The hydrodimerization of acrylonitrile

Nylon 66 (Fig. 6.1) is a polymer manufactured by the condensation of adipic acid with hexamethylenediamine and in the USA alone in excess of 10 ton yr are 

The electrochemical process for adiponitrile involves the cathodic hydrodimerization of acrylonitrile 

This electrolysis is the second step in the conversion of propylene, available from oil, to adiponitrile the first is the oxy-amidation of propylene which is carried out catalytically. Hence the complete process sequence is 

Other chemical routes to adiponitrile are available, namely OH 

The hydrodimerization of acrylonitrile is only one of many possible chemical reactions in the catholyte and, indeed, several have been observed during electrolysis. The first step in the hydrodimerization is the reduction of acrylonitrile to its anion radical, and to form adiponitrile dimerization and protonation must 

Nylon 66 is a polymer manufactured by the condensation of adipic acid with hexamethylcnediamine, and in excess of I million ton year are manufactured in the USA atone. Adiponitrile is a convenient intermediate for the production of both hexamethylenediamine and adipic acid, the former by hydrogenation and the Latter by hydrolysis. In addition, acrylonitrile is available from the gas phase, catalytic oxy-amination of propylene. Hence, to complete the manufacture of nylon from propylene and ammonia, a procedure for the conversion of acrylonitrile to adiponitrile is required (Fig. 6.1). 

The electrochemical route for the conversion of acrylonitrile to adiponitrile is a cathodic hydrodimerizationr 

The programme at Monsanto was commenced in 1959 by following up an observation by a Russian group that the reaction of acrylonitrile with potassium 

Fig 6 1 A route for the manufacture of nylon from basic feedstocks. In practice there are probably cheaper routes to adipic acid and all the adiponitrile ts now converted to hexamethylen iamine. 

On the other hand when the electrolyte was a neutral solution of a tctraalkyl- ammonium salt adiponitrile was indeed, formed (it was later shown that such ions also greatly increased the yield from the amalgam reduction). The formation of adiponitrile also depended on (he use of a high conceniralion of acrylonitrile and a cathode with a high hydrogen overpotentiaL In some electrolyses the yield of adiponitrile then exceeded 90%  

It was soon clear, however, that the conditions had to be controlled carefully. On the addition of acid, propionitrile again became the major product while the presence of base led to by-products by the route  

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Perhaps the best known example of adsorption effects in electrosynthetic reactions is the beneficial role of tetraalkylammonium ions in the hydrodimerization of acrylonitrile... 

Reactions of this type are called electrochemical hydrodimerization. They are of great value for the synthesis of various bifunctional compounds. A reaction that has found wide commercial nse is the hydrodimerization of acrylonitrile to adiponitrile (the dinitrile of adipic acid) ... 

Rate data have appeared (161) for the hydrodimerization of acrylonitrile to adiponitrile, which is catalyzed by various ruthenium-phosphine complexes (/, p. 101). 

Scheme 4. Possible reaction pathways for the hydrodimerization of acrylonitrile to adiponitrile. The asterisk indicates that electron transfer can be from the cathode or from [CH2CHCN] in homogeneous solution...

Adiponitrile may be produced from the hydrodimerization of acrylonitrile or from 1,3-butadiene via l,4-dicyanobutene-2. Adiponitrile is then hydrogenated forming 1,6-hexane-diamine. 

Cathodic hydrodimerization has been applied to a large number of electron-poor olefins. The most prominent example is the hydrodimerization of acrylonitrile to give adipodinitrile. This so-called Monsanto process has a production capacity of about 300,000 tons per year worldwide [20] ... 

The industrial organic electrosynthesis reaction of greatest impact must be the Monsanto process for the hydrodimerization of acrylonitrile to... 

For cells with continuous addition of reducible material and removal of product, the design depends on the techniques for the addition and removal. A cell designed for the hydrodimerization of acrylonitrile to adiponitrile is shown in Fig. 7.58... 

The hydrogen overvoltage of cadmium is rather large and cadmium may be used, plated on steel, as an alternative to lead. Cadmium has been used as cathode material in the hydrodimerization of acrylonitrile (Chapter 31). Magnesium may be used in reductive dimerization of aliphatic esters [459]. 

Baizer and coworkers established the most brilliant industrial electroorganic synthesis of the hydrodimerization of acrylonitrile to adiponitrile. They extended this hydrodimerization to a variety of activated olefins and in some cases [41 3] paid attention to the stereochemistry of products. However, their stereochemical data were not enough to discuss the stereochemical course of the reaction. Afterward, an attempt was made to provide a working hypothesis in the hydrodimerization of cinnamates by considering an orientated adsorption of radical anion intermediates on a cathode surface, but this was not persuasive because of a lack of experimental data on the stereochemistry of both the starting olefins and products. Recently Utley and coworkers [44-46] have reported stereochemical data of hydrodimers derived from a variety of cinnamic acid esters with chiral alcohol components. 

An oxidation using a nickel hydroxide electrode is shown in 15.8.433 Electrochemistry is also a way to produce radicals and anions. The hydrodimerization of acrylonitrile to adiponitrile just mentioned may involve the coupling of free radicals. The coupling of carbonyl compounds, such as p lolualdehyde, to form pinacols with up to 100% selectivity, by way of free radicals, can be done electrically.434 Anions can also be formed electrochemically and used in situ, as in example (15.9).435... 

Conjugated Alkenes. - By far the largest and best known industrial electro-organic reduction is the hydrodimerization of acrylonitrile to adiponitrile, an important precursor in nylon manufacture. Plants where this process... 

The complete study of the mechanism of the hydrodimerization of acrylonitrile is beyond the capability of the instrumental methods presently available. This is for three principal reasons (i) The concentration of electroactive species generally... 

While the hydrodimerization of acrylonitrile to adiponitrile is the only one to have been exploited commercially, it should be recognized that, using the conditions described above, hydrodimerization is a general reaction for activated olefins, i.e. olefins with an electron-withdrawing group on the double bond. The reaction... 

While the hydrodimerization of acrylonitrile to adiponitrile has been the most successful commercial organic process, a few other organic compounds are known to be prepared by electrolysis on an industrial scale. Many more possible processes have been examined on a pilot-plant scale and some of these may also have been extended to commercial operation. In addition, the literature describing laboratory-scale organic electrosynthesis is now very extensive. 

Another important process connected with propylene derivatives is the hydrodimerization of acrylonitrile, which has been developed commercially by Monsanto, up to a total installed capacity exceeding 2000.000 t y with plants located in the US and in Europe. This process easily withstands the competition with the concurrent chemical processes by using cheaper raw materials presenting minor problems of pollution and affording higher operational safety. Adiponitrile is hydrogenated to hexamethyl-enediamine, which is used in the manufacture of polyamides, and, to a lesser extent, of polyurethanes. 

The history of the Monsanto adiponitrile process demonstrates well the development of a synthetic process[1,23-25]. The first electrolytic process for the hydrodimerization of acrylonitrile was introduced on a 14,000 tons/year scale in 1965. The late 1970 s saw a large expansion of this process both in the UK and USA and it now produces some 200,000 tons adiponitrile/year. At least the recent cell houses are based on quite different technology to that used in the first plant and this has allowed large reductions in both capital investment and energy consumption. 

The reactions occurring at the cathode in the hydrodimerization of acrylonitrile to adiponitrile have been investigated in conditions close to those applied in the commercial electrolysis process [108]. 

Watson, M., Pletcher, D. and Sopher, D.W. (2000) A microelectrode study of competing electrode reactions in the commercial process for the hydrodimerization of acrylonitrile to adiponitrile. Journal of the Electrochemical Society, 147, 3751-3758. 

The mechanism of hydrodimerization is, however, likely to vary with both the electrolysis conditions and the structure of the olefin. Hence it may be inapprppnate to seek similarities in mechanism particularly between acrylonitrile and the more substituted molecules which give stable anion radicals. The high selectivity of the hydrodimerization of acrylonitrile would indicate that the desired reaction route is very favourable compared to all competitive pathways. 

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