07 - Chemistry 5.12 Spring 2003, Week 3 / Day 2 Handout #7:...

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1 Chemistry 5.12 Spri ng 2003, Week 3 / Day 2 Handout #7: Lecture 11 Outline IX. Free Radical Reactions (Read Chapter 4) A. Chlorination of Methane (4-2) 1. Mechanism (4-3) B. Review of Thermodynamics (4-4,5) C. Review of Kinetics (4-8,9) D. Reaction-Energy Diagrams (4-10) 1. Thermodynamic Control 2. Kinetic Control 3. Hammond Postulate (4-14) 4. Multi-Step Reactions (4-11) 5. Chlorination of Methane (4-7) Suggested Problems: 4-35–37,40,43 IX. A. Radical Chlorination of Methane C H H H H Cl Cl heat ( ) or light ( hv ) C H H Cl H H Cl C H H Cl H Cl Cl or hv C Cl H Cl H H Cl Cl Cl or hv C Cl Cl Cl H H Cl etc. Why is light or heat necessary? How fast does it go? Does it give off or consume heat? How fast are each of the successive reactions? Can you control the product ratio? To answer these questions, we need to: 1. Understand the mechanism of the reaction (arrow-pushing!). 2. Use thermodynamics and kinetics to analyze the reaction.
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2 1. Mechanism of Radical Chlorination of Methane (Free-Radical Chain Reaction) Free-radical chain reactions have three distinct mechanistic steps: initiation step: generates reactive intermediate propagation steps: reactive intermediates react with stable molecules to generate other reactive intermediates (allows chain to continue) termination step: side-reactions that slow the reaction; usually combination of two reactive intermediates into one stable molecule Initiation Step: Cl 2 absorbs energy and the bond is homolytically cleaved. (Remember to use fishhook arrows!) Cl Cl or hv Cl Cl Cl Cl The first-formed reactive intermediate is a chlorine radical. reactive intermediate: short-lived species that reacts quickly as it is formed radical: species with an unpaired electron (usually just draw the unpaired electron) chlorine radicals
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3 Propagation Steps: The Meat of the Reaction a) Chlorine radical abstracts a hydrogen from methane to form a methyl radical. b) Methyl radical abstracts a chlorine from Cl 2 to form product and another chlorine radical Cl Cl Cl C H H H H C H H H H Cl methyl radical C H H H methyl radical C Cl H H H Cl a) b) Sum: C H H H H C Cl H H H H Cl Cl Cl keeps the chain going The sum of the propagation steps is the overall chlorination reaction. Termination Steps: Unproductive Steps Consumption of reactive intermediates (radicals) without generation of new ones. Cl
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This note was uploaded on 11/27/2011 for the course CHEMICAL E 20.410j taught by Professor Rogerd.kamm during the Spring '03 term at MIT.

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07 - Chemistry 5.12 Spring 2003, Week 3 / Day 2 Handout #7:...

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