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Reactions of Alkanes: Chapter 12

Reactions of Alkanes: Chapter 12. Radicals. Contents of Chapter 12. Reactivity Considerations Chlorination and Bromination of Alkanes Reactivity – Selectivity Principle Radical Substitution of Benzylic and Allylic Hydrogens Stereochemistry of Radical Substitution

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Reactions of Alkanes: Chapter 12

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  1. Reactions of Alkanes:Chapter 12 Radicals Chapter 12

  2. Contents of Chapter 12 • Reactivity Considerations • Chlorination and Bromination of Alkanes • Reactivity–Selectivity Principle • Radical Substitution of Benzylic and Allylic Hydrogens • Stereochemistry of Radical Substitution • No Radical Reactions in Biological Systems Chapter 12

  3. Radicals • Sources include: • Hydrogen peroxide • Alkyl peroxides • Light causes homolysis of the weak O-O bond Chapter 12

  4. Heterolytic bond cleavage or heterolysis Homolytic bond cleavage or homolysis Heterolysis & Homolysis Homolysis produces radicals, which are very reactive species Chapter 12

  5. Radical Chain Reactions • Initiation turns stable species into radicals by breaking a bond. Chapter 12

  6. Radical Chain Reactions • Propagation causes products to form without resulting in net consumption of radicals. (production = consumption) Chapter 12

  7. Radical Chain Reactions • Termination results in net radical destruction. • Generally occurs when reactants are used up. Chapter 12

  8. Chlorination and Bromination of Alkanes Chapter 12

  9. Product Distribution Chapter 12

  10. Chlorination Rates Chapter 12

  11. Chlorination Product Distribution Chapter 12

  12. Bromination Rates Chapter 12

  13. Reactivity–Selectivity Principle • The very reactive chlorine atom will have lower selectivity and attack pretty much any hydrogen available on an alkane • The less reactive bromine atom will be more selective and tends to react preferentially with the easy targets, i.e. benzylic/allylic > 3° > 2° > 1° Chapter 12

  14. Radicals Stability of alkyl radicals is similar to stability of carbocations Chapter 12

  15. Crude Radical Stability Index • Add 1 for each attached carbon. • Add 3 for adjacent double bond or phenyl ring. • Radical equally stable on double bond carbon as on single bonded carbon • Profile similar to carbocations but resonance contributes more to radical stability, and radical is OK on C=C. Chapter 12

  16. Markovnikov Addition Anti-Markovnikov Addition Addition of Radicals Source of confusion regarding addition of HBr for years; can’t do ionic product analysis with radicals!!! Chapter 12

  17. Radical Addition is Unique to Hydrogen Bromide HBr and peroxide goes anti-Markovnikov! Chapter 12

  18. Hydrogen Halide Addition in the presence of Peroxide Hydrogen peroxide has same effect on hydrogen bromide addition to an alkyne as to an alkene (reversed regioselectivity). Chapter 12

  19. Benzylic/Allylic Radicals • Benzylic and allylic radicals are even more stable than tertiary alkyl radicals • It should be easy for a halogen radical to abstract a benzylic or allylic hydrogen Chapter 12

  20. Benzylic/Allylic Radicals Problem in making allylic radical is the greater likelihood that HBr or Br2 will add electrophilically to the double bond rather than making the allylic radical. Chapter 12

  21. Benzylic/Allylic Radicals • Problem solved with N-bromosuccinimide (NBS) is a good reagent for supplying low concentrations of bromine radical • NBS doesn’t produce much Br2 or HBr so it only replaces benzylic/allylic H’s Chapter 12

  22. Benzylic and Allylic Radicals • Allylic or benzylic radical stabilized by resonance. • Only H which leads to most stable set of resonance structures is removed (like C+). • Unlike C+ case no thermo or kinetic product exists; Br found on all radical-bearing carbons Chapter 12

  23. Stereochemistry of Radical Substitution Radicals give both enantiomers when they make products like carbocations Chapter 12

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