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Carey Chapter 4 – Alcohols and Alkyl Halides. Figure 4.2. YSU. 4.1 Functional groups – a look ahead. YSU. 4.2 IUPAC nomenclature of alkyl halides. YSU. Functional class nomenclature. pentyl chloride. cyclohexyl bromide. 1-methylethyl iodide. Substitutive nomenclature. - PowerPoint PPT Presentation
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Carey Chapter 4 – Alcohols and Alkyl Halides
Figure 4.2
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4.1 Functional groups – a look ahead
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4.2 IUPAC nomenclature of alkyl halides
• Functional class nomenclature
Cl
BrI
pentyl chloride cyclohexyl bromide 1-methylethyl iodide
• Substitutive nomenclature
Br
I
Cl
CH3
2-bromopentane 3-iodopropane 2-chloro-5-methylheptane
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4.3 IUPAC nomenclature of alcohols
OH
OHOH
1-pentanolcyclohexanol
2-propanol
OH OH
CH3
H3C OH
2-pentanol 1-methyl cyclohexanol 5-methyl-2-heptanol
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4.4 Classes of alcohols and alkyl halides
Primary (1o)
Secondary (2o)
Tertiary (3o)
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Cl OHBr
OH ICl
BrCH3
(CH3)3COHCH2CH3
Cl
4.5 Bonding in alcohols and alkyl halides YSUYSU
Figure 4.1
4.5 Bonding in alcohols and alkyl halides
Figure 4.2
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4.6 Physical properties – intermolecular forces
CH3CH2CH3 CH3CH2F CH3CH2OH
propane fluoroethane ethanol
b.p. -42oC -32 oC 78oC
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Figure 4.4
4.6 Physical properties – intermolecular forces YSUYSU
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
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R OH + H X R X + H O H
alcohol hydrogen halide alkyl halide water
OH H Br Br H O Hsolvent
OH
NaBr, H2SO4
heatBr
4.8 Mechanism of alkyl halide formation
4.8 Energetic description of mechanism - Step 1 : protonation
Figure 4.6YSUYSU
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 carbocation
Figure 4.9
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4.9 Full mechanism “pushing” curved arrows
H3C
CH3C
H3C
O H
H ClH3C
CH3C
H3C
Cl H O H
H Cl
H3C
CH3C
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 YSUYSU
4.10 Relative carbocation stability
Figure 4.12
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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
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)YSUYSU
RCH2 OH2X CH2 OH2
R
X+-
X CH2R + H2O
OH PBr3 BrSOCl2Cl
• 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
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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
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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
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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)
4.17 Mechanism of Methane Chlorination
4.17 Mechanism for Free Radical Chlorination of Methane
Cl Cl
Cl
CH3Cl Cl
2 Cl
CH3
Cl
Initiation:
H : CH3 Cl : H Propagation
: Cl : CH3
CH3CH3 CH3 : CH3 Termination
CH3Cl Cl : CH3 YSUYSU
4.18 Free Radical Halogenation of Higher Alkanes
CH3CH3 + Cl2420oC
CH3CH2Cl + HCl
78%
CH3CH2CH2CH3 + Cl2 CH3CHCH2CH3
Cl
+ HCl
28% 72%
hCH3CH2CH2CH2Cl
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4.18 Free Radical Halogenation of Higher Alkanes
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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
Cl H CHCH2CH3
CH3
Lower energy radical, formed faster
4.18 Free Radical Halogenation of Higher Alkanes
Figure 4.16
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4.18 Bromine radical is more selective than chlorine radical
Br2
Brh+ HBr
76%, only product
Consider propagation steps – endothermic with Br·, exothermic with Cl·
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4.18 Bromine radical is more selective than chlorine radical
Consider propagation steps – endothermic with Br·, exothermic with Cl·
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Bromination – late TS looks a lot like radical
Chlorination – early TS looks less like radical
