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Organic Chemistry
4th Edition
Paula Yurkanis Bruice
Chapter 24
Catalysis
Irene Lee
Case Western Reserve University
Cleveland, OH
©2004, Prentice Hall
Catalyst
A catalyst is a substance that increases the rate of a
reaction without itself being consumed or changed
A catalyst increases the rate of the reaction by lowering
the DG‡ of the reaction
A catalyst can decrease DG‡ of the reaction by one of
three different ways
The Catalyst Converts the Reactant to a Less
Stable Species
The Catalyst Stabilizes the Transition State
The Catalyst Changes the Mechanism of
the Reaction
A catalyst can provide a more favorable pathway for
an organic reaction by:
• increasing the susceptibility of an electrophile to
nucleophilic attack
• increasing the reactivity of a nucleophile
• increasing the leaving ability of a group by
converting it to a weaker base
Nucleophilic Catalysis
Both the formation of the acyl imidazole and its
subsequent hydrolysis are both faster than ester
hydrolysis
An Acid-Catalyzed Reaction
A proton is donated to the reaction
The acid increases the rates of both slow steps of the
reaction
In specific-acid catalysis, the proton is fully transferred
before the slow step of the reaction
In general-acid catalysis, the proton is transferred during
the slow step of the reaction
Compare Specific-Acid Catalysis with GeneralAcid Catalysis
A specific-acid must be a strong acid
A general-acid can be a weaker acid
Base Catalysis
A base catalyst increases the rate of the reaction by
removing a proton from the reaction
specific-base catalyzed dehydration
The rate of the reaction is accelerated by stabilization
of the transition state
In specific-base catalysis, the proton is completely
removed before the slow step of the reaction
general-base catalysis
In general-base catalysis, the proton is removed during
the slow step of the reaction
Metal-Ion Catalysis
A. The metal ion increases the susceptibility of electron
attack
B. The metal ion makes the leaving group a weaker base
C. The metal ion increases the nucleophilicity of water
An Example of a Metal-Ion-Catalyzed Reaction
Metal-Ion-Catalyzed Decarboxylation
Metal-Ion-Catalyzed Ester Hydrolysis
The metal-bound hydroxide is a better nucleophile than
water
The metal ion also decreases the basicity of the leaving
group
The relative rates are also called the effective molarity
The effective molarity is the advantage given to a
reaction
The relative rate of reactant D is higher than the relative
rate of B because the groups in D are less apt to adopt
an unfavorable conformation for the reaction
Putting a reacting group and a catalyst in the same
molecule increases the rate of the reaction
Intramolecular catalysis is also known as anchimeric
assistance
The trans isomer reacts much faster than the cis isomer
The rate of phenyl acetate hydrolysis is enhanced by
an intramolecular general base catalysis
In the presence of nitro groups, the ortho-carboxyl
substituent acts as an intramolecular nucleophilic
catalyst
An Intramolecular Metal-Ion Catalysis
Most Biological Catalysts Are Enzymes
The reactants are called substrates
The substrate specifically fits and binds to the active site
Hexokinase undergoes a conformational change upon
binding to a substrate
red: before substrate-binding
green: after substrate-binding
Important Features that Contribute to the
Catalytic Ability of Enzymes
• Reacting groups are brought together at the active site
in the proper orientation for reaction
• Some of the amino acids in the enzyme serve as
catalytic groups; many enzymes have metal ions as
catalysts
• Groups on the enzyme can stabilize the transition state
of the reaction
Carboxypeptidase A catalyzes the hydrolysis of the
C-terminal peptide
The binding pocket at the active site of serine proteases
dictates substrate specificity
The Proposed Reaction Mechanism of a
Serine Protease
Lysozyme Is an Enzyme that
Destroys Bacterial Cell Walls
The amino acids at
the active site of
lysozyme are
involved in binding
the substrate
The Proposed Reaction Mechanism
for Lysozyme
The pH-rate profile of an enzyme is a function of the
pKa values of the catalytic groups in the enzyme
a group is
catalytically
active in its basic
form
a group is
catalytically
active in its acidic
form
Glucose-6-phosphate Isomerase