Page 272 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

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Structural Effects on Stability

and Reactivity

Introduction

The concepts of stability and reactivity are fundamental to understanding chemistry.
In this chapter we consider first the thermodynamic definition of chemical stability.
We then consider chemical kinetics (Section 3.2) and how it can provide information
about reactivity. We also explore how structure influences stability and reactivity. We
want to learn how to make predictions about reactivity based on the structure of the
reactants and intermediates. We begin by reviewing the principles of thermodynamics
and kinetics, which provide the basis for understanding the relationship of structure to
stability and reactivity.
Reactions are usefully described in terms of potential energy diagrams such as
shown in Figure 3.1, which identify the potential energy changes associated with the
reacting molecules as they proceed to products. The diagram plots the free energy
of the system as a function of the progress of the reaction. For each individual step
in the reaction there is a transition state representing the highest energy arrangement
of the molecules for that step. The successive intermediates are the molecules that
are formed and then react further in the course of the overall reaction. The energies
of the transition states relative to the reactants determine the rate of reaction. The
energy difference between the reactants and products is G, the free-energy change
associated with the reaction. The free energy of a chemical reaction is defined by the
equation

G = H −T S (3.1)

where H is the enthalpy change and S is the entropy change for the reaction. The
enthalpy term is a measure of the stability of the molecule and is determined by the
strength of the chemical bonds in the structure. The entropy term specifies the change
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