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Chpt.8 Alkyl Halides & Radical Rx’s

Chpt.8 Alkyl Halides & Radical Rx’s. Structure Nomenclature Physical Properties Halogenation of Alkanes Mechanism of Halogenation Allylic Halogenation. Note the Chapter Summary and Key Rx’s. Haloalkane. R- X H 3 C- Cl. Haloalkene. X. H. Haloarene. sp 3 (alkyl halide). C. X. C.

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Chpt.8 Alkyl Halides & Radical Rx’s

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

  2. Haloalkane R-X H3C-Cl Haloalkene X H Haloarene sp3 (alkyl halide) C X C H H sp2 (vinyl halide) sp2 (aryl halide) 8.1 Structure of ‘R-X’

  3. 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 ?

  4. B r H C C H C 3 3 H (chloroform) vs trichloromethane H-CCl3 Common Names: (alkyl halide) or (special names) (isopropyl bromide) vs 2-bromopropane

  5. 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’

  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. + HBr + diBrs + > (92) (8) + HCl + diCls + > Regioselective for 3o > 2o > 1o C-H (57) (43)

  13. R H > R R C C R R > etc. > R H R R C C R R radical stability like carbocation - electron deficient > etc. R. is electron deficient (not charged)

  14. ‘Same’ for R. E Order of stability of R(+) E write condensed

  15. Order of stability of R(+) + H3C + > + > RCH2 R2CH E E . + > R3C H3C . > ‘Same’ for R. . > RCH2 R2CH . > R3C E

  16. 3o2o1o Br2 1600 80 1 Cl2 5 41 + HX + other RX Selectivity 3o > 2o > 1o, but Cl and Br are different major mono-X product

  17. Hammond’s Postulate

  18. 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

  19. In halogenation of an alkane, the rate-limiting step is hydrogen abstraction this step is endothermic for bromination • and exothermic for chlorination.  H° (kcal/mol) HBr CH CH Br CH CH + + • • 3 3 2 3 +98 +10.0 -88 CH CH + Cl CH CH + HCl • • 3 3 2 3 -5.0 +98 -103 Hammond’s Postulate

  20. 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):

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

  22. Halogenation (free radical substitution)

  23. H H H C C C H H Allyl Radical - resonance H H H H H C C H C C C C H H H H

  24. HBr + Br2 O O hv or B r X ROOR N + N  HBr O O R.+ HX X X X X + NBS for Br2

  25. NBS for Br2

  26. H C H Free Radical Stability

  27. H R C H H R C R R R C H R allylic H H C C C benzylic H H H C H Free Radical Stability H H C H > > >

  28. Addition of HBr to alkenes gives either Markovnikov addition or non-Markovnikov addition depending on reaction conditions Markovnikov addition occurs when radicals are absent non-Markovnikov addition occurs when peroxides or other sources of radicals are present Radical Addition of HBr to Alkenes

  29. addition of HCl and HI gives only Markovnikov products to account for the the non-Markovnikov addition of HBr in the presence of peroxides, chemists proposed a radical chain mechanism Chain initiation Radical Addition of HBr to Alkenes

  30. Chain propagation Radical Addition of HBr to Alkenes

  31. Chain termination This pair of addition reactions illustrates how the products of a reaction can often be changed by a change in experimental conditions polar addition of HBr is regioselective, with Br adding to the more substituted carbon radical addition of HBr is also regioselective, with Br adding to the less substituted carbon Radical Addition of HBr to Alkenes

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