Chapter 23 Catalysis in Organic Reactions and in

Catalyst A catalyst is a substance that increases the rate of a reaction without

The Catalyst Converts the Reactant to a Less Stable (More Reactive) Species © 2014

The Catalyst Stabilizes the Transition State © 2014 Pearson Education, Inc.

The Catalyst Changes the Mechanism of the Reaction The catalyzed reaction takes place by

How a Catalyst Can Provide a More Favorable Pathway for a Reaction It can

An Acid-Catalyzed Reaction An acid catalyst increases the rate of a reaction by donating

How an Acid Catalyzes the First Slow Step A catalyst must increase the rate

How an Acid Catalyzes the Second Slow Step The catalyst decreases the basicity of

Specific-Acid and General-Acid Catalysis In specific-acid catalysis, the proton is transferred to the reactant

Comparing Specific-Acid Catalysis with General-Acid Catalysis © 2014 Pearson Education, Inc.

Specific-Acid and General-Acid Catalysis A specific-acid catalyst must be a strong acid. A general-acid

Specific-Base Catalysis A base catalyst increases the rate of a reaction by removing a

General-Base Catalysis In general-base catalysis, the proton is removed from the reactant during the

How Base Catalysis Increases the Rate of the Reaction The transition state for elimination

Nucleophilic Catalysis A nucleophilic catalyst increases the rate of a reaction by acting as

Nucleophilic Catalysis Iodide ion is a nucleophilic catalyst. © 2014 Pearson Education, Inc.

The Mechanism for the Uncatalyzed Reaction © 2014 Pearson Education, Inc.

Mechanism for the Iodide-Ion Catalyzed Reaction © 2014 Pearson Education, Inc.

Nucleophilic Catalysis © 2014 Pearson Education, Inc.

Nucleophilic Catalysis Imidazole is a better nucleophile than water. An acyl imidazole hydrolyzes more

Metal-Ion Catalysis A A metal ion makes a reaction center more susceptible to receiving

p. Ka of Metal-Bound Water Metal-bound hydroxide ions are better nucleophiles than water. ©

Metal-Ion Catalyzed Decarboxylation © 2014 Pearson Education, Inc.

Metal-Ion Catalyzed Ester Hydrolysis Metal-bound hydroxide is a better nucleophile than water. The metal

The Relative Rate of a Reaction The relative rate is also called the effective

Why Five-Membered Ring Formation is Faster Than Six-Membered Ring Formation © 2014 Pearson Education,

Intramolecular Catalysis Putting a reacting group and a catalyst in the same molecule increases

An Uncatalyzed Reaction © 2014 Pearson Education, Inc.

The Thio Substituent is a Nucleophilic Catalyst If the thio substituent is trans, the

Intramolecular Nucleophilic Catalysis © 2014 Pearson Education, Inc.

The Carboxyl Group is a Catalyst © 2014 Pearson Education, Inc.

An Intramolecular General-Base Catalyst © 2014 Pearson Education, Inc.

An Intramolecular Nucleophilic Catalyst © 2014 Pearson Education, Inc.

An Enzyme Binds its Substrate at its Active Site The reactant is called a

Lock and Key versus Induced Fit © 2014 Pearson Education, Inc.

The Conformation Changes After the Enzyme Binds its Substrate red = the structure of

Factors That Contribute to the Catalytic Ability of Enzymes Reacting groups are brought together

Carboxypeptidase A (an exopeptidase) © 2014 Pearson Education, Inc.

Endopeptidases The binding pocket at the active site of serine proteases determines the substrate

Site-Specific Mutagenesis (a technique that replaces one amino acid with another) When Asp 102

Lysozyme (an enzyme that destroys bacterial cell walls) © 2014 Pearson Education, Inc.

The amino acids at the active site of lysozyme are involved in binding the

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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 decreasing the G‡ of the reaction. There are three ways a catalyst can decrease G‡. © 2014 Pearson Education, Inc.

How a Catalyst Can Provide a More Favorable Pathway for a 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 propensity of a group by converting it to a weaker base. It can increase the stability of a transition state. © 2014 Pearson Education, Inc.

How an Acid Catalyzes the First Slow Step A catalyst must increase the rate of a slow step. The catalyst increases the electrophilicity of the carbonyl carbon (makes it more susceptible to nucleophilic addition). © 2014 Pearson Education, Inc.

Specific-Acid and General-Acid Catalysis In specific-acid catalysis, the proton is transferred to the reactant before the slow step. In general-acid catalysis, the proton is transferred to the reactant during the slow step. © 2014 Pearson Education, Inc.

Specific-Base Catalysis A base catalyst increases the rate of a reaction by removing a proton. In specific-base catalysis, the proton is removed from the reactant before the slow step. © 2014 Pearson Education, Inc.

How Base Catalysis Increases the Rate of the Reaction The transition state for elimination of a hydroxide ion from a negatively charged intermediate is more stable because a positive charge does not develop on the oxygen. © 2014 Pearson Education, Inc.

Nucleophilic Catalysis A nucleophilic catalyst increases the rate of a reaction by acting as a nucleophile to completely change the mechanism of the reaction. A nucleophilic catalyst forms an intermediate by forming a covalent bond with the reactant. Nucleophilic catalysis is also known as covalent catalysis. © 2014 Pearson Education, Inc.

Nucleophilic Catalysis Imidazole is a better nucleophile than water. An acyl imidazole hydrolyzes more rapidly than an ester because imidazole is a better leaving group than the phenolate ion. © 2014 Pearson Education, Inc.

Metal-Ion Catalysis A A metal ion makes a reaction center more susceptible to receiving electrons. B A metal ion can make a leaving group a weaker base. C A metal ion can make water a better nucleophile. © 2014 Pearson Education, Inc.

The Relative Rate of a Reaction The relative rate is also called the effective molarity. The effective molarity is the advantage given to a reaction by having the reacting groups in the same molecule. © 2014 Pearson Education, Inc.

The Thio Substituent is a Nucleophilic Catalyst If the thio substituent is trans, the compounds react 70, 000 times faster than the unsubstituted compound. If the thio substituent is cis, the compounds react at about the same rate as the unsubstituted compound. © 2014 Pearson Education, Inc.

The Conformation Changes After the Enzyme Binds its Substrate red = the structure of the enzyme before it binds its substrate green = the structure of the enzyme after it has bound its substrate © 2014 Pearson Education, Inc.

Factors That Contribute to the Catalytic Ability of Enzymes Reacting groups are brought together at the active site in the proper orientation for reaction. The amino acid side chains that act as catalysts are positioned relative to the substrate precisely where they are needed. Amino acid side chains can stabilize transition states and intermediates. © 2014 Pearson Education, Inc.

Site-Specific Mutagenesis (a technique that replaces one amino acid with another) When Asp 102 of chymotrypsin is replaced by Asn 102, the catalysis drops to 0. 05%. Therefore, Asp 102 must be involved in the catalytic process. © 2014 Pearson Education, Inc.