Acrylonitrile, electrochemical reduction

Electrochemical reduction of oxazolinium salts 36 gives the anions 37, which add efficiently to alkyl halides or, in the presence of McsSiCl, to methyl acrylate, methyl vinyl ketone, and acrylonitrile. Simple acid hydrolysis then gives the ketone products 38 and 39, and this method is quite general since the starting salts are readily prepared from carboxylic acids, R C02H (87TL4411). 

Alkenes may be activated toward electrochemical reduction by electron-withdrawing sucstituents. Thus acrylonitrile and acrylate esters are easily reduced and, depending among other factors on the proton availability of the medium, they undergo either hydrogenation or hydrodimerisation. The basic character of the radical-anions of such substrates has been put to use in EGB promoted Michael additions of the type outlined in Scheme 15 the case where the probase is azobenzene has already been discussed. 

The electrohydrodimerisation of acrylonitrile to give adiponitrile (a one-electron process at high substrate concentrations, Scheme 1.8A and Chapter 6) is an example of how an industrially important electrosynthetic process has been investigated following recent instrumental developments, viz. the application of ultramicroelectrodes at low-voltage sweep rates. Use of conventional electrodes would have required substrate concentrations in the mM range but, under these conditions, acrylonitrile undergoes a different reaction - a two-electron electrochemical reduction of the alkene residue (Scheme 1.8B). The switchover between the two reactions occurs at about 1 mol dm-3 substrate concentration. 

Scheme 1.8 Different electrochemical reductions of acrylonitrile according to reaction conditions (A, high concentration B, low concentration).

Activated olefins such as acrylonitrile and methyl vinyl ketone are also effective as precursors of organometallic compounds. The electrochemical reduction of activated olefins using Sn [43,44], Bi [45], Se [46], Te [46], and Hg [47] cathodes gives the corresponding hydrometallated compounds as shown in Eq. (7). 

Ever since Monsanto commercialized their adiponitrile process in 1965 by electrolytic reductive coupling of acrylonitrile, a number of attempts at the commercialization of electrochemical reductions have been made. None of these attempts has succeeded in producing the tonnages involved in the adiponitrile process of approximately 100 000 tonne p.a. on plants both in the U.K and the U.S.A. However a number of significant tonnage production operations have been built and are outlined below. 

The electrochemical reduction of acrylonitrile was the first example of the formation of a covalent bond from vinylic compounds leading to the formation of thin polymeric layers on metallic cathode surfaces. The formation of a chemical... 

Cyanoethyl derivatives of tin, sulfur, selenium, and tellurium have been observed by electrolysis of acrylonitrile. Lead, mercury, and thallium do not enter into this reaction, but their cyanoethyl derivatives are obtained in satisfactory 3nleld by electrochemical reduction of iodopropionitrile. If it is supposed that formation... 

TABLE 5. Effect of Acrylonitrile on Electrochemical Reduction of Tin, Sulfur, Selenium, and Tellurium... 

In spite of the fact that the above mechanism for formation of organometallic compounds during electrolysis of nitriles appears most probable to us, it is impossible not to mention another possible explanationfor this reaction. In electrochemical reduction of acrylonitrile organometallic compounds have been obtained from tin, sulfur, selenium, and tellurium, which are themselves reduced electrochemically. 

The question arises as to whether the formation of organometallic compounds during electrolysis of aqueous solutions of acrylonitrile is not due to cyanoethylation of hydrides formed initially at the electrode. In Table 5 data on the electrochemical reduction of tin, sulfur, selenium, and tellurium in aqueous solutions of sodium sulfate with and without acrylonitrile are compared [40]. 

H2N (CH2)a NH2- Colourless solid when pure m.p. 4LC, b.p. 204 C. Manufactured by the electrochemical combination of two molecules of acrylonitrile to adiponitrile followed by catalytic reduction, or by a series of steps from cyclohexanone via adipic acid. Used in the production of Nylon [6, 6]. 

Atobe, M., Sasahira, M. and Nonaka, T., 2000, Ultrasonic effects on Electro-organic Processes part 42. Product selectivity control in reductive homo and cross couplng of acrylonitrile . Paper presented on the 197" meeting of the Electrochemical Society May 2000, Toronto Camab Abstract no. 108. 

A synthesis of great industrial interest is the electrochemical anodic reductive dimerisation of two molecules of acrylonitrile to give adiponitrile, from which adipic acid and 1,6-hexanediamine are prepared by hydrolysis and reduction, respectively, of the two nitrile groups. Polycondensation of the resulting products leads to Nylon 66 (Scheme 5.27). 

It is interesting to note that reduction of cinnamonitrile, related to aromatic nitriles, gives dimers. Electrochemical dimerization of acrylonitrile is a well-known industrial process of the... 

If the acrylonitrile surface concentration in the adsorbate layer is sufficiently enhanced, due to sufficiently high acrylonitrile concentrations in the bulk of the electrolyte, the Michael addition [Eq. (46d)], of the primarily formed radical anion of the coadsorbed acrylonitrile can compete effectively with (retarded) protonation [Eq. (46e)]. Eventually further reduction and protonation [Eq. (46f)] yields the desired hydrodimer, an important electrochemically produced commodity ... 

Electrochemical reactions may be carried out at any scale from the smallest to the largest and progress in nanotechnology has made it possible to address electron transfer at the single molecule level [26, 27]. Conversions at the laboratory scale are well established and have been addressed by numerous authors [1, 2] and, at the industrial scale, more than 50 electrochemical processes have reached a respectable level with the reductive hydrodimeri-sation of acrylonitrile to adiponitrile topping the list with an annual production of about 300 000 tons [28],... 

An electrochemical procedure for acrylonitrile coupling has been industrially successful. Here a reductive acrylonitrile coupling is effected by the electrons coming from the cathode of an electrolytic cell. The first formed radical carbanion attacks the second acrylonitrile molecule. Uptake of a new electron from the cathode and of two protons gives adiponitrile in high yield [M. Baizer and D. E. Danly, Chem. Ind. (London), 1979, 435, 439]. 

Electrochemical processes are often touted as being green chemistry because electricity is considered inexpensive, and toxic metal reagents are usually avoided. Electrochemical processes have produced tons of bulk chemicals [37], the best-known of which may be adiponitrile from reductive dimerization of acrylonitrile (Figure 13.17) [38]. An electrochemical synthesis to manufacture fenoprofen is shown in Figure 13.18, with the magnesium provided as a sacrificial electrode [39], Flow cell technology has been used for these operations on a commercial basis. 

Before moving on, one important exception to the SSR is worth noting, which arose from the work of Bewick, Pons and coworkers [92, 93] on the adsorption of acrylonitrile, and the later work of Ko-rzeniewsld and Pons [94-96]. In essence, the work showed that the vibrations of an adsorbed molecule that are parallel to the electrode surface may become activated as a result of the electric field, and this was termed the electrochemical Stark effect [94-96] as would be expected, this effect depends very strongly on the nature of the adsorbed molecule. Thus, for example, Pons and coworkers [96] observed a bipolar band centered near 1600 cm in the in situ infrared spectrum of anthracene on reducing it to its radical anion the band was attributed to the Ag C—C symmetric stretch of the anthracene shifting to lower V on reduction. As the molecule adsorbs... 

Hydrodimerization. The reductive dimerization of acrylonitrile can be done either chemically or electrochemically to form adiponitrile. Hydrodimerization with its derivatives also takes place. 

Baizer, M.M. (1964) Electrolytic reductive coupling, I acrylonitrile. Journal of the Electrochemical Society, 111, 215-222. 

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