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Alkyl Halides & Radical Rx’s

chapter 8. Alkyl Halides & Radical Rx’s. Structure Nomenclature Physical Properties Halogenation of Alkanes Mechanism of Halogenation Allylic Halogenation Examples Radical addition. Note the Chapter Summary and Key Rx’s. ss18 12. Chapter 8 . Chapter 7 . synthesis. synthesis.

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Alkyl Halides & Radical Rx’s

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  1. chapter 8 Alkyl Halides & Radical Rx’s Structure Nomenclature Physical Properties Halogenation of Alkanes Mechanism of Halogenation Allylic Halogenation Examples Radical addition Note the Chapter Summary and Key Rx’s ss18 12

  2. Chapter 8 Chapter 7 synthesis synthesis free radical substitution mechanism allylic benzylic Hammond radical addition

  3. haloalkene sp2 (a vinyl halide) 8.1 Structure of ‘R-X’ haloalkane R-X H3C-Cl haloarene sp3 (alkyl halide) sp2 (aryl halide)

  4. Haloalkane (alkyl halide) sp3 R " R-X H3C-Cl methyl chloride 3o halide R ' R C X H R ' 2o halide R C H X 1o halide R C H X 8.1 Structure of ‘R-X’

  5. 8.2 Nomenclature IUPAC - halides (X) are substituents Substituent names: halo fluoro, chloro, bromo, iodo #-haloalkane #-halocycloalkane (R)-4-bromo-1-chloro-4-fluoro-1-cyclopentene structure ?

  6. 8.3 Physical Properties H C 3   )  B r H C 3 “polar covalent bond” - dipole - mismatch of electronegativity -size H C

  7. 8.4 Halogenation of Alkanes substitution of X for H hv = ultraviolet light,  = heat X2 = Cl2, Br2 seldom F2 (too reactive - exothermic) or I2 (endothermic, unreactive)

  8. hv Cl2 hv Cl2 hv Cl2 Substitution, products and by-products + other R-X’s

  9. Br + Br2 + HBr + other Br's Generally halogenation not useful - mixtures (separate) A few rx’s are useful, e.g.: Others - allylic & benzylic

  10. + HBr + diBr + etc + HBr + diBr +... (92) (8) + HBr + diBr +... bromination favors 3o > 2o > 1o Substitution, products and by-products monobromination

  11. and/or and/or initiation propagation terminations

  12. > > Regioselective for 3o > 2o > 1o C-H (92) (8) (57) (43)

  13. Order of stability of R(+) / R. same order

  14. 3o2o1o Br2 1600 80 1 Cl2 5 41 Selectivity 3o > 2o > 1o, but Cl and Br are different major mono-X product Cl. more reactive less selective than Br.

  15. Hammond’s Postulate: the structure of the transition state: for an exothermic reaction looks more like the reactants of that step Hammond’s Postulate Cl vs Br • - for an endothermic reaction looks more like the products of that step

  16. In halogenation of an alkane, the rate-limiting step is hydrogen abstraction this step is endothermic for bromination and exothermic for chlorination. H°  (kcalmol) Hammond’s Postulate

  17. transition state resembles the alkane and chlorine atom little radical character on carbon in t.s. regioselectivity only slightly influenced by radical stability Hammond’s Postulate Forchlorination (hydrogen abstraction is exothermic):

  18. early t.s. - like SM PE site of collision important SMprog rx Prod late t.s. PE stability of R. important SM prog rx Prod Hammond

  19. Halogenation (free radical substitution)

  20. Allyl Radical - resonance

  21. initially with progress rx H-Br

  22. NBS for Br2

  23. Free Radical Stability

  24. allylic benzylic Free Radical Stability

  25. Markovnikov “normal” Radical ADDITION - note rx conditions!!! Add H-X in the presence of peroxides reverses normal addition “anti-Markovnikov”

  26. Radical ADDITION - note rx conditions!!! Add H-X in the presence of peroxides reverses normal addition “anti-Markovnikov” X = Cl, Br, I end

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