Chemical Education Today
Commentary: Reviewer Comments
A Discussion of “Can Reaction Mechanisms Be Proven?” Tehshik Yoon Department of Chemistry, University of Wisconsin–Madison, Madison, WI 53706;
[email protected] In their Commentary, Buskirk and Baradaran argue against the well-accepted axiom that reaction mechanisms can never be conclusively proven but only experimentally falsified. Their argument has both a philosophical basis (that Popper’s falsifiability standard is impractically strong) and a technological one (that contemporary high-resolution spectroscopic methods rescue us from our weak epistemological position). The crux of the authors’ philosophical objection is that “[r]epeated successful tests of a theory have no logical basis in Popper’s philosophy to increase our confidence in it.” Indeed, a strong version of Popper’s falsifiability standard could leave us unsure of any experimental observation. Scientific knowledge would become inaccessible, and theories would be mere conceptual models devoid of any concrete relationship to physical reality. Certainly, Popper had contemporaries who articulated similar critiques of his approach to the problem of inductive reasoning. On the other hand, I think that it’s fair to say that Popper is not the only modern philosopher who has had a significant influence on the way scientists conduct research. Kuhn (1) would tell us that the value of a scientific theory rests upon its ability to predict future outcomes. Theories that are supported by repeated experimental validation become accepted as “paradigms”, which succumb to falsification only with great difficulty. In Kuhn’s description of scientific inquiry, the practical value of a theory is independent of the ontological status of the theory itself. We can (and we do) behave as if a well-validated theory is true even if we have no epistemologically rigorous way of showing it to be so. I would argue that this is exactly the same standard we apply to the study of chemical mechanisms. As experimental chemists, we do indeed perform experiments designed to corroborate our mechanistic hypotheses. Nevertheless, there is a meaningful distinction between the “proof ” and “validation” of a scientific theory, and by extension of a reaction mechanism. For any given discrete set of data, there exist an infinite number of possible explanations consistent with all the data. No matter how well validated a mechanism is, we can always posit alternative mechanisms that also fit our experimental observations. We winnow through these alternatives using two tools—Occam’s razor, the indispensable but logically inconclusive principle that the simplest explanation consistent with the data is the one most likely to be true; and the careful design of experiments to falsify the reasonable alternative mechanistic interpretations of the existing data. The best and
most useful mechanistic hypotheses are those that have been experimentally validated and have survived falsification. The authors argue that techniques such as molecular beam and femtosecond spectroscopy allow us to gain insights into reaction mechanisms that were inaccessible using classical kinetic methods alone. This is certainly true, but I submit that no amount of technological sophistication rescues us from the overabundance of possible alternate explanations for any given discrete data set. Another well-accepted axiom of classical kinetics holds that the ability to observe an intermediate does not imply that this intermediate is on the productive pathway toward the product; there exist an infinite number of alternate possibilities. This continues to be true of data gathered by newer techniques, no matter how fleeting the lifetime of the intermediates observed. The intermediate may merely be en route to decomposition. It may exist in non-productive equilibrium with the resting state. There may be another kinetically invisible step following production of the intermediate. There may be another pathway kinetically accessible under experimentally relevant conditions. There may be an alternate structure consistent with the spectral data collected. And so on. New methods to probe reaction mechanisms in greater detail can certainly increase our confidence in our mechanistic hypotheses by enabling us to design more sophisticated corroborating and falsifying experiments, but they ultimately cannot provide us with affirmative proof. Finally, the authors state as a motivation for their argument that “the idea that mechanisms can never be proven or even supported by evidence will discourage students from using all the tools at their disposal.” My experience suggests the opposite problem. Far too often, I find that beginning graduate students accept the results of computation rather uncritically, treat important control experiments as afterthoughts, and accept the chemical literature as an inerrant source of truth. It seems to me that learning to adopt a position of healthy scientific skepticism is an important part of our intellectual development as research scientists. I maintain that the falsifiability standard, as used by chemists for decades, stands up to the critique articulated in Can Reaction Mechanisms Be Proven? and is as relevant and important as ever. Literature Cited 1. Kuhn, T. S. The Structure of Scientific Revolutions, 3rd ed.; University of Chicago Press: Chicago, 1996.
This article has been reformatted from its original appearance in the print Journal.
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Journal of Chemical Education • Vol. 86 No. 5 May 2009 • www.JCE.DivCHED.org • © Division of Chemical Education
