Carey Chapter 4 – Alcohols and Alkyl Halides

1. Carey Chapter 4 – Alcohols and Alkyl Halides Figure 4.2 YSU

2. 4.1 Functional groups – a look ahead YSU

3. 4.2 IUPAC nomenclature of alkyl halides YSU • Functional class nomenclature pentyl chloride cyclohexyl bromide 1-methylethyl iodide • Substitutive nomenclature 2-bromopentane 3-iodopropane 2-chloro-5-methylheptane

4. 4.3 IUPAC nomenclature of alcohols YSU 1-pentanol 2-propanol cyclohexanol 2-pentanol 1-methyl cyclohexanol 5-methyl-2-heptanol

5. 4.4 Classes of alcohols and alkyl halides YSU Primary (1o) Secondary (2o) Tertiary (3o)

6. 4.5 Bonding in alcohols and alkyl halides YSU Figure 4.1

7. 4.5 Bonding in alcohols and alkyl halides YSU Figure 4.2

8. 4.6 Physical properties – intermolecular forces YSU CH3CH2CH3 CH3CH2F CH3CH2OH propane fluoroethane ethanol b.p. -42oC -32 oC 78oC

9. 4.6 Physical properties – intermolecular forces YSU Figure 4.4

10. 4.6 Physical properties – water solubility YSU alcohols Figure 4.5 Alkyl halides are generally insoluble in water (useful)

11. 4.7 Preparation of alkyl halides from alcohols and HX YSU

12. 4.8 Mechanism of alkyl halide formation

13. 4.8 Energetic description of mechanism - Step 1 : protonation Figure 4.6 YSU

14. 4.8 Energetic description of mechanism - Step 2 : carbocation formation Figure 4.7 YSU

15. 4.8 Energetic description of mechanism - Step 3 : trapping carbocation Figure 4.9 YSU

16. 4.9 Full mechanism “pushing” curved arrows YSU

17. 4.9 Full SN1 mechanism showing energy changes Figure 4.11 YSU

18. 4.10 Carbocation structure and stability Figure 4.8 Figure 4.15 Hyperconjugation YSU

19. 4.10 Relative carbocation stability YSU Figure 4.12

20. 4.11 Relative rates of reaction of R3COH with HX YSU Relative Rates of Reaction for Different Alcohols with HX Related to the stability of the intermediate carbocation:

21. 4.11 Relative rates of reaction of R3COH with HX YSU Figure 4.16 Rate-determining step involves formation of carbocation

22. 4.12 Reaction of methyl and 1o alcohols with HX – SN2

23. 4.12 Substitution Reaction Mechanism - SN2 Transition state • Alternative pathway for alcohols that cannot form a good carbocation • Rate determining step is bimolecular (therefore SN2) • Reaction profile is a smooth, continuous curve (concerted) YSU

24. 4.13 Other methods for converting ROH to RX • Convenient way to halogenate a 1o or 2o alcohol • Avoids use of strong acids such as HCl or HBr • Usually via SN2 mechanism YSU

25. 4.14 Free Radical Halogenation of Alkanes YSU R-H + X2 R-X + H-X Types of bond cleavage: heterolytic homolytic

26. 4.15 Free Radical Chlorination of Methane YSU

27. 4.16 Structure and stability of Free Radicals Orbital hybridization models of bonding in methyl radical (Figure 4.17) YSU

28. 4.16 Bond Dissociation Energies (BDE)

29. 4.17 Mechanism of Methane Chlorination

30. 4.17 Mechanism for Free Radical Chlorination of Methane YSU

31. 4.18 Free Radical Halogenation of Higher Alkanes YSU

32. 4.18 Free Radical Halogenation of Higher Alkanes Radical abstraction of H is selective since the stability of the ensuing radical is reflected in the transition state achieved during abstraction. Lower energy radical, formed faster YSU

33. 4.18 Free Radical Halogenation of Higher Alkanes Figure 4.16 YSU

34. 4.18 Bromine radical is more selective than chlorine radical Consider propagation steps – endothermic with Br·, exothermic with Cl· YSU

35. 4.18 Bromine radical is more selective than chlorine radical Consider propagation steps – endothermic with Br·, exothermic with Cl· Bromination – late TS looks a lot like radical Chlorination – early TS looks less like radical YSU