Status of Ionic Reaction Mechanisms

The chemical behavior of ions, ion pairs, and polarizable molecules partakes of the same indistinctness as the definitions of these species. Any attempt to make a complete catalog of the reactions of ions will almost certainly include borderline reactions whose intermediates are in fact ion-pairs or even covalent molecules. For many purposes the identification of a reaction as carbonium ion-like, or what the Germans would call Krypto-ionenreaktion, is as useful as the certain knowledge that the intermediate is actually a carbonium ion. Many of the ionic reaction mechanisms in the literature do not represent actual free ions and were not so intended by their authors. The ionic representation is often merely a convenient simplification if it is an oversimplification it is one that is easily rectified when the pertinent data become available. The value of such approximate mechanisms is that 

The role of approximate mechanisms in organic chemistry is somewhat like that of the perfect gas laws in physical chemistry. The fact that an approximate mechanism has some value does not of course mean that precise mechanisms are not still better. 

Most of the discussion of ions in this book will be concerned with large complicated ions in the liquid phase rather than with small simple ions in the vacuum of the mass spectrometer. Organic chemistry is the chemistry of complicated molecules and for this reason the organic chemist will be most interested in the large radicals and ions whose usual habitat is the liquid phase. Perhaps this is why the boundary between physical and organic chemistry has somewhere been defined as the liquid-vapor interface. Certainly it is only in the amicable sense of a preoccupation with his natural habitat that the organic chemist should regard physical chemistry with a fishy eye. 

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