zhang chapter 23: catalysis chemchem 109c 109cfolk.ntnu.no/audunfor/7. semester/fysikalsk organisk...
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ZhangZhangChemChem 109C109CChapter 23: CatalysisChapter 23: Catalysis
2121--11
ZhangZhangChemChem 109C109CCatalyst and Catalysis: IntroductionCatalyst and Catalysis: Introduction
Catalyst is a compound that by its addition to a reaction increasesincreases Catalyst is a compound that by its addition to a reaction increases increases the rate of the reaction without itself being consumed or the rate of the reaction without itself being consumed or changed at the end of the reactionchanged at the end of the reaction (cycle).
The reaction is then called being catalyzed The reaction is then called being catalyzed. The phenomenon of catalyst in accelerating reaction is called
catalysis.
N O
Me
MeHN O
Me
Me
Ph3PAuNTf2 (5 mol %)MsOH (1.2 equiv), 4 Å MS
CH Cl
H+
Ph3PAu+
MeMe quantitative (NMR)/ 61% (isolated)CH2Cl2
HN O
Me
MeHN O
Me
H+ HN O
Me
Me
2323--22
Me
MeAu
PPh3
Me
Me AuPPh3
Me
MeMsOPh3PAu+
ZhangZhangChemChem 109C109CCatalyst and Catalysis: IntroductionCatalyst and Catalysis: Introduction
Catalyst increases the reaction rate by providing a pathway with Catalyst increases the reaction rate by providing a pathway with lower free energy of activation (reaction kinetics)
2323--33
ZhangZhangChemChem 109C109CCatalysis in Organic ReactionsCatalysis in Organic Reactions
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
2323--44
ZhangZhangChemChem 109C109CAcid Catalysis: Ester HydrolysisAcid Catalysis: Ester Hydrolysis
2323--55
ZhangZhangChemChem 109C109CAcid CatalysisAcid Catalysis
Specific-acid catalysis and general acid catalysis 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 slowGeneral acid catalysis: proton transfer to the reactant during the slow
step of the reaction (typically under weak acidic conditions, one stage)
2323--66
ZhangZhangChemChem 109C109CAcid CatalysisAcid Catalysis
General-acid catalysis vs specific-acid catalysis General-acid catalysis vs. specific-acid catalysis
2323--77
ZhangZhangChemChem 109C109CBase CatalysisBase Catalysis
General-base catalysis vs specific-base catalysis General-base catalysis vs. specific-base catalysis
2323--88
ZhangZhangChemChem 109C109CNucleophilic CatalysisNucleophilic Catalysis
The catalyst is nucleophile and forms a covalent bond with The catalyst is nucleophile and forms a covalent bond with reactants/intermediates
Also called covalent catalysis
2323--99
ZhangZhangChemChem 109C109CMetalMetal--Ion CatalysisIon Catalysis
Lewis acid catalysis (metal ions are Lewis acids) Lewis acid catalysis (metal ions are Lewis acids)
2323--1010
ZhangZhangChemChem 109C109CMetalMetal--Ion CatalysisIon Catalysis
Examples: Examples:
2323--1111
ZhangZhangChemChem 109C109CMetalMetal--Ion CatalysisIon Catalysis
2323--1212
ZhangZhangChemChem 109C109CIntramolecular ReactionsIntramolecular Reactions
1000 1000 times times fasterfasterfasterfaster
2323--1313
Effective Effective molaritymolarity (EM) is the concentration of the reactant that would (EM) is the concentration of the reactant that would be required in an intermolecular reaction for it to achieve the same be required in an intermolecular reaction for it to achieve the same rate as intramolecular reactionrate as intramolecular reaction
ZhangZhangChemChem 109C109CIntramolecular ReactionsIntramolecular Reactions
R = Me, EM = 2.3 x 10R = Me, EM = 2.3 x 104 4 MMR = R = iPriPr, EM = 1.3 x 10, EM = 1.3 x 106 6 M M
EM = 2.2 x 10EM = 2.2 x 105 5 MMEM 2.2 x 10EM 2.2 x 10 M M
EM = 1 x 10EM = 1 x 107 7 M M
2323--1414
ZhangZhangChemChem 109C109CIntramolecular CatalysisIntramolecular Catalysis
A catalyst is embedded or covalently bonded to substrates Also called anchimeric effect/assistance Due to intramolecularity, the catalysis should be more efficient.y, y
70000 times70000 timesFaster in the Faster in the Trans isomerTrans isomer
2323--1515
ZhangZhangChemChem 109C109CIntramolecular Catalysis: ExampleIntramolecular Catalysis: Example
2323--1616
ZhangZhangChemChem 109C109CIntramolecular Catalysis: ExampleIntramolecular Catalysis: Example
2323--1717
ZhangZhangChemChem 109C109CCatalysis in Biological ReactionsCatalysis in Biological Reactions
Most biological catalysts are enzymes (belongs to globular proteins) 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
HexokinaseRed: before binding
2323--1818
Blue: after binding
ZhangZhangChemChem 109C109CEnzyme CatalysisEnzyme Catalysis
How do enzymes do catalysis? 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 Some of the amino acid side chains as well as bounded metal ions as catalyst
Stabilizing the transition states and intermediates via van der WaalsStabilizing the transition states and intermediates via van der Waals interactions, electrostatic interactions, and H-bonding.
Naming of enzymes End typically with ‘ase’ indicating breaking the bondEnd typically with ase , indicating breaking the bond
Peptidase: breaking the peptide bond Esterase: breaking the ester bond
SynthaseSynthase Synthesizing instead of breaking it down.
2323--1919
ZhangZhangChemChem 109C109CCarboxypeptidaseCarboxypeptidase AA
Metalloenzyme y(enzymes containing tightly bound metal ions)
Hydrolyze C-terminal except lysine and arginine.
2323--2020
ZhangZhangChemChem 109C109CSerine ProteasesSerine Proteases
Trypsin, chymotrypsin and elastase are members of endopeptidases known as serine proteases
2323--2121
ZhangZhangChemChem 109C109CSerine ProteasesSerine Proteases
Proposed Proposed mechanism for chymotrypsin-catalyzedcatalyzed hydrolysis
2323--2222
ZhangZhangChemChem 109C109CLysozymeLysozyme
Catalyze the hydrolysis of the bacterial cell wall NAM-NAG bond Catalyze the hydrolysis of the bacterial cell wall NAM-NAG bond (NAM: N-Acetylmuramic acid)
2323--2323
ZhangZhangChemChem 109C109CProtein pHProtein pH--Rate ProfileRate Profile
lysozymelysozyme
The pHs at which the enzyme are 50% active corresponds to the pKa of enzyme’s catalytic groups (as long as those values are atpKa of enzyme 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?
2323--2424
its free form?
ZhangZhangChemChem 109C109CGlucoseGlucose--66--Phosphate Phosphate IsomeraseIsomerase
2323--2525
ZhangZhangChemChem 109C109CAldolaseAldolase
2323--2626
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