Organic Chemistry

Section 3.7 Thermodynamics and Kinetics 131

Free energy

ab

cd

Free energy

Progress of the reaction

−∆G°

−∆G°

+∆G°

+∆G°

Progress of the reaction

∆G‡

∆G‡

∆G‡

∆G‡

>Figure 3.4

Reaction coordinate diagrams for

(a) a fast exergonic reaction, (b) a

slow exergonic reaction, (c) a fast

endergonic reaction, and (d) a slow

endergonic reaction. (The four

reaction coordinates are drawn on

the same scale.)

anything about the energy barrier of the reaction, which is the energy “hill”that must

be climbed for the reactants to be converted into products. The higher the energy bar-

rier, the slower is the reaction. Kineticsis the field of chemistry that studies the rates

of chemical reactions and the factors that affect those rates.

The energy barrier of a reaction, indicated in Figure 3.4 by is called the free

energy of activation. It is the difference between the free energy of the transition state

and the free energy of the reactants:

The smaller the the faster is the reaction. Thus,anything that destabilizes the

reactant or stabilizes the transition state will make the reaction go faster.

Like has both an enthalpy component and an entropy component.

Notice that any quantity that refers to the transition state is represented by the double-

dagger superscript

Some exergonic reactions have small free energies of activation and therefore can

take place at room temperature (Figure 3.4a). In contrast, some exergonic reactions

have free energies of activation that are so large that the reaction cannot take place

without adding energy above that provided by the existing thermal conditions

(Figure 3.4b). Endergonic reactions can also have either small free energies of activa-

tion, as in Figure 3.4c, or large free energies of activation, as in Figure 3.4d.

¢S‡=(entropy of the transition state)-(entropy of the reactants)

¢H‡=(enthalpy of the transition state)-(enthalpy of the reactants)

¢G‡=¢H‡-T¢S‡

(‡):

¢G°,¢G‡

¢G‡,

¢G‡=(free energy of the transition state)-(free energy of the reactants)

¢G‡,

Notice that relates to the equilibrium constantof the reaction, whereas

relates to the rateof the reaction. The thermodynamic stabilityof a compound is in-

dicated by If is negative, for example, the product is thermodynamically

stable compared with the reactant, and if is positive, the product is

thermodynamically unstablecompared with the reactant. The kinetic stabilityof a

compound is indicated by If for a reaction is large, the compound is

kinetically stablebecause it does not undergo that reaction rapidly. If is small,

the compound is kinetically unstable—it undergoes the reaction rapidly. Generally,

when chemists use the term “stability,”they are referring to thermodynamic stability.

¢G‡

¢G‡. ¢G‡

¢G°

¢G°. ¢G°

¢G° ¢G‡

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