1. 8-1 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. 8-2 8.1 Structure of ‘R-X’ Haloalkene sp 2 (vinyl halide) C C H X H H Haloalkane sp 3 (alkyl halide) R-X H 3 C-Cl Haloarene sp 2 (aryl halide) X

3. 8-3 IUPAC - halides (X) are substituents Substituent names: halo fluoro, chloro, bromo, iodo 8.2 Nomenclature #-haloalkane #-halocycloalkane (R)-4-bromo-1-chloro-4-fluoro-1-cyclopentene structure ?

4. 8-4 Common Names: (alkyl halide) or (special names) (isopropyl bromide) vs 2-bromopropane (chloroform) vs trichloromethane H-CCl 3 H 3 C C Br CH 3 H

5. 8-5 8.1 Structure of ‘R-X’ Haloalkane (alkyl halide) sp 3 R-X H 3 C-Cl methyl chloride RC H X H 1 o halide RC H X R' 2 o halide X RC R" R' 3 o halide

6. 8-6 8.3 Physical Properties “polar covalent bond” - dipole - mismatch of electronegativity -size H 3 C C Br H 3 C H    

7. 8-7 8.4 Halogenation of Alkanes X 2 = Cl 2, Br 2 seldom F 2 (too reactive - exothermic) or I 2 (endothermic, unreactive) hv = ultraviolet light,  = heat Substitution of X for H

8. 8-8 Substitution, products and by-products + other R-X’s hv Cl 2 hv Cl 2 hv Cl 2

9. 8-9 Generally halogenation not useful - mixtures (separate) A few rx’s are useful, e.g.: + Br 2  Br + HBr + other Br's Others - allylic & benzylic

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

11. 8-11 initiation propagation terminations and/or

12. 8-12 + HBr + diBr s + (92) (8) Regioselective for 3 o > 2 o > 1 o C-H (57) (43) + HCl + diCl s + > >

13. 8-13 radical stability like carbocation - electron deficient R C R R R C H R > R. is electron deficient (not charged) > etc. R C R R R C H R >

14. 8-14 Order of stability of R (+) E E ‘Same’ for R. write condensed

15. 8-15 EE > R3CR3C + R2CHR2CH + RCH2RCH2 + H3CH3C + > > Order of stability of R (+) E > R3CR3C. R2CHR2CH. RCH2RCH2. H3CH3C. > > ‘Same’ for R.

16. 8-16 Selectivity 3 o > 2 o > 1 o, but Cl and Br are different Br 2 1600 80 1 Cl 2 5 4 1 3 o 2 o 1 o + HX + other RX major mono-X product

17. 8-17 Hammond’s Postulate

18. 8-18 Hammond’s Postulate Cl vs Br Hammond’s Postulate: the structure of the transition state: –for an exothermic reaction looks more like the reactants of that step - for an endothermic reaction looks more like the products of that step

19. 8-19 Hammond’s Postulate In halogenation of an alkane, the rate- limiting step is hydrogen abstraction –this step is endothermic for bromination  H° (kcal/mol) -88 +10.0+98 ++ CH 3 3 BrCH 3 2 HBr -5.0 -103 +98 ++CH 3 3 ClCH 3 2 HCl and exothermic for chlorination.

20. 8-20 Hammond’s Postulate transition state resembles the alkane and chlorine atom little radical character on carbon in t.s. regioselectivity only slightly influenced by radical stability For chlorination (hydrogen abstraction is exothermic):

21. 8-21 Hammond late t.s. PE SM prog rx Prod stability of R. important SM prog rx Prod PE early t.s. - like SM site of collision important

22. 8-22 Halogenation (free radical substitution)

23. 8-23 Allyl Radical - resonance C C C H H H H H C C C H H H H H C C C H H H H H

24. 8-24 NBS for Br 2 N O O N O O Br X + hv or ROOR  HBr + Br 2 X + X XX R. + HX

25. 8-25 NBS for Br 2

26. 8-26 Free Radical Stability C H H

27. 8-27 C C C H H H H H allylic Free Radical Stability C H H benzylic > > > R C R R R C H R R C H H H C H H

28. 8-28

29. 8-29

30. 8-30

31. 8-31

32. 8-32 Radical Addition of HBr to Alkenes 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

33. 8-33 Radical Addition of HBr to Alkenes –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

34. 8-34 Radical Addition of HBr to Alkenes Chain propagation

35. 8-35 Radical Addition of HBr to Alkenes 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