Chapter 4 - The Study of chemical reactions. In this chapter, students will be able to: Propose mechanisms and explain the steps for simple reactions such as free-radical halogenation; draw a reaction-energy diagram, and use it to identify the factors controlling the thermodynamics and kinetics of a reaction; use the mechanism, thermodynamics, and kinetics of a reaction to predict which of several possible products is the major product; identify reactive intermediates and explain their properties. | Chapter 4 ©2010, Prentice Hall Organic Chemistry, 7th Edition L. G. Wade, Jr. The Study of Chemical Reactions Chapter 4 Introduction Overall reaction: reactants products Mechanism: Step-by-step pathway. To learn more about a reaction: Thermodynamics Kinetics Chapter 4 Chlorination of Methane Requires heat or light for initiation. The most effective wavelength is blue, which is absorbed by chlorine gas. Many molecules of product are formed from absorption of only one photon of light (chain reaction). Chapter 4 The Free-Radical Chain Reaction Initiation generates a radical intermediate. Propagation: The intermediate reacts with a stable molecule to produce another reactive intermediate (and a product molecule). Termination: Side reactions that destroy the reactive intermediate. Chapter 4 Initiation Step: Formation of Chlorine Atom A chlorine molecule splits homolytically into chlorine atoms (free radicals). Chapter 4 Propagation Step: Carbon . | Chapter 4 ©2010, Prentice Hall Organic Chemistry, 7th Edition L. G. Wade, Jr. The Study of Chemical Reactions Chapter 4 Introduction Overall reaction: reactants products Mechanism: Step-by-step pathway. To learn more about a reaction: Thermodynamics Kinetics Chapter 4 Chlorination of Methane Requires heat or light for initiation. The most effective wavelength is blue, which is absorbed by chlorine gas. Many molecules of product are formed from absorption of only one photon of light (chain reaction). Chapter 4 The Free-Radical Chain Reaction Initiation generates a radical intermediate. Propagation: The intermediate reacts with a stable molecule to produce another reactive intermediate (and a product molecule). Termination: Side reactions that destroy the reactive intermediate. Chapter 4 Initiation Step: Formation of Chlorine Atom A chlorine molecule splits homolytically into chlorine atoms (free radicals). Chapter 4 Propagation Step: Carbon Radical The chlorine atom collides with a methane molecule and abstracts (removes) an H, forming another free radical and one of the products (HCl). Chapter 4 Propagation Step: Product Formation The methyl free radical collides with another chlorine molecule, producing the organic product (methyl chloride) and regenerating the chlorine radical. Chapter 4 Overall Reaction Chapter 4 Termination Steps A reaction is classified as a termination step when any two free radicals join together producing a nonradical compound. Combination of free radical with contaminant or collision with wall are also termination steps. Chapter 4 More Termination Steps Chapter 4 Lewis Structures of Free Radicals Free radicals are unpaired electrons. Halogens have 7 valence electrons so one of them will be unpaired (radical). We refer to the halides as atoms not radicals. Chapter 4 Equilibrium Constant Keq = [products] [reactants] For CH4 + Cl2 CH3Cl + HCl Keq
