THE REACTIVITY OF FREE RADICALS GENERAL INTRODUCTION BY E. W. R. STEACIE It is the custom at these Discussions for the President of the Society to welcome “ the distinguished overseas members and guests ”. I am sure the situation must have been a little confusing for the President to-day since the Society is itself over- seas, and the identification of the guests is somewhat difficult. I should, therefore, like to take this opportunity to welcome the Society on its first visit to Canada. I should also like to say how happy we are to have in Canada such a distinguished group of workers in the field, from Britain, from the United States, and from other countries. Faraday Society Discussions have a unique reputation, and we in Canada are particularly pleased at having the honour of being your hosts on the occasion of your first overseas visit.Free radicals have frequently been a subject of Faraday Discussions. Full- scale discussions were those on Free Radicals in 1934 and on the Labile MolecuIe in 1947. Free radical reactions have also figured largely in other discussions on hydrocarbons, oxidation, photochemistry, polymerization, etc. To-day’s title, “ The Reactivity of Free Radicals ” is, I think, appropriate since it emphasizes a definite change in outlook which has recently made itself felt. From a pessimistic point of view one might perhaps wonder what we have all been doing during the past eighteen years, since many of the questions asked in 1934 have still to be answered in 1952. Thus the role of free radicals in the decomposition of ethane was a burning question in 1934, and is still open to vigorous argument to-day.From a more optimistic point of view, however, I think that we can fairly say that much progress has been made in the meantime. In 1934 the emphasis was on the existence of radicals. Almost any type of mechanism could be, and was, postulated. To-day we know enough at least to predict the types of reaction which are likely to be of importance, and the maker of mechanisms has to exercise considerably more self-restraint. By the time of the 1947 Discussion the general state of affairs had, I think, become reasonably satisfactory from a qualitative standpoint. General agreement seemed to have been reached as to which types of reaction were fast and which were slow, and as to general orders of magnitude of activation cnergies.Also, there was fairly general agreement as to which overall reactions involved free radicals and which did not, even if there was by no means agreement on the quantitative aspects in many cases. Again, by 1947 at least the main features of the mechanisms of many important thermal and photochemical reactions had been worked out. In the last five years there has been a sharp shift in emphasis, with attention being focused on the rates of free radical reactions themselves, rather than on overall reactions with the elementary steps regarded as merely incidental stages. Along with this has come a demand for more quantitative data resulting in the inevitable reinvestigation of many old established reactions. As a result we are again to-day in a period of flux, with many previously accepted results now being regarded with suspicion.It is, therefore, an appropriate time for such a discussion as the present one, and in particular for emphasis on the reactivity of free radicals. At the moment there is much interest in an accurate knowledge of activation energies, and in a knowledge for a series of compounds of the way in which the activation energy varies with chemical structure. It looks at present as if a be- ginning has been made along these lines. This is a matter of great importance 910 GENERAL INTRODUCTION since it seems to the writer that the numerous efforts to make some such correlation in the past, and to compare the results with theory, have usually taken the form of a plot of f (activation energy) against f (bond dissociation energy) in which both sets of data were completely unreliable.There are also signs of a decided improvement in the bond dissociation energy situation. This has long been the scandal of kinetics, and it seems extraordinary that we are still by no means certain of many of the values for the simplest com- pounds. There has, however, been much accomplished in the past five years. I am by no means convinced that the present situation is as satisfactory as some workers assume, but it is certainly a great improvement. Another feature of recent work has been the successful determination of the absolute rates of certain recombination reactions. This has, in the first place, necessitated a change in outlook regarding third-body effects. In the second place, it has given standard reactions for comparison, and has led to a knowledge of the absolute values of frequency or steric factors for many reactions.The results have been surprising and have proved the misleading nature of calculations in which arbitrary values of steric factors have been assumed in the past. Much reinvestigation of simple reactions is needed before the matter can be cleared up. Much progress has been made in certain specific fields involving free radical reactions. In the field of polymerization there have been great advances in the formal kinetics of the process. Information on specific chemical steps is in by no means as satisfactory a state, and it is gratifying to see that a fundamental contribution to this question is part of the present discussion.Concerning oxidation reactions we are still in a position where nearly everyone is free to have his own mechanism. Great strides have been made in the way of formal mechanisms, but the identity of the chain carriers remains an open question in most cases. It may also be pointed out that the recent work on recombination, mentioned above, suggests that the wholesale postulation of third-body effects in oxidation reactions rests on doubtful ground. The question merits re-examin- ation. It is hoped that to-day’s discussion will be a step towards the final solution of some of these problems. In the liquid phase the problems are more complex, although in some ways the subject is further advanced. We have for our discussion a very good variety of problems on oxidation, polymerization, substitution, etc. All in all, I think the discussion is a timely one, and that we are unusually fortunate in the very distinguished group we have with us. We can hardly fail to effect some clarification of some of the issues.