Organic Chemistry

136 CHAPTER 3 Alkenes• Thermodynamics and Kinetics

Free energy

CH 2 CH 3

Br

+

CH 3 CH

CH 3 CHCH 2 CH 3

Progress of the reaction

−∆G°

intermediate

Br−

CH 3 CH CHCH 3

HBr

Figure 3.6N

Reaction coordinate diagram for

the addition of HBr to 2-butene.

diagram for the overall reaction (Figure 3.6). The for the overall reaction is the

difference between the free energy of the final products and the free energy of the ini-

tial reactants. The figure shows that for the overall reaction is negative. There-

fore, the overall reaction is exergonic.

¢G°

¢G°

Transition states have partially

formed bonds. Intermediates

have fully formed bonds.

A chemical species that is the product of one step of a reaction and is the reactant

for the next step is called an intermediate. The carbocation intermediate in this reac-

tion is too unstable to be isolated, but some reactions have more stable intermediates

that can be isolated. Transition states, in contrast, represent the highest-energy struc-

tures that are involved in the reaction. They exist only fleetingly and can never be iso-

lated. Do not confuse transition states with intermediates:Transition states have

partially formed bonds, whereas intermediates have fully formed bonds.

We can see from the reaction coordinate diagram that the free energy of activation

for the first step of the reaction is greater than the free energy of activation for the sec-

ond step. In other words, the rate constant for the first step is smaller than the rate con-

stant for the second step. This is what you would expect because the molecules in the

first step of this reaction must collide with sufficient energy to break covalent bonds,

whereas no bonds are broken in the second step.

The reaction step that has its transition state at the highest point on the reaction co-

ordinate is called the rate-determining step or rate-limiting step. The rate-

determining step controls the overall rate of the reaction because the overall rate

cannot exceed the rate of the rate-determining step. In Figure 3.6, the rate-determining

step is the first step—the addition of the electrophile (the proton) to the alkene.

Reaction coordinate diagrams also can be used to explain why a given reaction

forms a particular product, but not others. We will see the first example of this in

Section 4.3.

PROBLEM 24

Draw a reaction coordinate diagram for a two-step reaction in which the first step is ender-

gonic, the second step is exergonic, and the overall reaction is endergonic. Label the reac-

tants, products, intermediates, and transition states.

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