Structural Effects on Stability and Reactivity. Organic Chemstry Laboratory
Structural Effects on Stability and Reactivity. Organic Chemstry Laboratory
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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254
CHAPTER 3 Structural Effects on Stability and Reactivity
C Energy ΔG ‡
A+B D+E
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
253
254
CHAPTER 3 Structural Effects on Stability and Reactivity
C Energy ΔG ‡
A+B D+E