Figure 4.2 Carey Chapter 4 – Alcohols and Alkyl Halides

4.1 Functional groups – a look ahead

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

4.3 IUPAC nomenclature of alcohols 1-pentanolcyclohexanol 2-propanol 2-pentanol1-methyl cyclohexanol 5-methyl- 2-heptanol

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

4.5 Bonding in alcohols and alkyl halides Figure 4.1

4.5 Bonding in alcohols and alkyl halides Figure 4.2

4.6 Physical properties – intermolecular forces Figure 4.4 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 C

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

4.7 Preparation of alkyl halides from alcohols and HX

4.8 Mechanism of alkyl halide formation

4.8 Energetic description of mechanism Step 1 - protonation Figure 4.6

4.8 Energetic description of mechanism Step 2 – carbocation formation Figure 4.7

4.8 Energetic description of mechanism Step 3 – trapping the carbocation Figure 4.9

4.9 Full mechanism “pushing” curved arrows

4.9 Full S N 1 mechanism showing energy changes Figure 4.11

4.10 Carbocation structure and stability Figure 4.8 Figure 4.15 Hyperconjugation

Figure Relative carbocation stability

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:

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

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)

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 RX

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

4.15 Free Radical Chlorination of Methane

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

4.16 Bond Dissociation Energies (BDE)

4.17 Mechanism of Methane Chlorination

4.17 Mechanism for Free Radical Chlorination of Methane

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, formed faster

4.18 Free Radical Halogenation of Higher Alkanes Figure 4.16

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