Page 364 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

3.7. Catalysis 345

SECTION 3.7
Many chemical reactions involve a catalyst. A very general definition of a catalyst
Catalysis
is a substance that makes a reaction path available with a lower energy of activation.
Strictly speaking, a catalyst is not consumed by the reaction, but organic chemists
frequently speak of acid-catalyzed or base-catalyzed mechanisms that do lead to overall
consumption of the acid or base. Better phrases under these circumstances would be
acid promoted or base promoted. Catalysts can also be described as electrophilic or
nucleophilic, depending on the catalyst’s electronic nature. Catalysis by Lewis acids
and Lewis bases can be classified as electrophilic and nucleophilic, respectively. In
free-radical reactions, the initiator often plays a key role. An initiator is a substance
that can easily generate radical intermediates. Radical reactions often occur by chain
mechanisms, and the role of the initiator is to provide the free radicals that start the
chain reaction. In this section we discuss some fundamental examples of catalysis with
emphasis on proton transfer (Brønsted acid/base) and Lewis acid catalysis.

3.7.1. Catalysis by Acids and Bases

A detailed understanding of reaction mechanisms requires knowledge of the role
catalysts play in the reaction. Catalysts do not affect the position of equilibrium of a
o
reaction, which is determined by G and is independent of the path (mechanism) of
the transformation. Catalysts function by increasing the rate of one or more steps in the
reaction mechanism by providing a reaction path having a lower E . The most general
a
family of catalytic processes are those that involve transfer of a proton. Many reactions
are strongly catalyzed by proton donors (Brønsted acids) or proton acceptors (Brønsted
bases). Catalysis occurs when the conjugate acid or conjugate base of the reactant
is more reactive than the neutral species. As we discussed briefly in Section 3.4.4,
reactions involving nucleophilic attack at carbonyl groups are often accelerated by acids
or bases. Acid catalysis occurs when the conjugate acid of the carbonyl compound,
which is much more electrophilic than the neutral molecule, is the kinetically dominant
reactant. Base-catalyzed additions occur as a result of deprotonation of the nucleophile,
generating a more reactive anion.

+ H
Nu-H : Nu-H +
HO C Nu C O + H + C O Nu C OH + H
slow : : fast
acid catalysis of carbonyl addition by reactant protonation
B-H
:
B: – H-Nu B-H + Nu: – C O Nu C O – Nu C OH + B –
: fast
base catalysis of carbonyl addition by deprotonation of the nucleophile

Many important organic reactions involve carbanions as nucleophiles. The
properties of carbanions were introduced in Section 3.4.2, and their reactivity is
discussed in more detail in Chapter 6. Most C−H bonds are very weakly acidic and
have no tendency to ionize spontaneously to form carbanions. Reactions that involve
carbanion intermediates are therefore usually carried out in the presence of a base that
can generate the reactive carbanion intermediate. Base-catalyzed addition reactions of
carbonyl compounds provide many examples of this type of reaction. The reaction