Energetics, kinetics, and the investigation of mechanism

1 Reaction rate and free energy of activation, p. 37 2.2.2 Kinetics and the rate-limiting step, p. 39 2.2.3 Kinetic v. thermodynamic control, p. 42. 

We have now listed a number of electronic and steric factors that can influence the reactivity of a compound in a given situation, and also the types of reagent that might be expected to attack particular centres in such a compound especially readily. We have as yet, however, had little to say directly about how these electronic and steric factors, varying from one structure to another, actually operate in energetic and kinetic terms to influence the course and rate of a reaction. These considerations are of major importance, not least for the light they might be expected to throw on the detailed pathway by which a reaction proceeds. 

That this should be so is a corollary of the Second Law of Thermodynamics which is concerned essentially with probabilities, and with the tendency for ordered systems to become disordered a measure of the degree of disorder of a system being provided by its entropy, S. In seeking their most stable condition, systems tend towards minimum energy (actually enthalpy, H) and maximum entropy (disorder or randomness), a measure of their relative stability must thus embrace a compromise between H and S, and is provided by the Gibb s free energy, G, which is defined by, 

The AH factor for the change can be equated with the difference in energies between the bonds in the starting materials and the bonds in the products, and an approximate value of AH for a reaction can often be predicted from tables of standard bond energies which is hardly unexpected, as it is from AH data that the average bond energies were compiled in the first place  

AG will of course be even more negative, and the equilibrium constant, K, correspondingly larger still. Where the number of participating species decreases on going from starting materials to products there is likely to be a decrease in entropy (AS negative) hence, 

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Energetics, kinetics, and the investigation of mechanism stable (lower energy), intermediate ... 

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