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CHEM 109A Organic 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 (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: , 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 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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