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CHEM 109A Organic Chemistry https://labs.chem.ucsb.edu/zakarian/armen/courses.html
Chapter 5 Alkene: Introduction Thermodynamics and Kinetics
Midterm 2.....
Grades will be posted on Tuesday, Feb. 27th .
Tests can be picked up outside room CHEM 2138 starting Tuesday, Feb. 27th
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A Reaction Coordinate Diagram
time A reaction coordinate diagram shows the energy changes that take place in each step of a reaction.
Thermodynamics and Kinetics
Thermodynamics: • are products more stable than starting materials? • overall, are new bonds in products stronger than the old in starting materials? • deals with equilibria
Kinetics: • which reaction is faster? Reaction rates • How high is the energy of the transition state? • deals with mechanism of reactions
or is Z more stable than Y?
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The Equilibrium Constant (thermodynamics)
The equilibrium constant gives the relative concentration of reactants and products at equilibrium.
Exergonic and Endergonic Reactions
∆G° = free energy of the products - free energy of the reactants
the relationship between ∆G° and Keq :
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Exergonic and Endergonic Reactions
∆G° = free energy of the products - free energy of the reactants
the relationship between ∆G° and Keq : ∆G° negative: exergonic reaction, products more stable ∆G° positive: endergonic reaction, products less stable
Gibbs Free-Energy Change (∆G°) (thermodynamics)
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Increasing the Amount of a Product Formed in a Reaction (thermodynamics)
Le Chatelier’s Principle: if an equilibrium is disturbed, the system will adjust to offset the disturbance
Calculating ∆H° (thermodynamics)
Coupled Reactions: an endergonic reaction followed by an exergonic reaction
Total ∆H° (A!C) = ∆H°(rxn1) + ∆H° (rxn2)
rxn1 rxn2
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Calculating ∆H° (thermodynamics)
Coupled Reactions: an endergonic reaction followed by an exergonic reaction
Total ∆H° (A!C) = ∆H°(rxn1) + ∆H° (rxn2)
DH(rxn) = DH(broken bonds) + DH (formed bonds)
rxn1 rxn2
Calculating ∆H° (thermodynamics)
Coupled Reactions: an endergonic reaction followed by an exergonic reaction
Total ∆H° (A!C) = ∆H°(rxn1) + ∆H° (rxn2)
DH(rxn) = DH(broken bonds) + DH (formed bonds)
rxn1 rxn2
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Catalytic Hydrogenation
reduction reactions (the number of C-H bonds increases)
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Catalytic Hydrogenation
Using ∆H° Values to Determine the Relative Stabilities of Alkenes are we talking about kinetics or thermodynamics here?
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Using ∆H° Values to Determine the Relative Stabilities of Alkenes are we talking about kinetics or thermodynamics here?
The Relative Stabilities of Alkenes
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A Trans Alkene is More Stable Than a Cis Alkene
The Relative Stabilities of Alkenes
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Reaction Coordinate Diagrams for Fast and Slow Exergonic and Endergonic Reactions (kinetics)
Free Energy of Activation (∆G‡) (kinetics)
The free energy of activation is the energy barrier of the reaction.
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Kinetic and Thermodynamic Stability
Kinetic Stability is indicated by ∆G‡
If ∆G‡ is large, the reactant is kinetically stable because it reacts slowly. If ∆G‡ is small, the reactant is kinetically unstable because it reacts rapidly.
Thermodynamic Stability is indicated by ∆G°
If ∆G° is negative, the product is thermodynamically stable compared to the reactant. If ∆G° is positive, the product is thermodynamically unstable compared to the reactant.
Rate of a Reaction
Increasing the concentration increases the rate. Increasing the temperature increases the rate.
( The rate can be increased by a catalyst )
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The Rate of a Reaction versus the Rate Constant for a Reaction
The Arrhenius Equation
The value of A pertains to the frequency and orientation of collisions.
e−Ea/RT e–Ea/RT is the fraction of the collisions with the minimum energy needed for a reaction.
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How are Rate Constants Related to the Equilibrium Constant?
The Structure of the Transition State
Transition states have partially formed bonds.
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Reaction Coordinate Diagram for the Addition of HBr to 2-Butene
Reaction Coordinate Diagram for the Addition of HBr to 2-Butene
another ∆G‡ ∆G‡ kinetics kinetics
• The rate-limiting step has its transition state at the highest point on the reaction coordinate diagram. • The rate limiting step determines the rate of the whole reaction
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Reaction Coordinate Diagram for the Addition of HBr to 2-Butene
another ∆G‡ ∆G‡ kinetics kinetics
thermodynamics:
• The rate-limiting step has its transition state at the highest point on the reaction coordinate diagram. • The rate limiting step determines the rate of the whole reaction
A Catalyst
• makes a reaction faster
• provides a pathway for a reaction with a lower energy barrier. (changes the kinetics)
• does not change the energy of the starting point (the reactants) or the energy of the end point (the products). (does not change thermodynamics of the reaction
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Enzymes
Most biological reactions require a catalyst.
Most biological catalysts are proteins called enzymes.
The reactant of a biological reaction is called a substrate.
The Active Site of an Enzyme
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Enzyme Side Chains That Bind the Substrate
Enzyme side chains bind the substrate using hydrogen bonds, London dispersion forces, and dipole-dipole interactions.
Enzyme Side Chains That Catalyze the Reaction
Some enzyme side chains are acids, bases, and nucleophiles that catalyze the reaction.
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