I 76 INTRODUCTORY SURVEY

111.-THE LIQUID PHASE.

INTRODUCTORY SURVEY.

B Y W. A. WATERS.

Received 12th September, 1947.

Only ten years have elapsed since the possibility that free radicals might be concerned in a few, somewhat abnormal, reactions in solution was first reviewed,' and in this period knowledge of homolytic reactions in solution has advanced at an astonishing rate. The greatest development of the subject lies in the field of polymerisation chemistry, which is being accorded specialised treatment a t this meeting, but it is worth while to recall now that the present quantitative development of this technically important subject has depended upon the outcome of those exploratory studies of the reactions of dibenzoyl peroxide which indicated that it yielded free neutral phenyl radicals in solution, and which showed that these active radicals then initiated reaction-chains which, with olefines involved radical addition to one end of a double bond, and with saturated mole- cules involved atom transfer whereby one free radical was converted into another. The respective roles of solvent and solute molecules then became clear, and it was seen that the circumstances under which dissolved sub- stances might act as retarders, or inhibitors, of the reaction-chain depended essentially upon the intrinsic energy contents of the free radicals which they might generate.2

To-day the study of polymerisation chemistry has developed so rapidly that it has become the main source of exact information concerning the degree of reactivity of any specific free radical in solution. It is to be hoped, therefore, that the intense specialisation which is occurring in this field will not be such as to obscure the general applicability of its quanti- tative conclusions concerning free-radical kinetics. Many other important reactions, as for instance oxidation processes, can be explained by the use of the same fundamental theories, and their development will largely depend upon the extent to which their quantitative aspects can be elucid- ated. Already papers presented at this meeting are indicating how this can be done. The contribution of Dr. Bolland and Dr. ten Have (p. 252)

Hey and Waters, Chem. Rev., 1937, 2 1 , 186. Waters, Trans. Faraday Soc., 1941, 37, 770.

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W. A. WATERS I 7 7 gives us a very striking instance of the way in which an exact kinetic study can identify as R-0-0' the essential free radical concerned in what is after all a very complicated reaction process, whilst the paper by Miss Stead and Dr. Denbigh (p. 263) deserves consideration in that it may give us a helpful new technique for investigating labile molecules of interest.

New methods of producing free radicals in solution should always receive particular attention, since they constitute the starting-points of new avenues of research. Photolytic processes in solution, such as those described by Mr. Olearts and Prof. Jungers (p. 222) have as yet received far too little study, whilst the decompositions of very unstable organo- metallic compounds which are described by Dr. Bawn and Mr. Whitby (p. 228), offer almost unlimited scope to the organic chemist. Coming from quite another angle, Dr. Mann's paper (p. 236) gives us a reminder that there can be many types of electron-deficient labile molecules besides the analogues of free methyl or free phenyl with which we are now fairly well acquainted.

We are still in the exploratory stage of free-radical chemistry, in which certain reactions, as, for instance, the Cannizzaro reaction are for a time thought to be free-radical proce~ses,~ and then turn out to be substantially otherwise whilst others, somewhat unexpectedly, do seem to have some free-radical character. It is as exploratory studies that we should view the contributions of Prof. Fieser (p. 242) and of Dr. Birch (p. 246), which deal with oxidation and reduction processes respectively. Both these papers give valuable indications as to what are the critical centres at which complex molecules can be attacked by electron-abstracting and electron- supplying reagents, and though the observable reaction-product is rarely of free-radical character there'is still a good reason to believe that the initial change may be a one-electron process.

Only two years ago the Faraday Society, a t a meeting in London, heard an interesting exploratory paper which dealt with a process. of " reduction-activation " whereby peroxides and persulphates could be- come polymerisation catalysts. One of these recuction-activations was then shown by Baxendale, Evans and Park to be a one-electron transfer process in which a ferrous ion disrupted the weak 0-0 link of hydrogen peroxide, and set free the intensely reactive free radical hydroxyl, 'OH.

had postulated the same electron-transfer reaction, viz.,

Several years before this Haber and Weiss

Fe++ + HO-OH -+ Fe+++ + (H0:)- + 'OH

as one of the chain-starting processes involved in the catalase type of decomposition of hydrogen peroxide to oxygen and water, and in a series of speculative papers Weiss had developed the view that peroxidase oxida- tions also involve this radical, but not until Evans and his colleagues showed that free hydroxyl was a typical polymerisation catalyst, which became incorporated into the end groups of the resulting polymerised products* were these views firmly substantiated in such a way that the Haber-Weiss conception could be developed quantitatively. To-day we can see quite clearly the great significance of one-electron transfer processes.

Dr. George (p. 196) has shown that the original Haber-Weiss scheme for the catalase decomposition of hydrogen peroxide is not quite correct by studying the particular properties of the HO; radical, which can be

3Cf. Weiss, Trans. Faraday SOC., 1941, 37, 782. 4Alexander, J. Amer. Chem. SOC., 1947, 69, 289.

Bacon, Trans. Faraday Sbc., 1946, 42, 140. Ibid., 1946, 42, 155. Haber and Weiss, Proc. Roy. SOC., A . 1934, 147, 332.

sEvan~, J . Chem. SOC., 1947, 266.

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1 78 INTRODUCTORY. SURVEY obtained from that curious substance, potassium peroxide, KOz, and his work will interest all those who are concerned with the decompositions of organic peroxides.

Dr. Weiss (p. 188) and the writer (p. 179) have concerned themselves with specific reactions of the free hydroxyl radical, and in consequence have established the occurrence, in aqueous solution, of a whole series of new free radicals, such as '00C-COOH from oxalic acid, R'CH-OH and R"&--OH from alcohols, and R--6=0 from aldehydes ; the last of these had previously been encountered (though not hydrated) as a transient photochemical decomposition product of aldehyde and ketone vapours. In addition to demonstrating the free-radical nature of oxidations in- volving peroxides, the quantitative aspects of these papers are valuable, for they indicate how the rates of the various reactions of free hydroxyl, such as addition to C=C or abstraction of hydrogen from H-C, can all be compared by reference to one standard reaction, i.e. the electron transfer :

Fe++ + 'OH -+ Fe+++ + (:OH)-. Thus, in the past two years quantitative chemistry has made another big stride forward towards the interpretation of what has hitherto been one of the most puzzling of all groups of catalysed reactions ; i.e. oxidase systems. So many metallic, and other ions, can undergo one-electron oxidation or reduction, that it may not be too rash a prediction to suggest that here, 'in this meeting's papers, we shall find the underlying theoretical explana- tions of the actions of most of those trace elements which are so important in biochemistry, and so vitally significant in relation to both medicine and agriculture.

Still another feature of these papers on free-radical reactions in aqueous solution deserves special comment. The free hydroxyl radical is particu- larly important in water, because any more active radical which we may manage to generate in water would immediately be converted to free hydroxyl by chain transfer of the type :

(compare, C1' (atomic) + H-OH -+ HC1 + 'OH) whereupon hydroxyl will have an apparentZy long existence on account of solvent exchange :

R' + H-OH + R-H + 'OH

HO' + H-OH + HO-H + 'OH. If, however, our aqueous solution contains a solute molecule, R-0-H

(e.g. an alcohol) or an anion X- (e.g. I-) which by hydrogen, or electron abstraction gives a less reactive free radical than hydroxyl then a similar chain transfer will occur,g e.g.

Employing the terminology of investigators of polymerisation, we would describe these molecules, or ions, as retarders, or, if they are sufficiently distinct, as inhibitors of our oxidase systems. Effects of inhibiting or poisoning agents are recorded in abundance in the vast literature of enzyme chemistry, and i t may well be that we shall soon be able to explain them, and perhaps also be able to account for much of the specificity of enzyme oxidations, in the light of our studies of chain transfer.10 Polymerisation research is showing us that even a small change of an activation energy, or of a reaction velocity constant, can vastly change the nature of the final product in a system in which there are only two competing chain reactants (e.g. two monomeric olefines in a co-polymerisation process), and that in consequence careful quantitative work is needed before com- petitive chain reactions can really be understood. In future it will there- fore be essential to pay an equally careful attention to the quantitative aspects of electron-transfer chemistry.

HO' + I- -+ (HO:) + I".

Cf. ref. 2. lo Cf. Waters , Trans. Faraday SOC., 1943, 39, 142.

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W. A. WATERS I 7 9 Our theoretical information as to the energy contents of free radicals

is undoubtedly of great value, but it is only by measuring reaction velocities, and activation energies, for free-radical processes in solution that we can hope t9 answer the vital question as to how much the reactivity of a free radical is a function of its environment. It has been suggested that the electrical neutrality of a radical makes its reactivity far less dependent upon the polar nature of the solvent which surrounds it than is the case with a reacting ion, but though it may be reasonably justifiable to neglect the environmental influence of free radicals in non-polar solvents, such as hydrocarbons, this simplification is certainly inadmissible for reactions in polar solvents such as water, which may even solvate our labile mole- cules. For these reasons we must beware of letting speculations run far ahead of experimental research in this most promising, new field of chemistry. The following papers, however, present so much that is novel that i t may not be long before we find that its subject matter forms the basis of an entire future meeting of the Faraday Society.

Dyson Perrins Laboratories, Oxford.

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