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The Electrosynthesis of Organic Compounds

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The Electrosynthesis of Organic Compounds The Electrosynthesis of Organic Compounds By Professor Martin Fleischmann, Ph.D., and Derek Pletcher, Ph.D. Department of Chemistry, University of Southampton reactions such as substitutions and cyclisa- The decelopment of electrochemical tions. routes in the industrial production of In order to explain the interest in electro- organic compounds is beginning to chemistry it is convenient to contrast electro- attract considerable interest. Already chemical reactions with homogeneous or two large-scale processes are in opera- heterogeneous reductions and oxidations tion, one producing adiponitrile from using hydrogen and air or oxygen. The free acrylonitrile, an intermediate step in the energy change, AGO, of these processes is manufacture of Nylon 66, and the other equivalent to a cell potential, E”, given by producing lead tetra-ethyl anti-lcnock AG= --nFE’ compounds. This article reviews the By referring to a scale of free energies or basic electrochemistry involved in this electrode potentials, Figure I, it is evident type OJ; synthesis and outlines the great that such spontaneous reactions are only pos- possibilities being opened up by ad- sible within the potential range limited by the vances in technique, in reactor design reduction of oxygen or the oxidation of and in the development of new types of hydrogen. This driving force only amounts electrode structures in which platinum to approximately 0.5 eV or 10 kcals/mole. By and its associated metuls will play an contrast, it is possible to carry out electro- important part. chemical reactions at potentials between +3.5 V and -2.5 V even in aqueous solution, if suitable electrolytes and electrodes are In recent years research in the field of chosen. Thus, the driving force for an elec- organic electrosynthesis has received consider- trode process is of the order of 3eV or 60 able stimulation from the forecast that, with kcalslmole. Electrochemical reactions there- the advent of nuclear power, electricity will fore enable one to introduce considerable become cheaper compared to chemical oxi- energy into molecules at low temperature and dants and reductants. Furthermore, the the order of magnitude of this energy is, in reasons for the failure of earlier workers to fact, that required to break chemical bonds. achieve selective reactions have become ap- Figure 2 compares the energy ranges in which parent and by controlling the electrode different methods of activating molecules will potential, the solution conditions (solvent, be effective. pH, concentration of species, etc.) and by Clearly, in view of the energy which may choice of suitable electrode materials, some be introduced, it is not surprising that many measure of selectivity is now possible. It “high energy” chemicals that are used as therefore seems likely that electrochemical oxidmts and reductants in synthesis are pre- techniques will have an increasing role in pared electrochemically (e.g. sodium, man- preparative organic chemistry, both in indus- ganese dioxide, chlorine) and there are try and in the laboratory as a standard method obvious advantages in avoiding such inter- for oxidations and reductions as well as other mediates. Platinum MetalsTev., 1969, 13, (2), 46-52 46 + 4.0 0,+ ~H,O+ 6e --+60H- + 3.0 C10, + e _jC10,- carbornurn cO3- -+ e +Co2- i2.0 C10- + H,O + 2e +C1- + 20H- T :z - H- radical OL- 2Hz0 + 4e __j_ 40H radical cation T 1.0 C1, - 2e __j 2C1 Te 2H ~2e+HA 0 substrate 1 I -e - L radical Zn2' + 2e 4 Zn 1.0 radical anion +HT i - 2.0 Na- + e dNa +e -3.0 carbanion1 Solvated electrons Fig. 1 The scale of free energies or electrode potentials. This shows the approxi- mate potentials (in volts) at which various electron transfer reactions take place and indicates the intermediates that may be formed in electrode processes The intermediates formed in electrode pro- of nitrobenzene where phenylhydroxylamine cesses are indicated in Figure I and are may be produced at low negative potentials entirely consistent with the large driving and aniline at more negative potentials (I). force of such reactions. While the figure is not drawn to scale, it nevertheless shows the approximate points at which radical ions, - H,O radicals, carbanions and carbonium ions would be expected to form. Thus it can be seen that by controlling the electrode potential - H,O a measure of selectivity may be achieved. The -aNHz classical example of this idea is the reduction A further example is the oxidation of 9, IO- 0 1.0 3.5 6.0 8.0 12.0 ev I I I I I photochemistry radiation chemistry f electrochemistry corona discharge <- <- - - - - -- -Fig. 2 The diagram illustrates the range in energy (I electron volt = 23 k.cal male-') which may be introduced into molecules by various 'high energy' techniques Platinum MetalsTev., 1969, 13, (2) 47 diphenylanthracene at platinum electrodes in trode activity rapidly falls with time. In view aprotic solvents. At low potentials this leads of thc wide range in potential over which the to the radical cation and at high potentials to inert metals may be used the polarisation can the dication (2). be reversed for short controlled periods of time and the electrode reactivated cathodic- ally. In this way controlled successful oxida- tion of hydrocarbons may be achieved. Intermediates of the kind shown in Figure I will be formed in many electrode reactions. For example, carbonium ions will be gener- Q ated at platinum electrodes during the oxida- tions of alkyl halides (4), carboxylic acids (5), hydrocarbons (6), and amines (7). ? RI- >R BIL ,e RCOO--+R-I CO, t 2e RH --R- I H-' i2e Carbanions and dianions are generated in the It is also possible to exert a measure of con- reduction of alkyl halidcs, aromatic hydro- trol over electrode reactions by maintaining carbons (g), quinones ( IO), nitrocompounds the electrode at a series of potentials for con- (I I), and activated olefms (12), e.g. trolled durations. In this way it is possible, RI , 2e- .R-i I- for example, to switch successively from 0H-H+ e--0H,- oxidation to reduction and thus to change the + products of the reaction (3). CH, :CH - CN 2e-- - Cg2-cH - CN Radicals arc also frequently produced as in the oxidation of carboxylic acids (13) and of carbanions (14, IS), and the re- duction of some alkyl halides (8), ketones (13)~quinones (17) and tetra-alkylammonium salts (IS), e.g. RCOO.-+R' +CO,+e 4Hi+4c @N-XG - xm CH(COOEt),--->CH(COOEt), +e H,K SII Al(CH,),----Al(CH,), CCHs'+e A further important application of pulse electrolysis is likely to be the control of ele- 0. trode activity by control of the electrode 0 history. For example, in the use of platinum 0,CO -H+'++O,C.OH metal electrodes for the oxidation of hydro- carbons in non-aqueous solutions the elec- RI -ie--R' +I- Platinum MetalsTev., 1969, 13, (2) 48 In order to study these highly reactive The intermediates which are formed react intermediates, it is clearly necessary to use in similar ways to those observed when they unreactive solvents and electrode materials. are generated in homogeneous processes, Indeed, it is the advent of aprotic polar although their reactions will often be modi- solvents in which the intermediates have an fied by the adsorption on the electrode. In appreciable half life that has allowcd the typical reactions, radicals will dimerise (Z I), characterisation of many intermediates by electrochemical and spectroscopic methods z EtOOC (CH,), COO- -*e > (in particular ESR). In many other cases the zCO,+EtOOC (CH,), COOEt nature of the intermediates has been inferred attack double bonds (zz), from the products of the reaction and by analogy to known reaction mechanisms. EtOOCCOO e->C02t-COOEt butaAe%-t EtOOC -CH, -CH =CH-CH;-+dimer Reactions at Platinum Electrodes or react with certain electrode metals (23), The generation of the reactive species naturally also demands inert electrode sub- R Mg I --e+MgI : R. 'b+ PbR, strates and platinum metals have been widely Carbanions or dianions formed from aro- used in aprotic solvents over the range +3.0 matic hydrocarbons will protonate to form di- to -3.oV. On the other hand, in water or hydroaromatic compounds (9) or undergo other protic solvents the use of platinum other typical coupling reactions such as that metals has been conked to anodic reactions with carbon dioxide (17). because the decomposition of solvent to form hydrogen is strongly favoured. However, at H COO- the present time a considerable research ze + zCOz effort is being devoted to investigations of the oxidation and substitution of organic com- H 'coo- pounds and for these processes the use of inert platinum electrodes is of key import- Similarly, the dianions formed from activated ance. Indeed, most of the oxidative reactions olefines will react with unsaturated com- listed in this article have been carried out on pounds as in platinum metal electrodes. Cg, - CH -CN + CH, = CHCN +2H I--+ The formation of the intermediates has NC -(CHJ-CN been written as if the electrode surface is not directly involved in the reaction step. In and intramolecular reactions leading to many cases, however, the reactive species will cyclisation are also possible (24), be adsorbed on the surface and the catalytic role of the electrode will be of key importance. For example, the reductive hydrogenation of olefines is dependent on the use of platinum blacks (19) as is the dissociative adsorption of Carbonium ions are naturally highly reactive saturated hydrocarbons or methanol as an essential step in the overall reactions on fuel and usually attack the solvent or deprotonate to form olefines. For example, in acetonitrile cell electrodes (20). Again the coupling of radicals produced from carboxylic acids in amides are formed (4) the Kolbe or Brown-Walker reaction is R++CH,CN--+CH,-C-+=N-R % dependent on the electrode surface, being CHa--CO -NHR favoured by platinum in aqueous solutions (13).
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