Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
2000-11-16
2004-02-10
Rotman, Alan L. (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
C562S594000
Reexamination Certificate
active
06689907
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a new process for the manufacture of mercaptocarboxylic acids represented by the formula (I):
in which:
R
1
represents hydrogen or
and
R
2
represents hydrogen or methyl.
BACKGROUND OF THE INVENTION
These mercaptocarboxylic acids are useful as intermediates for synthesis into the esters (for example with 2-ethylhexanol, pentaerythritol) used either to obtain tin salts (stabilizers for PVC), or as hardening or modifying agents for epoxide resins (adhesives) and for polyurethanes (optical glasses). Mercaptopropionic acid can also serve to modify acrylic polymers (paint, paper industry).
Various routes are known for producing mercaptocarboxylic acids:
(1) One of these routes consists in reacting halogenated saturated carboxylic acids with alkali metal hydrosulphides and under an H
2
S pressure. This route is widely described in the literature for thioglycolic acid from monochloroacetic acid, and for mercaptopropionic acid from 3-chloropropionic acid (see American patent U.S. Pat. No. 3,927,085). It is known that this reaction is very selective if the operating conditions are well chosen. However, the stoichiometry of the reaction requires the use of 2 mol of alkali metal hydrosulphide per mol of chlorinated carboxylic acid and the consequent formation of 2 mol of inorganic salt to be removed in the aqueous effluent.
(2) Reactions for opening the lactone rings by alkali metal hydrosulphides have also been proposed; thus, the formation of &bgr;-mercaptopropionic acid from &bgr;-propiolactone and NaSH is described in British Patent No. 639 679. It should be indicated here that propiolactone is not commercially available.
(3) The preparation of 3-mercaptopropionic acid can also be carried out from widely available raw materials such as unsaturated organic acids and, in particular, the acrylic derivatives.
The reaction of H
2
S in a basic medium or of alkali metal hydrosulphides makes it possible to obtain as an intermediate, starting with acrylonitrile, mercaptopropionitrile which can be subsequently hydrolyzed to 3-mercaptopropionic acid.
However, very special conditions should be observed in order to avoid the very easy reaction of the mercaptopropionitrile formed with the unreacted acrylonitrile. The thiodipropionitrile sulphide is indeed very easy to form and it is then necessary to subsequently carry out a sulphohydrolysis of the sulphide to mercaptan, thereby complicating the preparation process.
Moreover, in addition to going via toxic and unstable products (mercaptopropionitrile), this process has the disadvantage that 2 mol of salt will be further formed per mol of 3-mercaptopropionic acid.
Another possibility is that consisting of using, as raw material, methyl acrylate, and to form, by reaction with H
2
S, methyl mercaptopropionate, which, by hydrolysis, will lead to 3-mercaptopropionic acid. This route is economically unattractive because it occurs in two stages, with the loss of one mol of methanol.
A direct route for preparing mercaptopropionic acid consists in starting with acrylic acid, which is a cheap raw material. The reaction of acrylic acid in liquid phase, with a large excess of H
2
S in the presence of an organic base as catalyst, is not easy to carry out. The other technique, consisting in reacting acrylic acid with CS
3
Na
2
, makes it possible to enhance the selectivity in relation to mercaptopropionic acid and to carry out the procedure without an H
2
S pressure. Unfortunately, the use of CS
2
and of methanol as cosolvent makes the process cumbersome, especially with regard to the treatment of effluents.
GB-A-670 702 describes, in its example I, the following reaction:
In fact, thiodisuccinic acid
which undergoes sulphohydrolysis to mercaptan because of the basic pH, is first prepared. In other words, generally speaking a sulphide is first formed, which sulphide undergoes sulphohydrolysis to mercaptan according to the reaction scheme:
A, R
1
, R
2
being as defined below.
Applicant has, moreover, experimentally confirmed this reaction scheme. The article in Collection of Czechoslovak Chemical Communications, Vol 21, No. 6, 1956, Prague CS, pages 1651-1653 presents what is described in GB-A-670 702.
GB-A-1 358 019 describes a process for the preparation of &bgr;-mercaptopropionic acid by reaction of acrylic acid with MSH where M=ammonium group or alkali or alkaline-earth metal in the presence of carbon disulphide CS
2
. CS
2
plays the role of coreagent and prevents the sulphide from being formed:
In accordance with the documents which have just been cited, the aim is to obtain a better selectivity in relation to mercaptocarboxylic acids and to avoid the formation of sulphides. In fact, the aim would be to avoid the formation of sulphides by reaction of the mercaptan already formed and the nonconverted acrylic compound:
with
or HOOC—and R
2
=H or CH
3
.
DESCRIPTION OF THE INVENTION
The present invention is essentially centred on the beneficial effect of the solubility of H
2
S in the reaction medium which promotes the formation of mercaptan. In accordance with the present invention, carrying out the procedure with a supply of H
2
S leads directly to the desired mercaptan, the sulphohydrolysis of the sulphide
becoming impossible under these conditions.
According to the invention, hydrosulphides are therefore reacted with unsaturated carboxylic acids, according to the following overall reaction (case where the hydrosulphide is represented by ASH):
A=ammonium or alkali metal cation;
R
1
=H or
and
R
2
=H or CH
3
which is, in fact, the sum of the following two reactions:
It was surprising to observe that, under the abovementioned conditions, the mercaptan formed reacts weakly with the unsaturation of the acid R
1
—CH═CR
2
—COOH, whereas the reaction of formation of the mercaptan is itself highly favoured. This results in very good selectivities in relation to the desired compound of formula (I) which are higher than those of known processes.
The subject of the present invention is therefore a process for the manufacture of a mercaptocarboxylic acid represented by the formula (I) as defined above, according to which an unsaturated carboxylic acid of formula (II):
in which R
1
and R
2
are as defined above, is reacted with a hydrosulphide of formula ASH, A being an alkali metal cation or a cation NR
3
R
4
R
5
R
6+
, R
3
to R
6
each representing H or a hydrocarbon radical, or of formula Q(SH)
2
, Q being an alkaline-earth metal cation, and the resulting reaction medium is acidified in order to obtain the desired compound (I), characterized in that the reaction is carried out with a supply of H
2
S other than that supplied by the neutralization of the acid (II).
The starting acid (II) is chosen especially from acrylic acid, methacrylic acid, maleic acid and fumaric acid.
In general, the reaction according to the invention may be carried out in an aqueous medium, but an enhancement of the selectivity can be obtained using an aqueous-alcoholic or a purely alcoholic medium. This alcoholic medium, or the alcohol fraction of the aqueous-alcoholic medium, is formed, for example, by methanol, propanol, isopropanol or a mixture of these alcohols.
An important parameter which governs the selectivity in relation to mercaptocarboxylic acid (I) is the H
2
S/acid (II) molar ratio in solution. The higher the H
2
S concentration in solution, the better the selectivity in relation to the acid (I). If the procedure is carried out without a supply of H
2
S other than that supplied by the neutralization of the acid (II), this ratio is 1. If H
2
S is supplied to the reaction system, its solubility in the reaction medium depends on several physical factors:
dilution of the medium (true H
2
S/acid (II) molar ratio in solution will be higher);
use of an alcoholic or aqueous-alcoholic medium;
increase in the H
2
S pressure; and
to a lesser degree, reduction in the temperature; as well as on the H
2
S/acid (II) ratio in solution.
To increase this ratio, as is e
Atofina
Oh Taylor V
Rotman Alan L.
Smith , Gambrell & Russell, LLP
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