Carey Chapter 4 – Alcohols and Alkyl Halides Figure 4.2 YSU

4.1 Functional groups – a look aheadYSU

4.2 IUPAC nomenclature of alkyl halides Functional class nomenclature pentyl chloridecyclohexyl bromide1-methylethyl iodide Substitutive nomenclature 2-bromopentane3-iodopropane2-chloro-5-methylheptaneYSU

4.3 IUPAC nomenclature of alcohols 1-pentanol cyclohexanol 2-propanol 2-pentanol 1-methyl cyclohexanol 5-methyl-2-heptanolYSU

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

4.5 Bonding in alcohols and alkyl halidesYSU Figure 4.1

4.5 Bonding in alcohols and alkyl halides Figure 4.2YSU

4.6 Physical properties – intermolecular forces CH 3 CH 2 CH 3 CH 3 CH 2 FCH 3 CH 2 OH propane fluoroethane ethanol b.p. -42 o C -32 o C 78 o CYSU

Figure Physical properties – intermolecular forcesYSU

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

4.7 Preparation of alkyl halides from alcohols and HXYSU

4.8 Mechanism of alkyl halide formation

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

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

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

4.9 Full mechanism “pushing” curved arrowsYSU

4.9 Full S N 1 mechanism showing energy changes Figure 4.11 YSU

4.10 Carbocation structure and stability Figure 4.8 Figure 4.15 HyperconjugationYSU

4.10 Relative carbocation stability Figure 4.12YSU

4.11 Relative rates of reaction of R 3 COH with HX Relative Rates of Reaction for Different Alcohols with HX Related to the stability of the intermediate carbocation:YSU

4.11 Relative rates of reaction of R 3 COH with HX Rate-determining step involves formation of carbocation Figure 4.16YSU

4.12 Reaction of methyl and 1 o alcohols with HX – S N 2

4.12 Substitution Reaction Mechanism - S N 2 Transition state Alternative pathway for alcohols that cannot form a good carbocation Rate determining step is bimolecular (therefore S N 2) Reaction profile is a smooth, continuous curve (concerted) YSU

Convenient way to halogenate a 1 o or 2 o alcohol Avoids use of strong acids such as HCl or HBr Usually via S N 2 mechanism 4.13 Other methods for converting ROH to RXYSU

4.14 Free Radical Halogenation of Alkanes R-H + X 2 R-X + H-X Types of bond cleavage: heterolytic homolyticYSU

4.15 Free Radical Chlorination of MethaneYSU

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

4.16 Bond Dissociation Energies (BDE)

4.17 Mechanism of Methane Chlorination

4.17 Mechanism for Free Radical Chlorination of MethaneYSU

4.18 Free Radical Halogenation of Higher AlkanesYSU

YSU 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

4.18 Free Radical Halogenation of Higher Alkanes Figure 4.16 YSU

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

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