XII. Free Rad Notes 135 S16

XII. Free Rad Notes 135 S16 - 1 XII Free-Radical Reactions...

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1 XII. Free-Radical Reactions Alkanes Alkanes have only strong non-polar sigma bonds They are very unreactive They are neither electrophiles nor nucleophiles Called “paraffins” – Latin for “little affinity” Halogenation of Methane Alkanes do not have a pi bond or polar sigma bonds so heterolytic cleavage is a very high energy process.
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2 Experimental Observations 1. No reaction in the dark at low temps 2. UV light or high temp (>400˚C) initiates the reaction 3. Once initiated the reaction is quite rapid 4. Many molecules react for each photon of light Mechanism Relative Reactivities of the halogens
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3 Fluorination is too fast (explosive and difficult to control) Chlorination and Bromination react at reasonable and controllable rates Iodination is too slow (overall endothermic) Rate Determining Step Clorination E act = 3.8 kcal/mol Bromination E act = 18.6 kcal/mol Halogenation of Higher Alkanes Some regioselectivity with 2˚ favored CH 4 + X CH 3 + HX
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4 Bromination
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