Acrylonitrile electrochemical reactions

Many such driven systems are known in electrochemistry and some of them will be explained in the chapters of this volume. One of them, indeed, is a step in the synthesis of nylon. This is the polymerization of acrylonitrile by means of the following electrochemical reaction ... 

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],... 

In terms of sustainability, the process starting from propene would be preferable, since it avoids the risks connected with the use of HCN in the butadiene route, even if produced on demand. However, the butadiene route to produce adiponitrile (ADN) is more cost-effective, owing to the need to use an electrochemical reaction for acrylonitrile dimerization. The problem of cost, however, is highly dependent on several factors, including sensitivity to natural gas prices (which influences butadiene cost), the market for acrylonitrile, and soon. The acrylonitrile route is used by Solutia, BASF and Asahi Kasei. New plants to make caprolactam, using ADN as intermediate, are under construction in Asia. 

The adsorption of polymers, poly(vinyl pyridine) or poly(acrylonitrile) either to coordinate metal atoms or to adsorb biopolymers has been used to prepare chemically modified electrodes for immobihzation of enzymes either by physical or by chemical adsorption (carrier binding), cross-linking, and entrapping at lattice sites or in microcapsules [43]. A wide application of these types of electrodes has been made for electrochemical reactions of biological interest [44]. 

Reaction of thioacetamide with benzaldehyde in ethanol, NaOH gave 783 that upon electrochemical oxidation afforded acrylonitrile derivative 785 together with the thiadiazoloquinoline derivative 784 (82MI199) (Scheme 137). 

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) ... 

There are many possible reaction pathways between acrylonitrile and adiponitrile and, in each, there are several possible rate-determining steps. None of the reaction intermediates has yet been detected electrochemically or spectroscopically thus indicating very fast chemical processes with intermediates of half-lives of < 10-5 s. Bard and Feiming Zhou [104a] have recently detected the CH2 = CHCNT radical by Scanning Electrochemical Microscopy (SCEM) using a 2.5 fim radius Au electrode (1.5 mol CH2 = CHCN in MeCN/TBAPF6). The dimerization rate has been determined to 6.107 M-1 S l. 

Most industrially desirahle oxidation processes target products of partial, not total oxidation. Well-investigated examples are the oxidation of propane or propene to acrolein, hutane to maleic acid anhydride, benzene to phenol, or the ammoxidation of propene to acrylonitrile. The mechanism of many reactions of this type is adequately described in terms of the Mars and van Krevelen modeE A molecule is chemisorbed at the surface of the oxide and reacts with one or more oxygen ions, lowering the electrochemical oxidation state of the metal ions in the process. After desorption of the product, the oxide reacts with O2, re-oxidizing the metal ions to their original oxidation state. The selectivity of the process is determined by the relative chances of... 

Development of the industrial process for electrochemical conversion of acrylonitrile to adiponitrile led to extensive investigation into the mechanism of the dimerization process. Reactions of acrylonitrile radical-anion are too fast for investigation but the dimerization step, for a number of more amenable substrates, has been investigated in aprotic solvents by electrochemical techniques. Pulse-radiolysis methods have also been used to study reactions in aqueous media. 

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).

In general, an alternating eopolymer is formed over a wide range of monomer compositions. It has been reported that little chain transfer occurs, and in some cases, conventional free radical retarders are ineffective. Reaction occurs with some combinations, like styrene-acrylonitrile, when the monomers are mixed with a Lewis acid, but addition of a free-radical source will increase the rate of polymerization without changing the alternating nature of the copolymer. Alternating copolymerizations can also be initialed photochemically and electrochemically. The copolymerization is often accompanied by a cationic polymerization of the donor monomer. 

These latter workers liave demonstrated, by electrochemical experiments, the different reactivities of 125 and its cis isomer.Thus, whereas 125 is formed essentially reversibly and lost in a slow dimerization reaction and a slow coupling with unreduced thioindigo, its cis isomer is rapidly consumed in isomerization, radical-radical, and radical-parent coupling reactions. Similar differential reactivities with carbon dioxide, acrylonitrile, and cin-namonitrile were also observed. For 6,6 -diethoxytliioindigo, it was found that the electroreduction of the cis and trans isomers occurred at different... 

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. 

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