Heuristics in EROS

However, there are still important reactivity features which have so far been neglected by the reactivity functions, but yet which must be accounted for even at this stage of development if a sensible overall approach is to result. An important case concerns the special position of the hydrogen atom, and its ion, the proton. Its peculiar role in chemistry is reflected particularly in the way that even weakly basic solvents are able to interact with, and stabilize, it to a degree sufficient to render it a common and feasible independent entity in chemical reactions. This is in marked contrast to simple alkyl group ions, such as the methyl cation, whose electronic properties in many respects are very similar to those of the proton. Our current level of model development does not reflect this difference, and so specific allowance must be made artificially for the proton. 

This is achieved by the inclusion of heuristic rules . These are general rules-of-thumb which owe nothing to theory or an understanding of basic underlying principals, but which emanate from empirical observations of relevant systems (i.e. chemical reactions). (For further discussion of heuristics, see Sect. 11.) 

The most important heuristics relate to reaction conditions, in particular, to acid-base catalysis. Depending on whether acidic or basic conditions are specified, the reactivity of certain bonds is changed. As an example, under basic conditions the breakability of H-X bonds is increased in comparison with other bonds. In fact, the relative acidities of all H-X bonds (X = any other element) can be rapidly calculated in EROS, and this allows further distinction within this class of bonds. 

EROS also contains other less important, more formal heuristics which together 

The approach which is now available is sufficiently general to be applied to organic reactivity in a much broader sense. The examples in the next section show how the reactivity functions perform in EROS when confronted with reactivity problems of several types. 

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