purification of the thiogly-
colic acid by distillation.
The following reaction equation expresses the release of
the thioglycolic acid salt formed according to (1).
HS-CH2-COO (CH3)3HN+HCl->HS-CH2-COOH+(CH3)3N HCl (2)
It was especially surprising to find that in the process in
accordance with the invention relatively few by-products
are formed, even if higher concentrations of the starting
substances are employed. This was not expected, since in
the known reactions in which a salt of the monochloroacetic
acid is converted with a hydrogen sulfide such as sodium
hydrogen sulfide or potassium hydrogen sulfide, with in-
creased concentrations the proportion of by-products such
as thiodiglycolic and dithiodiglycolic acid also increases
sharply. In the process in accordance with the invention,
on the other hand, the selectivity remains very high even
in concentrated solution.
It is also not necessary to work with high hydrogen sulfide
pressures.-~ery high selectivities can also be achieved
with low H2S pressures e.g. 5 bar.
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Especially when trimethylamine is used the process in ac-
cordance with the invention is free of waste. I.e. thiogly-
colic acid and trimethylamine hydrochloride are produced,
both of which are valuable products, which can be used di-
rectly. The process is thus free of waste, as is seen in
the above reaction equation.
Even when other tertiary amines are used, working up is
easily possible without inorganic waste products being pro-
duced as is the case in conventional processes, e.g. when
trioctylamine is used the HCl salt formed can be broken
down thermally into HCl gas and amine.
The process enables thioglycolic acid of high purity to be
manufactured, so that the product per se can be directly
employed or can be transformed directly into the desired
secondary products such as salts or esters etc.
The process in accordance with the invention is especially
suitable for continuous execution.
Because of the high reaction rate this process is also ex-
cellently well suited to continuous process execution in a
cascade of stirred tanks. The efficient reaction system
means that only a few stirred tanks are required to bring
about the complete conversion of the monochloroacetic acid
to thioglycolic acid.
Steel autoclaves are employed which are equipped with an
effective agitator and a pressure control or pressure regu-
lator mechanism. The agitated autoclaves are arranged such
that first the complete conversion of monochloroacetic acid
to the trimethyl ammonium salt of the thioglycolic acid and
then the release of the thioglycolic acid with hydrochloric
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1()
acid takes place. Separation of the products also takes
place continuously, as described above.
The invention will be explained in more detail using the
following examples:
Example 1
216.7 g aqueous trimethylamine solution is placed in a 1
liter Buchi steel autoclave, whereby the content of tri-
methylamine is 45 % (1.65 mol trimethylamine). This solu-
tion is then saturated with hydrogen sulfide until the de-
sired pressure of 15 bar has been reached. The temperature
is set to 40 C with the aid of a thermostat and the re-
ceiver mixed through with a gas dispersion agitator at 700
r.p.m. The reaction begins when the 70.9 g monochloroacetic
acid (0.75 mol) dissolved in 100 g water is metered into
the reaction vessel within a minute with the aid of a pis-
ton pump. During~the reaction the pressure and temperature
are regulated. After 10 minutes the monochloroacetic acid
has been completely converted and a 96.0 % yield of thio-
glycolic acid has been achieved.
Example 2
Execution as in Example 1, but at 20 C. After 30 minutes
the monochloroacetic acid has been completely converted and
the yield of thioglycolic acid is 97.5 %.
Example 3
Execution as in Example 1, but with 207.8 g aqueous trieth-
ylamine solution (152.0 g triethylamine + 55.8 g water).
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After 30 minutes the monochloroacetic acid has been com-
pletely converted and the yield of thioglycolic acid is
94.5 %.
.
Example 4
Execution as in Example 1, but at 5 bar hydrogen sulfide.
After 30 minutes the monochloroacetic acid has been com-
pletely converted and the yield of thioglycolic acid is
94.0 %.
Example S
Execution as in Example 1, but with only 1.05 mol trimethy-
lamine (mol ratio of trimethylamine to monochloroacetic
acid = 1.5 : 1). After 120 minutes the monochloroacetic
acid has been completely converted with an 80% yield of
thioglycolic acid.
Example 6
197 g aqueous trimethylamine solution is placed in a 1 li-
ter Buchi steel autoclave, whereby the content of trimethy-
lamine is 45 %. This solution is then saturated with hydro-
gen sulfide until the desired pressure of 15 bar has been
reached. The autoclave is then separated from the hydrogen
sulfide supply. The temperature is set to 30 C with the
aid of a thermostat and the receiver mixed through with a
gas dispersion agitator at 700 r.p.m.
The reaction begins when the 70.9 g monochloroacetic acid
(0.75 mol) dissolved in 100 g water is metered into the re-
action vessel within a minute with the aid of a piston
-
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pump. By the metering of the monochloroacetic acid part of
the hydrogen sulfide bound in the solution is released,
by means of which the pressure in the reaction chamber in-
creases to 25 bar. At the end of the reaction the partial
pressure of hydrogen sulfide is 23 bar.
After 30 minutes the monochloroacetic acid has been com-
pletely converted and a 96.7 % yield of thioglycolic acid
has been achieved.
Example 7
192 g methyl tert-butyl ether MTBE and 39 g anhydrous
trimethylamine are placed in a 1 liter Buchi steel auto-
clave. This solution is then saturated with hydrogen sul-
fide until the desired pressure of 10 bar has been reached.
The temperature is set to 20 C with the aid of a thermo-
stat and the receiver mixed through with a gas dispersion
agitator at 700 r.p.m.
The reaction begins when the 28.4 g monochloroacetic acid
dissolved in 60 g methyl tert-butyl ether MTBE is metered
into the reaction vessel within a minute with the aid of a
piston pump. After 20 minutes the monochloroacetic acid has
been completely converted and a 92.0 % yield of thiogly-
colic acid has been achieved.
Example 8
Execution as in Example 7, but in place of MTBE diisobutyl
ketone DIBK is used as the solvent. After 25 minutes the
monochloroacetic acid has been completely converted, the
yield of thioglycolic acid is 91 %.
