2/23/18 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 1 2/23/18 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 2/23/18 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 : 3 2/23/18 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) 4 2/23/18 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 2/23/18 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 6 2/23/18 © 2017 Pearson Education, Inc. Catalytic Hydrogenation reduction reactions (the number of C-H bonds increases) 7 2/23/18 Catalytic Hydrogenation Using ∆H° Values to Determine the Relative Stabilities of Alkenes are we talking about kinetics or thermodynamics here? 8 2/23/18 Using ∆H° Values to Determine the Relative Stabilities of Alkenes are we talking about kinetics or thermodynamics here? The Relative Stabilities of Alkenes 9 2/23/18 A Trans Alkene is More Stable Than a Cis Alkene The Relative Stabilities of Alkenes 10 2/23/18 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 2/23/18 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 2/23/18 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. 13 2/23/18 How are Rate Constants Related to the Equilibrium Constant? The Structure of the Transition State Transition states have partially formed bonds. 14 2/23/18 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 15 2/23/18 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 16 2/23/18 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 2/23/18 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 .