Chapter 4 - Wade - Introduction to General and Organic...

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Chapter 4 The Study of Chemical Reactions Dr. David P. Brown Department of Chemistry Introduction to General and Organic Chemistry II
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Chapter 4 2 Tools for Study To determine a reaction’s mechanism , look at: Equilibrium constant Free energy change Enthalpy Entropy Bond dissociation energy Kinetics Activation energy =>
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Chapter 4 3 Chlorination of Methane Requires heat or light for initiation . Halogenation occurs by the chain mechanism involving free radicals . Reaction has a high quantum yield - Lots of product formed from absorption of only one photon of light (chain reaction). C H H H H + Cl 2 heat or light C H H H Cl + HCl
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Chapter 4 4 Steps in the Chain Mechanism Initiation generates a reactive 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.
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Chapter 4 5 Initiation Step A chlorine molecule splits homolytically into chlorine atoms (free radicals) =>
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Chapter 4 6 Propagation Step (1) The chlorine atom collides with a methane molecule and abstracts (removes) a H, forming another free radical and one of the products (HCl).
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Chapter 4 7 Propagation Step (2) The methyl free radical collides with another chlorine molecule, producing the other product (methyl chloride) and regenerating the chlorine radical .
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Chapter 4 8 Overall Reaction C H H H H Cl + C H H H + H Cl C H H H + Cl Cl C H H H Cl + Cl C H H H H + Cl Cl C H H H Cl + H Cl The sum of the propagation steps
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Chapter 4 9 Termination Steps Collision of any two free radicals Combination of free radical with contaminant or collision with wall. C H H H Cl + C H H H Cl Note : Free radicals are being consumed !!
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Chapter 4 10 Termination Reactions
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Chapter 4 11 Equilibrium constant K eq = [Products]/[Reactants]
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Chapter 4 12 Free Energy Change G = free energy of (products - reactants), amount of energy available to do work. Negative values indicate spontaneity.
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Chapter 4 13 Factors Determining G ° Free energy change depends on enthalpy entropy 2200 H ° = (enthalpy of products) - (enthalpy of reactants) 2200 S ° = (entropy of products) - (entropy of reactants) 2200 G ° = H ° - T S °
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Chapter 4 14 Enthalpy
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This note was uploaded on 04/06/2008 for the course CHEM 10765 taught by Professor Brown during the Spring '08 term at St. Johns Duplicate.

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Chapter 4 - Wade - Introduction to General and Organic...

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