The Tyranny of lnterconversion
provocative opinion
For a long time now the teaching of introductory organic chemistry has sufferedthe tyrrany of interconversions. For every 100 texts introducing organic chemistry to students, there cannot be more than one or two that have really tried to break away from this tyranny. Early in the history of organic chemistry little else could have been expected. But for many years now we have known quite a lot about the fundamentals of molecular behavior and i t is reasonable to ask if there is not a more fundamental and desirable emphasis that could be given to introductory organic chemistry. Many teachers have, of course, felt the need to incorporate our more recent knowledge into their treatment of organic chemistry. For example, many have realized the importance of ideas about the nature of organic reactioneoften loosely called mechanistic ideas. However, in all but a few circumstances these ideas have been simply plugged into an existent framework based on the dominant importance of interconversions. The marvellous exception to these pathetic attempts to broaden the scope of organic chemistry is publicized in the text of Cram and Hammond. Many teachers also have realized the importance of ideas about structure and bonding, thermodynamics, and kinetics. It is recognized that the principles established in these areas are the basic principles controlling all molecular behavior. Correspondingly, many organic chemistry texts devote some pages at the beginning to a review of these principles. Almost invariably, however, after this reverent reference to the fundamental laws of Nature, organic chemistry texts then plunge into the familiar lawless jungles of the "real" organic chemistry, which to a beginner, we feel sure, does not appear to follow logically at all.' In our view both synthesis and mechanism are relatively derivative aspects of organic chemistry and neither, therefore, is suitable as a framework of organization for introducing organic chemistry. The fundamentals of organic chemistry are the same as the fundamentals of any part of chemistry-structure and bonding, thermodynamics, and kinetics-and it is these which should properly provide the framework for Similar remarks apply to many inorganic chemistry texts. We are not aware, however, of any organic chemistry text which does this! * I t is not a historical accident, in other words, although few texts actually suggest why they begin with alkanes. Hetero atoms are meant to be stoms other than carbon and hydrogen. Our approach demands that one explores the Periodic Table as widely as possible. Every day that passes shows that there is less and less reason for restricting discussion of hetero atoms to those conventionally treated in organic chemistry texts. We are all for establishing strong links with inorganic chemistry. 1
174 / Journal o f Chemicol Educofion
-CH%-CH-X
-CH=CHs
and
Coordination no. of C = 4
N; v%it orbitals Coordination no. ofC=2or3
-CH2--CH=X snd - C H n - m Weak C-X s bond IV Lone pa~r(s)and/or vacant orhitalb) on Coordnmtion no. of C = 2 or 3
x
chwacteri$tisr of the four fundamental closrer of organic cornIln I!, the C-F and C-8 a bonds ore exceptionr.1
discussion. This conclusion appears to be unavoidable but, of course, it still remains to consider how i t might be implemented. Our own solution at this stage is set out briefly below. The unique chemistry of carbon is a chemical phenomenon whose exploration provides the logical starting point for any introductory discussion of organic chemi ~ t r y . This ~ exploration, conducted with the principles of bonding, thermodynamics, and kinetics as tools, reveals that saturated hydrocarbon structures are unique in their conjunction of restraints on reaction. The basic reasons for the remarkable occurrence of organic chemistry are thereby exposed. But more than that, the discussion itself suggests that in a logical review of organic chemistry, the alkanes are the natural. starting point.a It also suggests that it is logical to investigate how molecular behavior changes as the structural, thermodynamic, and kinetic restraints are progressively relaxed. This relaxation occurs wherever hetero atoms or multiple bonds i n t r ~ d ebecause ,~ these are structural units which are invariably associated with either weaker bonds, or the occurrence of non-bonding orbitals, or lower coordination numbers, or combinations of these features. These considerations lead to the identification of four fundamental classes of organic compounds namely
I. saturated compounds containing no hetero atoms (hydrocarbons) 11. saturated monofunctional compounds containing a. hetero atom (X) sinelv bonded to carbon 111. unsaturated coApbun&"contsining no hetero atoms (hydrocarbons) IV. unsaturturated monofunctional compounds containing a. hetero atom (X) multiply bonded to carbon
The essential characteristics of these classes of compounds are summarized in the figure. All other organic compounds are combinations of two or more of these types. The effects of these features on a saturated hydrocarbon skeleton are explored for their structural, thermodynamic, and kinetic consequences, and the themes are exemplified to a modest extent in the known behavior of particular molecules. We do not refrain from mentioning the synthetic utility of a reaction if it is generally useful; nor do we avoid discussion of mechanism where it seems fruitful. But our emphasis is not in these areas. To our minds
themes such as synthesis and mechanism are better handled after the basic general facts of organic molecular behavior are established. The theme of synthesis is not covered by memorizing an arbitrary selection of interconversions; the theme of mechanism is not covered by arrow-pushing in a vacuum. They are complex themes if they are decently treated, and they are unsuitable for forming the basic framework for an introductory treatment of organic chemistry. We believe that the approach we recommend is the most logical for the introduction of organic chemistry. We believe also that this approach is ideally suited for showing organic chemistry in the context of chemistry as a whole, an aspect of increasing importance in these times. J. D. Bradley G. C. Gerrans University of Witwatersrand Johannesburg, South Africa
Volume 48, Number 3, March 1971
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