CHE201 - Chapter 4

CHE201 - Chapter 4 - Organic Chemistry, 6th Edition L. G....

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Chapter 4 The Study of Chemical Reactions Organic Chemistry , 6 th Edition L. G. Wade, Jr.
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Chapter 4 2 Introduction • Reactants Products: overall reaction. • Mechanism: step-by-step pathway. • To learn more about a reaction: ¾ Thermodynamics ¾ Kinetics.
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Chapter 4 3 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).
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Chapter 4 4 The Free-Radical Chain Reaction • 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). Free radicals: • Contain unpaired electron • Are highly reactive
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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
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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. Can you suggest others?
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Chapter 4 10 Equilibrium Constant • For chlorination K eq = 1.1 x 10 19 • Large value indicates reaction “goes to completion.” A + B C + D ] ][ [ ] ][ [ ] [ ] [ B A D C reactants products K eq = =
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Chapter 4 11 Free Energy Change Δ G = free energy of (products - reactants), amount of energy available to do work. • Negative values indicate spontaneity. Δ G o = - RT (ln K eq ), where R = 8.314 J/Kmol T = temperature in kelvins • Since chlorination has a large K eq , the free energy change is large and negative.
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Chapter 4 12 Problem • Given that -X is -OH, the energy difference for the following reaction is -4.0 kJ/mol.
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CHE201 - Chapter 4 - Organic Chemistry, 6th Edition L. G....

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