Ch_322a_10.010

Ch_322a_10.010 - Halogenation of Higher Alkanes Higher...

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Halogenation of Higher Alkanes Higher alkanes react with the halogens by the same free radical chain mechanism. An important difference is the possibility of multiple reaction sites that leads to competing reactions and the possibility of selectivity in product formation. The Chlorination of Ethane: six equivalent H mechanism initiation Cl 2 2Cl . h ! or heat propagation (1) CH 3 CH 2 -H + Cl . CH 3 CH 2 + HCl . (DH o = -98) (DH o = -104) ! H o = -6 E a = 1.0 (2) CH 3 CH 2 . + Cl 2 CH 3 CH 2 Cl + Cl . (DH o = -58) (DH o = -81.5)
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Chlorination of Ethane: Reactivity Ethane is more reactive than methane by a factor of about 400 to 1. CH 3 CH 3 CH 4 relative reactivity in free radical chlorination 400 1 There is a statistical factor favoring ethane (6H/4H ) which accounts for a reactivity difference of 1.5 to 1 . The remaining difference in reactivity (266/1) is due to the intrinsic greater reactivity of each H in ethane as revealed by the activation energies: CH 4 CH 3 CH 3 E a = 3.8 kcal/mol E a = 1.0 kcal/mol This intrinsic reactivity difference results from the greater stability of the ethyl radical relative to the methyl radical.
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This note was uploaded on 07/22/2009 for the course CHEM 322AL taught by Professor Jung during the Summer '07 term at USC.

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Ch_322a_10.010 - Halogenation of Higher Alkanes Higher...

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