Organic compounds -- part of the class 532-570 series – Organic compounds – Nitriles
Reexamination Certificate
2000-08-31
2001-10-02
McKane, Joseph K. (Department: 1626)
Organic compounds -- part of the class 532-570 series
Organic compounds
Nitriles
Reexamination Certificate
active
06297393
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an improved process for the preparation of malononitrile from cyanoacetamide. This invention particularly relates to an eco-friendly process for the synthesis of malononitrile from cyanoacetamide by using POC1
3
as the dehydrating agent, pyridine or an organic amine as catalyst and dispensing the use of solid absorbent, which replaces inorganic salt, to reduce polymerisation of meta-phosphoric acid by-product.
BACKGROUND OF THE INVENTION
Malononitrile is a versatile intermediate used extensively in synthetic organic chemistry. In industry, it is largely used as building block for a variety of pharmaceuticals and pesticides, such as thiamine (vitamin B
1
), adenine, minoxidil (anti-hypertensive, Upjohn), thiopurinol (gout remedy), diuretic triamterene (Smith Kline), aminopeterin, bensulfuron-methyl (herbicide, Du Pont), etc. A variety of important methine dyes, in particular, aminoaryl cyanine based polyester dyes, characterized by high light fastness, are derived from malononitrile. Several important dyes belonging to this class are produced by major dye manufacturers such as Bayer, BASF, Ciba-Geigy, and Kodak. Malononitrile is also used in making tetracyanoquinodimethane (TCNQ), which forms with tetrathiafulvalene charge-transfer complex suitable for the production of conducting films in photocopiers and three-dimensional memories. On the other hand, o-chlorobenzylidene malononitrile (CS gas) is well-known tear gas with high safety factor.
Extensive literature search reveals that the preparation of malononitrile has largely relied upon dehydration of cyanoacetamide in a batch process utilizing a variety of dehydrating agents. It is also obtained in a continuous process involving gas phase reaction of cyanogen chloride and acetonitrile over a suitable catalyst at high temperature in a tube reactor.
Reference may be made to patents, such as U.S. Pat. No. 4,136,108, 1979; Neth. Appl. 80,04516, 1980; Swiss Appl. 68/6944, 1968; Ger. Often 3,006,492, 1981 wherein high conversion is attained in the transformation of acetonitrile to malonotirile. The drawbacks in the above described processes are the requirement of high reaction temperatures and low selectivity due to formation of maleic, succinic and fumeric acid by-products, which require efficient separation to obtain malononitrile in 60% yield (based on CNC1 or MeCN).
Reference may be made to the patent U.S. Pat. No. 5,959,136, 1999 wherein malononitrile is made from an isonitrile, optionally in the presence of a nitrile by using a similar procedure. The drawback in the above process is the requirement of high temperature for the activation of isonitrile, which isomerizes to nitrile, the reactive ingredient in the previous process.
Reference may be made to publication by E. M. Gal and A. T. Shulgin J. Amer. Chem. Soc. 73, 2938, 1951 wherein P
2
O
5
is employed as dehydrating agent. The drawback in the above described process is that the dehydrating agent is less effective in terms of selectivity and yield as compared to POC1
3
.
Reference may be made to publications by B. B. Corson, R. W. Scott and C. E. Vose Org. Synth. 10, 66, 1930; A. J. Fatidadi Synthesis 165, 1978 and patents, such as U.S. Pat No. 2,802,857, 1957; Brit. 1,163,397, 1969, wherein PC1
5
is employed as dehydrating agents. The drawback in the above described process is that the dehydrating agent is less effective in terms of selectivity and yield as compared to POC1
3
.
Reference may be made to publications by Surrey, Org. Synth. 25, 63, 1945; J. Amer. Chem. Soc 65, 2471, 1943 wherein the preparation of malononitrile by elimination of water from cyanoacetamide, is extensively studied by employing POC1
3
as dehydrating agent.
In POC1
3
based procedures, a suspension of cyanoacetamide in dichloroethane is refluxed in the presence of POC1
3
. Nevertheless, this dehydration protocol possesses a severe process limitation when applied to large-scale synthesis due to formation of metastable phosphoric acid as side product, which, under the reaction conditions, tends to form hard polymeric crust along the reactor wall inhibiting uniform stirring and heating.
Moreover, a considerable portion of suspended starting material remains embedded within the coating causing substantial lowering of the yield. In order to avoid polymerisation of meta-phosphoric acid, inorganic salts are employed to convert it into its salt, which is precipitated from the reaction mixture. Several processes, employing alkali metal salts and alkaline earth metal salts, are disclosed.
Reference may be made to patents, for example, U.S. Pat. No. 2,389,217, 1945 and U.S. Pat. No. 2,799,697, 1957 wherein alkali metal salts are used for the purpose. The drawback in the above described process is that they involve consumption of a large quantity of inorganic salt, which is required in stoichiometric amount and can not be easily recovered due to the presence of tarry polymeric products. Thus, in the patent U.S. Pat. No. 2,389,217, 1945 1.00 Kg NaC1 per 1.26 Kg cyanoacetamide is recommended and no salt recovery is prescribed.
Reference may be made to publication by R. Malinowski and J. Legocki Organika 53, 1977 and U.S. Pat. No. 3,459,783, 1969 wherein alkaline earth metal salts are used. The drawback in the above described processes is that they involve consumption of a large quantity of inorganic salt, which is required in stoichiometric amount and can not be easily recovered due to the presence of tarry polymeric products.
However, in synthesizing malononitrile by utilizing above procedure, only a marginal improvement is noted since the formation of the polymeric product cannot be completely prevented. Therefore, notwithstanding the foregoing, the art has not heretofore taught or suggested a methodology that ensured process compatibility as well as offered environmentally clean technology.
OBJECT OF THE INVENTION
The principal objective of the present invention is to provide an alternate, efficient technology for the synthesis of malononitrile from the reaction of cyanoacetamide and POC1
3
in a batch process, which obviates the drawbacks as detailed above.
Another objective of the present invention is to disclose a strategy in which a smooth process operation is secured by replacing inorganic salt with porous solid material to eliminate formation of polymeric by-product in the reaction medium.
Still another objective of the present invention is to develop an eco-friendly process by avoiding the use of large quantities of inorganic salts, which require proper disposal.
Still another objective of the present invention is to provide an improved method for the isolation of pure malononitrile from crude product by recommending the use of appropriate stabilizer during vacuum distillation.
Still another objective of the present invention is to provide a method for storage of isolated malononitrile by prescribing the use of a suitable stabilizer so that a prolonged shelf-life is achieved.
SUMMARY OF THE INVENTION
Accordingly, the present invention describes an improved process for the preparation of malononitrile characterised in using a porous particulate solid substance as efficient absorbent, wherein the said process comprises reacting cyanoacetamide with POCl
3
as a dehydrating agent in a suitable solvent in the presence of an organic N-donor base as catalyst and a porous particulate solid substance as efficient absorbent at a temperature in the range 80-120° C. for 6-8 hours and distilling the product under vacuum from the crude in the presence of a stabilizer into a receiver containing the same stabilizer, to obtain the desired product of high purity and shelf-life.
In one embodiment of the present invention, the solid absorbent used may be silica gel of 60-120 mesh size employed in 38-70 parts per 250 parts of cyanoacetamide.
In another embodiment of the present invention the N-donor organic base catalyst is preferably selected from pyridine or sec- or tert-amine present in an optimal concentration ranging between 2.3-3.0 mol% with respect to t
Aziz Khathija
Bandyopadhyay Ananda Kumar
Choudary Boyapati Manoranjan
Likhar Pravin Raybaji
Council of Scientific and Industrial Research
Ladas & Parry
McKane Joseph K.
Murray Joseph
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