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Home , Electrosynthesis

The of Organic Compounds

By Professor Martin Fleischmann, Ph.D., and Derek Pletcher, Ph.D. Department of , 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 and air or . The free , 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 tetra-ethyl anti-lcnock AG= --nFE’ compounds. This article reviews the By referring to a scale of free energies or basic involved in this 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 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 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, 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 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 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 (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 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 (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 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 cell electrodes (20). Again the coupling of radicals produced from carboxylic acids in 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). and in methanol ethers are the products (25)

Platinum MetalsTev., 1969, 13, (2) 49 as shown by the reaction

MeOH -Me0 (o> OMC I OhIc In other cases carbonium ions show their generation of CO". In this way reaction typical rearrangements as in the oxidation of occurs within a zone adjacent to the electrode neopentyl iodide (4), ring formation (26), and the actual electrode process e.g. CO"-+CO~~' te CH, -CH, -CH,-COO- LZe% serves to "drive" the homogeneous catalytic CO, I CH, -CH,-CH, - H++~~2-~~2reaction (30). Other examples of such \/ CHZ reactions are the oxidation of propylene by Hgrl ring expansion (27), or ring contraction (28), CH,-CH-CH,i 4HgZ t HzO---t e.g. CH, = CH -CHO T 4H- TaHgq ' CH ,COO- CH where the mercuric ion is regenerated electro- chemically (31). These indirect reactions (yH -25 @ - co, which involve highly oxidising conditions can again only be achieved by the use of inert platinum metal electrodes. The oxidation of propylene to propylene oxide CH,-CCH-CH, +ClO-- The substitution, for example cyanation or CH2-CH CH, 1 C1 acetoxylation, of aromatic compounds also \/ 0 takes place via cationic intermediates since by electrochemically generated hypochlorite the reaction will only take place at potentials (32) and the reaction of olefines with carbon at which the hydrocarbons are oxidised even monoxide and methanol in the presence of if the substituting anion is oxidised at lower platinum carbonyls to give methyl of potentials (29). p unsaturated acids (33) Industrial Processes in Operation C,H,CR-CH,ICO I -0Me- z It will be seen that many of these reactions C,H,CR CHCOOMe lead to industrially useful products, for are further interesting examples of indirect example, diterminal substituted compounds. processes. Indeed, the formation of lead tetra-alkyl and Some of the most extreme oxidations of adiponitrile have been made the basis of carried out at present are the perfluorinations new commercial synthetic processes by Nalco of aliphatic hydrocarbons in anhydrous and Monsanto respectively. . It is not clear, however, Figure I also indicates the potential regions whether these proceed via the generation of in which a number of inorganic intermediates oxidised aliphatic species or of fluorine (34, are generated. For example, Col'I may be 35). generated in situ at an electrode and allowed Figure I also shows that highly reducing to react with an organic substrate with re- species may be generated and the ultimate

Platinum MetalsTev., 1969, 13, (2) SO limit is, in fact, the formation of solvated simplifying the work-up by avoiding the intro- electrons in suitable solvents such as ammonia, duction of reagents which must subsequently amines, or hexamethyl phosphoramide (36, be removed. 37). The electrons will react with aromatic hydrocarbons and give di- or tetrahydro Development of Reactors derivatives. and Electrodes HH A further major development is likely to be the construction of new electrochemical reactors which will permit an easier scale-up. In recent years the bulk of the effort has been H H devoted to the development of highly active It may be noted that benzene cannot be catalytic electrodes for fuel cells, the aim being directly reduced at electrodes. the complete oxidation of hydrocarbons to carbon dioxide. It is likely that further re- Importance for search will lead to structurcs capable of giving, Large-scale Synthesis for example, controlled partial oxidations but It will be apparent that many of the inter- utilising many of the advantages inherent in mediates which have been described are of the design of the porous fuel cell electrodes. considerable importance for large-scale syn- The development of these and other new thesis. A major aim of these synthetic types of electrode structures such as fluidised procedures in the coming years will un- beds and packed beds and the development of doubtedly be the build-up of complex mole- membranes suitable for separating electrode cules from relatively simple starting materials compartments will undoubtedly lead to the as might be achieved by the reaction of construction of low cost reactors having a radicals with olefines. high throughput. The operation of many of Linked to this aim will be the activation of these electrodes will be dependent on the use unreactive substrates such as carbon dioxide of finely divided platinum metals imbedded in and saturated hydrocarbons by utilising the the structure to give a high surface area for high driving force of electrochemical pro- the surface reactions at low cost. There is cesses. Of equal importance to the develop- every prospect, therefore, that the chemical ment of new synthetic routes will be the use engineering technology to make use of the of electrochemical procedures to increase the chemical advantages of electrosynthetic pro- selectivity of existing reactions and to reduce cedures on the industrial scale will be avail- the number of steps in a synthesis as well as able.

References F. Haber, Z. Electrochem., 1898, 90, 3263 9 M. E. Peover, “Electroanal. Chemistry”, M. E. Peover and B. S. White, J. Electroanal. Edward Arnold, London, 1967, vol. 11, p. I Chem., 1967,13,93 10 M. E. Peover, J. Chem. SOL.,1962, 4540 M. Fleischmann, I. N. Petrov and W. F. K. w~~~-J~~~~,“Electrochemistry-proceed- I I A. H. Maki and D- H. Geske, 7. Am. Chem. ings of 1st Australian Conference”, Pergamon 1961J 837 Press, London, 1964, p. 500 12 M. M. Baizer,J. Electrochem. Soc., 1964, 111, L. L. Miller and A. K. Hoffmann, J. Am. 215 Chem. Soc., 1967,89,593 13 B. C. L. Weedon, Advances in Organic Chem., L. Ebersonand K. Nyberg, ActaChim. Scand., 1960, I, I 1964, 18, I567 14 T. Okubo and S. Tsutsumi, Bull. Chem. SOC. M. Fleischmann and D. Pletcher, Tetrahedron Letters, 1968, 6255 japan,1964, 3,, 1794 K. K. Barnes and C. K. Mmn, J, Org. Chem., I5 H. Lehmkuhl, R. Schaefer and K. Ziegler, Chem.-Ing.-Tech., 1964, 36, 612 1967, 32, 1474 F. Lantelme and M. Chemla, Electrochim. 16 P. Zuman, D. Barnes and A. RyvolovA- Acta, 1965, 10, 657 Kejharova, Disc. Farada.y Soc., 1968, 45, 202

Platinum MetalsTev., 1969, 13, (2) 51 I7 R. Berkey, S. Wawzonek, E. W. Blaha and 27 E. J. Corey, N. L. Bauld, R. T. La Londe, M. E. Runner, J. Electrochem. SOC.,1956,103, J. Casanova and E. T. Kaiser, J. Am. Chem. 456 Soc., 1960, 82, 2645 I8 M. Finkelstein, R. C. Peterson and S. D. 28 A. J. Baggerley and R. Brettle,?. Chem. SOC., Ross, Electrochim. Acta, 1965, 10, 465 c, org., 1968, 2055 19 S. H. Langer and H. P. Landi, J. Am. Chem. 29 N. L. Weinberg and H. R. Weinberg, Chem. SOC.9 1963, 85, 3043 Rev., 1968, 68, 449 20 E. Gileadi and B. Piersma. “Modern Asuects -70 M. Fleischmann et al,.- unuublished work of Electrochemistry”, Butierworths, LoLdon, 31 F. Goodridge, R. E. Plirnley, I. D. Robb and 1966, vol. IV, p. 47 J. M. Coulson, Brit. Pat. Appln. 16364j67 21 M. Ya. Fioshin, A. J. Kamneva, Sh. M. 32 J. A. M. LeDuc, U.S. Pat. 3,288,692 (1966); Itenburg, L. I. Kazakova and Yr. A. Ershov, Chem. Abs., 1967,24,43238 p. Khim. Prom., 1963, 263 T. Inoue and S. Tsutsumi, Am. Chem. SOC., 22 M. Ya. Fioshin, L. A. Mirkin, L. A. Salrnin 33 J. and A. Kornienko, Zh. Vxes. Obshchestva im 1965,87,3525 D. I. Mendeleeva, 1965,10,238 34 J. Burdon and J. C. Tatlow, “Advances in 23 E. M. Martlett, Ann. N.Y. Acad. Sci., 1965, Fluorine Chemistry”, Butterworths, London, 125, 12 1960, I, 129 24 J. D. Anderson, M. M. Baizer and J. P. 35 S. Nagase, Fluorine Chem. Rev., 1967, I, 77 PetrovichJ. Org. Chem., 1966, 31, 3890 36 H. W. Sternberg, R. E. Markby and I. 25 B. Wladislaw and H. Viertler, Chem. Ind. Wender,J. Electrochem. SOC.,1966, 113, 1060 (London), 1965, 39 37 R. A. Benkeser and E. M. Kaiser,J. Am. Chem. 26 W. J. Koeh1,J. Am. Chem. SOC.,1964,86,4687 SOC.,1963,852858

Measurement of Electrodeposit Thickness The use of the beta back-scatter technique thickness by reference to a calibration graph. for the non-destructive measurement of the Variations of this equipment provide the thickness of electrodeposited coatings - and facility to pre-set a minimum thickness or a the basic design of a commercial instrument, range of thicknesses, pass or fail lights being the Beta 750 - were described some two years activated by the count figure obtained. ago (Platinum Metals Rev., 1967, 11, 13). A direct reading instrument, the Beta 752, The success of this equipment has now led incorporates a meter reading of deposit to the introduction of an extended range of thickness in micro-inches, based upon cali- instruments to meet specialised needs and to brated scales for specific combinations of satisfy the increasingly stringent demands for deposit and basis metal. This instrument is consistency and accuracy, particularly where particularly suitable where large numbers of electrodeposits such as rhodium, palladium tests must be carried out and maximum and gold are involved. accuracy is not a vital consideration. The Beta 751 provides a four-figure digital The units are produced by Panax Equip- read out of the beta count over a pre-selected ment Limited and marketed by Johnson time, this figure being related to deposit Matthey.

The Beta 752 unit is fitted with a calibrated scale for the direct reading of electro- deposit thickness. Zero and full-scale deflection points are set by adjusting the two potentiometers

Platinum MetalsTev., 1969, 13, (2) 52