Organic Chemistry

When we say that a catalyst is neither consumed nor changed by a reaction, we do

not mean that it does not participate in the reaction. A catalyst mustparticipate in the

reaction if it is going to make it go faster. What we mean is that a catalyst has the same

form after the reaction that it had before the reaction. Because the catalyst is not used

up during the reaction—if it is used up in one step of the reaction, it must be regener-

ated in a subsequent step—only a small amount of the catalyst is needed. Therefore, a

catalyst is added to a reaction mixture in small catalyticamounts, much less than the

number of moles of reactant.

Notice that the stability of the original reactants and final products is the same in both

the catalyzed and uncatalyzed reactions. In other words, the catalyst does not change the

equilibrium constant of the reaction. (Observe that is the same for the catalyzed and

uncatalyzed reactions in Figures 24.1a, 24.1b, and 24.2.) Because the catalyst does not

change the equilibrium constant, it does not change the amountof product formed dur-

ing the reaction. It changes only the rateat which the product is formed.

PROBLEM 1

Which of the following parameters would be different for a reaction carried out in the pres-

ence of a catalyst, compared with the same reaction carried out in the absence of a catalyst?

(Hint:See Section 3.7.)

24.1 Catalysis in Organic Reactions

There are several ways a catalyst can provide a more favorable pathway for an organ-

ic reaction:

• It can increase the susceptibility of an electrophile to nucleophilic attack.


• It can increase the reactivity of a nucleophile.


• It can increase the leaving ability of a group by converting it into a weaker base.

In this chapter, we will look at some of the most common catalysts—nucleophilic cat-

alysts, acid catalysts, base catalysts, and metal-ion catalysts—and the ways in which

they provide an energetically more favorable pathway for an organic reaction. We will

then see how the same modes of catalysis are used in enzyme-catalyzed reactions.

¢G°, ¢H‡, Ea, ¢S‡, ¢H°, Keq, ¢G‡, ¢S°, krate

¢G°

3. The catalyst can completely change the mechanismof the reaction, providing an
   alternative pathway with a smaller than that of the uncatalyzed reaction
   (Figure 24.2).

¢G‡

1000 CHAPTER 24 Catalysis

Free energy

Progress of the reaction

∆G‡catalyzed ∆G‡uncatalyzed

Figure 24.2N
Reaction coordinate diagrams for
an uncatalyzed reaction and for a
catalyzed reaction. The catalyzed
reaction takes place by an
alternative and energetically more
favorable pathway.