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Free Radical Chlorination

Free Radical Chlorination. Experimental Evidence Helps to Determine Mechanism. Chlorination does not occur at room temperature in the dark . The most effective wavelength of light is blue that is strongly absorbed by Cl 2 gas.

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Free Radical Chlorination

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  1. Free Radical Chlorination

  2. Experimental Evidence Helps to Determine Mechanism • Chlorination does not occur at room temperature in the dark. • The most effective wavelength of light is blue that is strongly absorbed by Cl2 gas. • The light-initiated reaction has a high quantum yield (many molecules of product are formed from each photon of light).

  3. Free Radical Species are Constantly Generated Throughout the ReactionPropagation

  4. Termination: Reaction of any 2 Radicals

  5. Chlorination of Propane

  6. H’s are not abstracted at the same rate. For secondary: 70%/4H = 17.5 For primary: 30%/6H = 5.0 Therefore… Relative rate of H abstraction for 1o : 2o is 1 : 3.5

  7. Chlorinationof Methylpropane

  8. 3o Radicals are Easiest to Form

  9. Tertiary H’s removed five times more readily than primary H’s in chlorination reactions Relative reactivity of 1o H abstraction: 65% / 9H = 7.2 Relative reactivity of 3o H abstraction: 35% / 1H = 35 Therefore… Relative rate of H abstraction for 1o : 3o 1:5

  10. Stability of Free Radicals

  11. Bromination is Very Selective

  12. RDS in Bromination is highly endothermic

  13. Consider the free radical monochlorination of 2,2,5-trimethylhexane. Draw all of the unique products (ignore stereoisomers; use zig-zag structures please) and predict the ratio or percent composition of the products.The relative reactivity of H abstraction in a chlorination reaction: 1o: 2o: 3o = 1: 3.5: 5

  14. 2.(12) Consider the free radical monochlorination of 1,4-dimethylcyclohexane. The reaction affords three different products (all C8H15Cl). In this reaction, the rates of hydrogen abstraction of primary : secondary : tertiary are 1 : 3.5 : 5. Draw the three products and predict the percent composition (or ratio) of the product mixture. (Disregard cis / trans isomers for this problem.)

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