Post on 03-Jul-2015
Organic Chemistry 4th Edition
Paula Yurkanis Bruice
Chapter 24
Catalysis
Irene LeeCase Western Reserve
UniversityCleveland, OH
©2004, Prentice Hall
A catalyst is a substance that increases the rate of areaction without itself being consumed or changed
A catalyst increases the rate of the reaction by loweringthe ∆G‡ of the reaction
A catalyst can decrease ∆G‡ of the reaction by one of three different ways
Catalyst
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
Both the formation of the acyl imidazole and its subsequent hydrolysis are both faster than esterhydrolysis
Nucleophilic Catalysis
An Acid-Catalyzed Reaction
A proton is donated to the reaction
The acid increases the rates of both slow steps of thereaction
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 General-Acid 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 stabilizationof 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 duringthe 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 thanwater
The metal ion also decreases the basicity of the leavinggroup
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 relativerate of B because the groups in D are less apt to adoptan unfavorable conformation for the reaction
Putting a reacting group and a catalyst in the samemolecule 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-carboxylsubstituent 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 uponbinding to a substrate
red: before substrate-bindinggreen: 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 theC-terminal peptide
The binding pocket at the active site of serine proteasesdictates 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 thepKa values of the catalytic groups in the enzyme
a group iscatalytically
active in its basicform
a group iscatalytically
active in its acidicform
Glucose-6-phosphate Isomerase