Additivity of Effective Charge in Reactants and Products

Knowledge of the transition structure of a reaction is the goal of mechanistic studies but it presents an immediate problem. The transition 

Structure is an average structure which is taken up by molecular systems as they pass from reactant to product it cannot be studied in the same way as for a molecule because it is not a discrete species and cannot be isolated even in principle (see Chapter 1). It is impossible to measure attributes of the transition structure in the same way that can be done for regular collections of molecules. Since the transition state can be considered as if it were an equilibrium state it is possible to define its effective charges in the same way as those just considered for reactant and product molecules in equilibrium reactions. Equation (28) has rate constants for breakdown of the transition state species (represented as J ) forward kf) and return kf) which are essentially invariant because they register the collapse of the transition structure. These rate constants are independent of substituent changes and are therefore associated with zero P values. The equilibrium constants for formation of the transition state kfk and for its breakdown to products kjk f vary only according to changes in k and A , . 

The polar effects on the rate constants A , and k x (corresponding to pi and P, respectively) therefore measure changes in effective charge from reactant or product to the transition structure. 

The effective charge in a hypothetical general base-catalysed reaction (Equations 29 and 30) is given by Equations (31) and (32) = d, = 

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