Download Chapter 23: Catalysis

Chapter 23: Catalysis
Zhang
Chem 109C
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Zhang
Chem 109C
Catalyst and Catalysis: Introduction
 Catalyst is a compound that by its addition to a reaction increases
the rate of the reaction without itself being consumed or
changed at the end of the reaction (cycle).
 The reaction is then called being catalyzed
catalyzed.
 The phenomenon of catalyst in accelerating reaction is called
catalysis.
Me
Me
HN
Ph3PAuNTf 2 (5 mol %)
MsOH (1.2 equiv), 4 Å MS
O
N
O
Me
Me
CH2Cl2
Me
Me
quantitative (NMR)/ 61% (isolated)
H+
Ph3PAu+
Me
HN
Me
Me
Me
Me
O
HN
Au
PPh3
O
H+
Me
Me
Ph3PAu+
Au
PPh3
HN
O
Me
Me
MsO
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Catalyst and Catalysis: Introduction
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Chem 109C
 Catalyst increases the reaction rate by providing a pathway with
lower free energy of activation (reaction kinetics)
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Catalysis in Organic Reactions
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Chem 109C
 In organic reactions, several ways to accelerate reactions by
catalysts:




Increase electrophilicity
Increase nucleophilicity
Increase leaving ability as a nucleofuge (decrease electrophilicity)
Increase the stability of a transition state
 Four types of catalysis:




acid catalysis
base catalysis
nucleophilic catalysis
metal-ion catalysis
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Acid Catalysis: Ester Hydrolysis
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Chem 109C
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Acid Catalysis
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Chem 109C
 Specific-acid catalysis and general acid catalysis
 Specific-acid catalysis: proton is fully transferred before the slow step of
the reaction (typically strong acidic conditions, two-stages)
 General acid catalysis: proton transfer to the reactant during the slow
step of the reaction (typically under weak acidic conditions, one stage)
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Acid Catalysis
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Chem 109C
 General-acid catalysis vs
vs. specific-acid catalysis
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Base Catalysis
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Chem 109C
 General-base catalysis vs
vs. specific-base catalysis
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Nucleophilic Catalysis
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Chem 109C
 The catalyst is nucleophile and forms a covalent bond with
reactants/intermediates
 Also called covalent catalysis
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Metal--Ion Catalysis
Metal
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Chem 109C
 Lewis acid catalysis (metal ions are Lewis acids)
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Metal--Ion Catalysis
Metal
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Chem 109C
 Examples:
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Metal--Ion Catalysis
Metal
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Chem 109C
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Intramolecular Reactions
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Chem 109C
1000
times
faster

Effective molarity (EM) is the concentration of the reactant that would
be required in an intermolecular reaction for it to achieve the same
rate as intramolecular reaction
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Intramolecular Reactions
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Chem 109C
R = Me, EM = 2.3 x 104 M
R = iPr
iPr,, EM = 1.3 x 106 M
EM = 2.2 x 105 M
EM = 1 x 107 M
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Chem 109C
Intramolecular Catalysis
 A catalyst is embedded or covalently bonded to substrates
 Also called anchimeric effect/assistance
 Due to intramolecularity,
y, the catalysis
y should be more efficient.
70000 times
Faster in the
Trans isomer
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Intramolecular Catalysis: Example
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Chem 109C
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Intramolecular Catalysis: Example
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Chem 109C
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Catalysis in Biological Reactions
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Chem 109C
 Most biological catalysts are enzymes (belongs to globular proteins)
 Substrates and active sites (where the reaction occurs)

Substrates bound to the active site
 Specificity (molecular recognition)
 Lock-and-key model
 Induced-fit model
Hexokinase
Red: before binding
Blue: after binding
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Enzyme Catalysis
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Chem 109C
 How do enzymes do catalysis?
 Reacting groups are brought together in a proper orientation
 Some of the amino acid side chains as well as bounded metal ions as
catalyst
 Stabilizing the transition states and intermediates via van der Waals
interactions, electrostatic interactions, and H-bonding.
 Naming of enzymes
 End typically with ‘ase’
ase , indicating breaking the bond
 Peptidase: breaking the peptide bond
 Esterase: breaking the ester bond
 Synthase
 Synthesizing instead of breaking it down.
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Carboxypeptidase A
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Chem 109C
 Metalloenzyme
y
(enzymes containing
tightly bound metal
ions)
 Hydrolyze C-terminal
except lysine and
arginine.
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Serine Proteases
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 Trypsin, chymotrypsin and elastase are members of endopeptidases
known as serine proteases
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Serine Proteases
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Chem 109C
 Proposed
mechanism for
chymotrypsincatalyzed
hydrolysis
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Lysozyme
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Chem 109C
 Catalyze the hydrolysis of the bacterial cell wall NAM-NAG bond
(NAM: N-Acetylmuramic acid)
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Protein pH
pH--Rate Profile
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Chem 109C
lysozyme
 The pHs at which the enzyme are 50% active corresponds to the
pKa of enzyme
enzyme’s
s catalytic groups (as long as those values are at
lease 2 units apart.
 Why pKa of Asp52 is 3.8 but pKa of Glu35 is 6.7 instead of 4.25 in
its free form?
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Glucose--6-Phosphate Isomerase
Glucose
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Aldolase
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