Organic Chemistry

Section 3.7 Thermodynamics and Kinetics 125

PROBLEM 12

Use curved arrows to show the movement of electrons in each of the following reaction steps:

a.

b.

c.

d.

3.7 Thermodynamics and Kinetics

Before we can understand the energy changes that take place in a reaction such as the

addition of HBr to an alkene, we must have an understanding of thermodynamics,

which describes a reaction at equilibrium, and an appreciation of kinetics, which deals

with the rates of chemical reactions.

If we consider a reaction in which Y is converted to Z, the thermodynamicsof the

reaction tells us the relative amounts of Y and Z that are present when the reaction has

reached equilibrium, whereas the kineticsof the reaction tells us how fast Y is convert-

ed into Z.

Reaction Coordinate Diagrams

The mechanism of a reaction describes the various steps that are believed to occur as

reactants are converted into products. A reaction coordinate diagramshows the

energy changes that take place in each of the steps of the mechanism. In a reaction

coordinate diagram, the total energy of all species is plotted against the progress of

the reaction. A reaction progresses from left to right as written in the chemical equa-

tion, so the energy of the reactants is plotted on the left-hand side of the x-axis and

the energy of the products is plotted on the right-hand side. A typical reaction coor-

dinate diagram is shown in Figure 3.2. The diagram describes the reaction of A—B

with C to form A and B—C. Remember that the more stable the species, the lower is

its energy.

As the reactants are converted into products, the reaction passes through a

maximumenergy state called a transition state. The structure of the transition state

lies somewhere between the structure of the reactants and the structure of the products.

Bonds that break and bonds that form, as reactants are converted to products, are

partially broken and partially formed in the transition state. Dashed lines are used to

show partially broken or partially formed bonds.

reactants products

AB+ CABC+

YZ

CH 3

CH 3

CH 3

CH 3

CH 3 CClCl CH 3 C+ + −

H

CH 3 COH HHO

O

+ + H 2 O

+

CH 3 COH

+OH

Br+

Br

+

+

CH OH HO−

3 C

O

+ CH 3 C O− H 2 O

O

+

The more stable the species,

the lower is its energy.

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