CHEM 109A Organic Chemistry

Transcription

1 CHEM 109A Organic Chemistry Chapter 5 Alkene: Introduction Thermodynamics and Kinetics Midterm 2... Grades will be posted on Tuesday, Feb. 27 th. Tests can be picked up outside room CHEM 2138 starting Tuesday, Feb. 27 th 1

2 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? 2

3 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 K eq : 3

4 Exergonic and Endergonic Reactions G = free energy of the products - free energy of the reactants the relationship between G and K eq : G negative: exergonic reaction, products more stable G positive: endergonic reaction, products less stable Gibbs Free-Energy Change ( G ) (thermodynamics) 4

5 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 5

6 Calculating H (thermodynamics) Coupled Reactions: an endergonic reaction followed by an exergonic reaction Total H (A!C) = H (rxn1) + H (rxn2) rxn1 rxn2 DH(rxn) = DH(broken bonds) + DH (formed bonds) Calculating H (thermodynamics) Coupled Reactions: an endergonic reaction followed by an exergonic reaction Total H (A!C) = H (rxn1) + H (rxn2) rxn1 rxn2 DH(rxn) = DH(broken bonds) + DH (formed bonds) 6

7 2017 Pearson Education, Inc. Catalytic Hydrogenation reduction reactions (the number of C-H bonds increases) 7

8 Catalytic Hydrogenation Using H Values to Determine the Relative Stabilities of Alkenes are we talking about kinetics or thermodynamics here? 8

9 Using H Values to Determine the Relative Stabilities of Alkenes are we talking about kinetics or thermodynamics here? The Relative Stabilities of Alkenes 9

10 A Trans Alkene is More Stable Than a Cis Alkene The Relative Stabilities of Alkenes 10

11 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. 11

12 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 ) 12

13 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 E a/rt e Ea/RT is the fraction of the collisions with the minimum energy needed for a reaction. 13

14 How are Rate Constants Related to the Equilibrium Constant? The Structure of the Transition State Transition states have partially formed bonds. 14

15 Reaction Coordinate Diagram for the Addition of HBr to 2-Butene Reaction Coordinate Diagram for the Addition of HBr to 2-Butene G kinetics another G 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 15

16 Reaction Coordinate Diagram for the Addition of HBr to 2-Butene G kinetics another G 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 16

17 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 17

18 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. 18