1. Chapter 9 Reactions of Alkanes Radicals Irene Lee Case Western Reserve University Cleveland, OH ©2004, Prentice Hall Organic Chemistry 4 th Edition Paula Yurkanis Bruice

2. Petroleum is a complex mixture of alkanes and cycloalkanes that can be separated by distillation

3. Alkanes are very unreactive compounds because they have only strong  bonds and atoms with no partial charges However, alkanes do react with Cl 2 and Br 2

4. Reaction of Alkane with Cl 2 or Br 2

6. Factors that determine product distribution … The rate-determining step of the overall reaction is hydrogen abstraction

7. Consider the relative stabilities of alkyl radicals, The stable alkyl radical is formed faster, therefore 2-chlorobutane is formed faster

8. pg 341 middle

9. In determining the relative amounts of products obtained, both probability and reactivity should be considered probability: the number of hydrogens that can be abstracted that will lead to the formation of the particular product reactivity: the relative rate at which a particular hydrogen is abstracted

12. The Reactivity–Selectivity Principle A bromine radical is less reactive and more selective than a chlorine radical

13. Why are the relative rates of radical formation so different between the bromine versus the chlorine radical?

14. The more reactive a species is, the less selective it will be

15. What about fluorination and iodination? Alkanes undergo chlorination and bromination, but not iodination Fluorination is too violent of a reaction to be useful

16. Consider the relative stabilities of radicals

17. The more stable radicals form faster Radical Substitution of Benzylic Hydrogens

18. Because of the reactivity of allylic hydrogens, a milder brominating reagent can be used

19. Advantage: the low concentration of Br 2 and HBr present cannot be added to the double bond

20. The allylic radical has two resonance contributors

21. Stereochemistry of Radical Substitution Reactions

22. Why are both enantiomers formed? Consider the first propagation step

23. What happens if the reactant already has an asymmetric carbon and the radical substitution reaction creates a second asymmetric carbon?

24. Reactions of Cyclic Compounds

25. Reactions of Cyclopropane

26. Radical Reactions in Biological Systems

28. Radicals and Stratospheric Ozone ozone is a major constituent of smog ozone shields the Earth from harmful radiation

29. Chlorofluorocarbons remain very stable in the atmosphere until they reach the stratosphere The chlorine radicals are ozone-removing agents