SLIDE 1
Carey Chapter 4 Alcohols and Alkyl Halides Figure 4.2 4.1 - - PowerPoint PPT Presentation
Carey Chapter 4 Alcohols and Alkyl Halides Figure 4.2 4.1 - - PowerPoint PPT Presentation
Carey Chapter 4 Alcohols and Alkyl Halides Figure 4.2 4.1 Functional groups a look ahead 4.2 IUPAC nomenclature of alkyl halides Functional class nomenclature Br I Cl pentyl chloride cyclohexyl bromide 1-methylethyl iodide
SLIDE 2
SLIDE 3
4.2 IUPAC nomenclature of alkyl halides
- Functional class nomenclature
Cl Br I
pentyl chloride cyclohexyl bromide 1-methylethyl iodide
- Substitutive nomenclature
Br I Cl CH3
2-bromopentane 3-iodopropane 2-chloro-5-methylheptane
SLIDE 4
4.3 IUPAC nomenclature of alcohols OH OH OH
1-pentanol cyclohexanol 2-propanol
OH OH CH3 H3C OH
2-pentanol 1-methyl cyclohexanol 5-methyl- 2-heptanol
SLIDE 5
4.4 Classes of alcohols and alkyl halides
Cl OH Br
Primary (1o) Secondary (2o)
OH I Cl
Tertiary (3o)
Br CH3 (CH3)3COH CH2CH3 Cl
SLIDE 6
4.5 Bonding in alcohols and alkyl halides
Figure 4.1
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4.5 Bonding in alcohols and alkyl halides
Figure 4.2
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4.6 Physical properties – intermolecular forces
Figure 4.4
CH3CH2CH3 CH3CH2F CH3CH2OH
propane fluoroethane ethanol
b.p. -42oC
- 32 oC
78oC
SLIDE 9
4.6 Physical properties – water solubility Alkyl halides are generally insoluble in water (useful)
alcohols Figure 4.5
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4.7 Preparation of alkyl halides from alcohols and HX
R OH + H X R X + H O H alcohol hydrogen halide alkyl halide water
OH H Br Br H O H solvent
OH NaBr, H2SO4 heat Br
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4.8 Mechanism of alkyl halide formation
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4.8 Energetic description of mechanism Step 1 - protonation
Figure 4.6
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4.8 Energetic description of mechanism Step 2 – carbocation formation
Figure 4.7
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4.8 Energetic description of mechanism Step 3 – trapping the carbocation
Figure 4.9
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4.9 Full mechanism “pushing” curved arrows
H3C C H3C H3C O H H Cl H3C C H3C H3C Cl H O H H Cl H3C C H3C H3C O H H C CH3 H3C CH3 Cl H O H Cl
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4.9 Full SN1 mechanism showing energy changes
Figure 4.11
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4.10 Carbocation structure and stability
Figure 4.8 Figure 4.15
Hyperconjugation
SLIDE 18
Figure 4.12
4.10 Relative carbocation stability
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4.11 Relative rates of reaction of R3COH with HX
Relative Rates of Reaction for Different Alcohols with HX
C R R R OH C R R H OH C R H H OH C H H H OH > > >
Related to the stability of the intermediate carbocation:
CH3 H3C CH3 CH3 H3C H CH3 H H H H H > > >
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4.11 Relative rates of reaction of R3COH with HX Rate-determining step involves formation of carbocation
Figure 4.16
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4.12 Reaction of methyl and 1o alcohols with HX – SN2
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4.12 Substitution Reaction Mechanism - SN2
X RCH2 OH2 CH2 OH2 R X X CH2R
δ+ δ-
+ H2O
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)
SLIDE 23
Cl SOCl2 OH PBr3 Br
- Convenient way to halogenate a 1o or 2o alcohol
- Avoids use of strong acids such as HCl or HBr
- Usually via SN2 mechanism
4.13 Other methods for converting ROH to RX
SLIDE 24
4.14 Free Radical Halogenation of Alkanes
R-H + X2 R-X + H-X
Types of bond cleavage:
X : Y X : Y
heterolytic
X : Y X Y
homolytic
SLIDE 25
CH4 + Cl2 CH3Cl + HCl ∆ (~400oC) CH3Cl + Cl2 CH2Cl2 + HCl ∆ (~400oC) CH2Cl2 + Cl2 CHCl3 + HCl ∆ (~400oC) CHCl3 + Cl2 CCl4 + HCl ∆ (~400oC)
4.15 Free Radical Chlorination of Methane
SLIDE 26
CH3 H3C CH3 CH3 H3C H CH3 H H H H H
> > >
4.16 Structure and stability of Free Radicals
Orbital hybridization models of bonding in methyl radical (Figure 4.17)
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4.16 Bond Dissociation Energies (BDE)
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4.17 Mechanism of Methane Chlorination
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Cl Cl Cl CH3 Cl Cl 2 Cl CH3 Cl Initiation : H : CH3 Cl : H Propagation : Cl : CH3
4.17 Mechanism for Free Radical Chlorination of Methane
CH3 CH3 CH3 Cl CH3 : CH3 Termination Cl : CH3
SLIDE 30
4.18 Free Radical Halogenation of Higher Alkanes
CH3CHCH2CH3 Cl CH3CH3 + Cl2 420oC CH3CH2Cl + HCl 78% CH3CH2CH2CH3 + Cl2 + HCl 28% 72% hν CH3CH2CH2CH2Cl
Radical abstraction of H is selective since the stability of the ensuing radical is reflected in the transition state achieved during abstraction.
Cl H CH2CH2CH2CH3 δ δ Cl H CHCH2CH3 δ δ CH3
Lower energy, formed faster
SLIDE 31
4.18 Free Radical Halogenation of Higher Alkanes
Figure 4.16
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