Ions and Ion Pairs. Their Meaning and Significance in Organic Reactions

A brief survey of the developments leading to the concept of ion pairs and of still higher ionic aggregates is followed by detailed discussion of its physical meaning. The distinction between contact ion pairs and covalently bonded molecules is clarified by comparing gaseous dissociation processes with those taking place in solution. Such a comparison reveals clearly the significance and the role of solvent molecules in homolytic and heterolytic dissociations. In the following discussion it is pointed out that a variety of ion pairs may be formed from the pair of oppositely charged ions. An attempt is made to explain under what conditions the structurally different ion pairs may be treated as distinct species and when such a distinction fails. Finally, examples are given to demonstrate how the structural changes of ion pairs nature may affect the rates and the equilibria of reactions in which they participate, an effort being made to provide examples pertaining to polymerization processes. The Symposium on Ring-Opening Polymerization is concerned with reactions belonging to the class of ionic processes. The active carriers responsible for such reactions include ions, ion pairs, or their higher aggregates, and hence the discussion of the nature and behaviour of these species provides an appropriate subject for the opening remarks to this gathering. It is my intention to clarify the meaning and the significance of some concepts used in the description of ionic species, and characterize their nature and their behaviour especially under the conditions encountered in polymerizing systems. The concept of free ions—molecules or molecular fragments endowed with some electric charge—was envisaged by Arrhenius about a century ago. He correctly deduced that neutral salts, e.g. sodium chloride, dissociate into positive cations and negative anions when dissolved in an appropriate solvent like water. Since in solution the cations and anions become independent of each other, they move in opposite directions under the influence of any electric field, the cations drifting towards cathode while anions tend to move towards anode. Their directional motion represents an electric current, and thus, the salt solution becomes conducting whereas the solvent is not. This conversion of a non-conducting solvent into a conducting solution upon dissolution of a salt is the striking verification of the idea of ionic dissociation. Subsequent studies led to the conclusion that the ionic dissociation of inorganic salts in water is quantitative. The variation of the equivalent conductance of such solutions with salt's concentration was then accounted for in terms of electrostatic interactions between the ions, resulting in creation of "ionic atmosphere"—an increase in the local concentration of cations around an anion and vice versa. In contrast to the behaviour of inorganic salts in water, the aqueous solutions of organic acids, bases and some other compounds revealed the phenomenon of partial ionic dissociation, its degree being governed by the masslaw as originally proposed by Arrhenius. This diverse behaviour of salts on the one hand and organic acids on the other led to the classification of the compounds capable of forming free ions into two classes, namely, strong electrolytes and weak electrolytes. The former 247 quantitatively dissociate into ions when dissolved in an appropriate solvent, whereas the degree of ionic dissociation of the latter is given by the mass-law. This classification led Fuosst to suggest the term of ionophores for the compounds forming ionic crystals, and belonging therefore to the class of strong electrolytes, and the term ionogenes for those forming molecular crystals and liquids. This terminology, although highly useful, is not without a flow. There are compounds forming ionic crystals which exist nevertheless in solutions in molecular form and not as ions. For example, crystals of nitrogenpentoxide are built from positive NO2 and negative N03 ions, but the solutions of nitrogenpentoxide contain the covalently bonded N205 molecules. It should be also emphasized that a hypothetical slow expansion of an ionic lattice, a process simultaneously increasing the distances between all the ions, leads ultimately to the formation of separated atoms and notions. The formation of free ions on dissolving such a crystal in water results from hydration of the ions that prevents the redistribution of the charges occuring in the previous process. Continuation of studies of ionic solutions led to unexpected observations. For example, Kraus2 reported that liquid ammonia solutions of typical ionophores like NaC1 behave like those of weak electrolytes. These observations led Bjerrum3 to postulate that two oppositely charged ions may form in a solution an ion pair, a species which does not contribute to the conductance. Indeed, the early studies of ion pairs utilized the conductance data as a source of information about ion pairs. Thus, ion pairs were recognized not by their action but by the lack of action, namely, their inability to conduct electric current—surely, a highly unsatisfactory approach to characterization of a new species. This negative approach to ion pairs is reflected in a somewhat unsatisfactory definition of ion pairs originally introduced by Bjerrum and refined by Fuoss and others.4 Such a definition overemphasizes the Coulombic interaction between the positive and negative ions, and deprives ion pairs, at least to some extent, of their identity as distinct chemical species. Positive recognition of ion pairs through their characteristic properties was reported 20 yr after their existence