Abstract
Organic chemists have been able to develop a robust, theoretical understanding of the phenomena they study; however, the primary theoretical devices employed in this field are not mathematical equations or laws, as is the case in most other physical sciences. Instead it is diagrams, and in particular structural formulas and potential energy diagrams, that carry the explanatory weight in the discipline. To understand how this is so, it is necessary to investigate both the nature of the diagrams employed in organic chemistry and how these diagrams are used in the explanations of the discipline. I will begin this paper by characterizing some of the major ways that structural formulas used in organic chemistry. Next I will present a model of the explanations in organic chemistry and describe how both structural formulas and potential energy diagrams contribute to these explanations. This will be followed by several examples that support my abstract account of the role of diagrams in the explanations of organic chemistry. In particular, I will consider both the appeal to ‘hyperconjugation’ in the explanation of alkene stability and how the idea of ‘ring strain’ was developed to explain the relative stability of cyclic compounds.
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Notes
Another use of structural formulas that also depends on their one-to-one correspondence with the compounds that they denote is in explaining or predicting the number of isomers of particular sorts. For example, by noticing that there are only four distinct structural formulas with a particular composition and connectivity, one can predict that there should be at most four stereoisomers with this composition and connectivity. It was explanatory/predictive successes of this sort that originally gave credibility to the idea that structural formulas represent the ‘real’ physical structure of molecules. See (Brock 2000, pp. 257–269) for some of the early explanatory successes of structural formulas.
As we shall see later on, there are important ways in which the structural formula is more versatile than the ball-and-stick model. After electronic theories of bonding were developed early in the 20th century, structural formulas were supplemented so that they could be used to represent (to a certain extent) the electronic distribution of a compound. It is much more difficult to conveniently supplement physical models with representations of the electronic distribution. See (Vollhardt 1994, pp. 14–19) for a presentation of one such supplement.
Hyperconjugation is itself explained as an effect due to the delocalization of electrons in chemical bonding (Lowry 1987, p. 68). Organic chemists frequently make use theories of chemical bonding in order to support (in the case of something like resonance stabilization) or rationalize (in the case of something like hyperconjugation) energy differences correlated with these robustly applicable concepts. These sorts of explanations do not fit comfortably within the model of explanation described in the text. Instead they are a supplement to the explanations characterized by that model.
Baeyer’s explanation was produced before the development of the background theoretical model that underwrites the direct answers of contemporary explanations in organic chemistry. Still, his explanation depends on the idea that more ‘stable’ compounds (those with less strain) are more likely to be found in nature and easier to produce. If stability is understood in terms of potential energy, this is perfectly consistent with the predictions of the contemporary theoretical model.
The low yields can be explained without invoking strain. Essentially, the ends of long chains are less likely to find one another, and ring closure competes with a lot of other possible outcomes; see (Ihde 1966, p. 151) for details.
As an anonymous reviewer helpfully pointed out, the concept of ‘strain’ has undergone substantially more refinement than that briefly mentioned in the text. In particular, organic chemists now distinguish additional varieties of strain, such as torsional and steric strain, which–like the angle strain described in this paper—can also be used to provide structural accounts of the energy differences relevant to explanations in organic chemistry.
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Goodwin, W.M. Structural formulas and explanation in organic chemistry. Found Chem 10, 117–127 (2008). https://doi.org/10.1007/s10698-007-9033-2
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DOI: https://doi.org/10.1007/s10698-007-9033-2