Conditions to Create an LFER

In principle, the reaction constant Q does not have to be a constant for all substituents, because LFERs do not derive from any thermodynamic law. However, chemists find empirically that it most often is. When Q is constant, it means that two different reactions respond to all substituents in the same manner, but just to differing extents. To understand how this can be possible, we must substitute AG = AH -TAS into Eq. 8.60, giving Eq. 8.63. 

63 has two independent variables, AAH and AAS AAH and AAS are already determined) and therefore can only hold when one of three conditions are met. 

I Second, the AASs are coincidentally the same for both reactions and the AAHs are linearily proportional. 

This is actually the most common situation, and is referred to as the enthalpy-entropy compensation effect (see Chapter 4). 

Before exploring why enthalpy-entropy compensation effects exist, it is instructive to delineate the temperature dependence of LFERs. Because temperature dependence has been mostly explored with regard to kinetics, we present this discussion below in terms of activation parameters, but it also applies to reaction free energies. 

---