Organic Chemistry (8th Edition)

Published by Pearson
ISBN 10: 0321768418
ISBN 13: 978-0-32176-841-4

Chapter 4 - The Study of Chemical Reactions - Problems - Page 144: Problem 4-9 a

Answer

The free-radical chlorination of ethane mechanism consists in three main steps: initiation, propagation and termination. The initiation step generates the Cl radicals, the propagation steps lead to the products formation and the termination steps lead to non-radical compounds.

Work Step by Step

To begin with, the initiation reaction can be deduced from the bond-dissociation enthalpies of C-C, C-H and Cl-Cl bonds. As the Cl-Cl bond has the lowest bond-dissociation enthalpy (242 kJ/mol), the initiation reaction will consist in the homolytic cleavage of the Cl-Cl bond, the necessary energy being supplied by light. The propagation reaction consists in two steps: The first step consists in the C-H abstraction by the Cl radical in order to form the ethyl radical and HCl. This step is enthalphy-favoured as the H-Cl bond-dissociation energy (431 kJ/mol) is higher than the C-H bond-dissociation energy (410 kJ/mol). The second step is represented by the ethyl radical reaction with the Cl2 molecule, generating the product and the Cl radical which will continue the chain. This process is enthalphy-favoured as the C-Cl bond-dissociation energy (339 kJ/mol) is higher than the Cl-Cl bond dissociation energy (242 kJ/mol). There are multiple termination reactions which can take place, three of them consisting in the product formation via ethyl radical and Cl radical reaction, Cl2 formation via Cl radicals reaction and butane formation via ethyl radicals reaction.
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