Organic Chemistry

Key Terms 1029

Key Terms

acid catalyst (p. 000)
active site (p. 1015)
acyl-enzyme intermediate (p. 1020)
base catalyst (p. 1006)
catalyst (p. 999)
covalent catalysis (p. 1001)
effective molarity (p. 1010)
electrophilic catalyst (p. 1008)
electrostatic catalysis (p. 1020)

enzyme (p. 1015)

general-acid catalysis (p. 1004)

general-base catalysis (p. 1006)

induced-fit model (p. 1016)

intramolecular catalysis (p. 1012)

lock-and-key model (p. 1015)

metal-ion catalysis (p. 000)

molecular recognition (p. 1015)

nucleophilic catalysis (p. 1001)

nucleophilic catalyst (p. 1001)

pH–activity profile (p. 1024)

pH–rate profile (p. 1024)

relative rate (p. 1010)

site-specific mutagenesis (p. 1020)

specific-acid catalysis (p. 1004)

specific-base catalysis (p. 1006)

substrate (p. 1015)

Summary

A catalystincreases the rate of a chemical reaction but is not
consumed or changed in the reaction. A catalyst changes the
rate at which a product is formed, not the amount of product
formed. A catalyst must increase the rate of a slow step. It
does this by providing a pathway with a lower To pro-
vide a lower a catalyst can convert the reactant into a
less stable species, make the transition state more stable, or
completely change the mechanism of the reaction. Some of
the ways a catalyst provides a more favorable pathway for a
reaction is by increasing the susceptibility of an electrophile
to nucleophilic attack, increasing the reactivity of a nucleo-
phile, or increasing the leaving ability of a group.
A nucleophilic catalystincreases the rate of a reaction
by acting as a nucleophile: It forms an intermediate by form-
ing a covalent bond with a reactant. Stabilization of a charge
by an opposite charge is called electrostatic catalysis. An
acid catalyst increases the rate of a reaction by donating a
proton to a reactant. There are two types of acid catalysis: In
specific-acid catalysis, the proton is fully transferred to the
reactant before the slow step of the reaction; in general-acid
catalysis, the proton is transferred during the slow step. A
base catalystincreases the rate of a reaction by removing a
proton from the reactant. There are two types of base cataly-
sis: In specific-base catalysis, the proton is completely
removed from the reactant before the slow step of the
reaction; in general-base catalysis, the proton is removed
during the slow step.
A metal ioncan increase the rate of a reaction by making
a reaction center more susceptible to receiving electrons, by
making a leaving group a weaker base, or by increasing the
nucleophilicity of water. An electrophilic catalystis a metal
ion with the same catalytic effect as a proton.
The rate of a chemical reaction is determined by the
number of collisions between two molecules or between

¢G‡,

¢G‡.

two intramolecular constituents with sufficient energy and

with the proper orientation in a given period of time. An

intramolecular reaction that forms a five- or a six-

membered ring occurs more readily than the analogous

intermolecular reactionbecause both the frequency of the

collisions and the probability that collisions will occur in

the proper orientation increases. Effective molarityis the

concentration of the reactant that would be required in an

intermolecular reaction for it to have the same rate as the

corresponding intramolecular reaction. When a catalyst is

part of the reacting molecule, the catalysis is called

intramolecular catalysis; intramolecular nucleophilic

catalysis, intramolecular general-acid or general-base catal-

ysis, and intramolecular metal-ion catalysis all are possible.

Essentially all organic reactions that occur in biological

systems require a catalyst. Most biological catalysts are

enzymes. The reactant of an enzyme-catalyzed reaction is

called a substrate. The substrate specifically binds to the

active siteof the enzyme, and all the bond-making and

bond-breaking steps of the reaction occur while it is at that

site. The specificity of an enzyme for its substrate is an ex-

ample of molecular recognition. The change in conforma-

tion of the enzyme when it binds the substrate is known as

induced fit.

Two important factors contributing to the remarkable cat-

alytic ability of enzymes are that reacting groups are brought

together at the active site in the proper orientation for reac-

tion and that the amino acid side chains and a metal ion in

the case of some enzymes are in the proper position relative

to the substrate needed for catalysis. Information about the

relationship between the structure of a protein and its func-

tion has been determined by site-specific mutagenesis. A

pH–rate profileis a plot of the activity of an enzyme as a

function of the pH of the reaction mixture.

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anchimeric assistance (p. 1012)

metal ion (p. 1007)

gem-dialkyl effect (p. 1012)