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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
Br
I
Cl
pentyl chloride
cyclohexyl bromide
1-methylethyl iodide
• Substitutive nomenclature
Cl
Br
I
2-bromopentane
3-iodopropane
CH 3
2-chloro-5-methylheptane
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4.3 IUPAC nomenclature of alcohols
OH
OH
OH
1-pentanol
OH
2-propanol
cyclohexanol
H 3C OH
OH
CH 3
2-pentanol
1-methyl cyclohexanol
5-methyl-2-heptanol
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4.4 Classes of alcohols and alkyl halides
Br
Primary (1o)
Cl
OH
Secondary
OH
I
(2o)
Tertiary (3o)
Cl
CH3
Br
(CH3)3COH
Cl
CH2CH3
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
CH3CH2CH3
propane
b.p. -42oC
CH3CH2F
CH3CH2OH
fluoroethane
ethanol
-32 oC
78oC
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4.6 Physical properties – intermolecular forces
Figure 4.4
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4.6 Physical properties – water solubility
alcohols
Figure 4.5
Alkyl halides are generally insoluble in water (useful)
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4.7 Preparation of alkyl halides from alcohols and HX
solvent
OH
H O H
Br
H Br
NaBr, H 2SO 4
OH
Br
heat
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4.8 Mechanism of alkyl halide formation
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 carbocation
Figure 4.9
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4.9 Full mechanism “pushing” curved arrows
H3 C
H3 C C O H
H Cl
H3 C
H3 C C Cl
H3 C
H3 C
H Cl
H 3C H
H 3C C O H
H 3C
H O H
Cl
H O H
CH 3
C
H 3C
CH 3
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
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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
R
R
R
H
R C OH > R C OH > H C OH > H C OH
R
H
H
H
Related to the stability of the intermediate carbocation:
CH3
CH3
>
H3C
CH3
CH3
>
H3C
H
H
>
H
H
H
H
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4.11 Relative rates of reaction of R3COH with HX
Figure 4.16
Rate-determining step involves formation of carbocation
4.12 Reaction of methyl and 1o alcohols with HX – SN2
4.12 Substitution Reaction Mechanism - SN2
-
X
RCH2
OH2
X
R
CH2
+
OH2
X CH2R + 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)
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4.13 Other methods for converting ROH to RX
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
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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
X:Y
X
Y
heterolytic
homolytic
4.15 Free Radical Chlorination of Methane
CH4
+
Cl2

o
CH3Cl
+
HCl
(~400 C)
CH3Cl
+
Cl2

o
CH2Cl2
+
HCl
CHCl3
+
HCl
(~400 C)
CH2Cl2
+
Cl2

o
(~400 C)
CHCl3
+
Cl2

o
(~400 C)
CCl4
+
HCl
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4.16 Structure and stability of Free Radicals
CH3
CH3
>
H3C
CH3 H3C
CH3
>
H
H
>
H
H
H
H
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
Cl : Cl
H : CH3
CH3
2
Initiation
Cl
Cl : H
CH3
Cl : CH3
CH3
CH3
CH3 : CH3
Cl
CH3
Cl : CH3
Propagation
Cl
Termination
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4.18 Free Radical Halogenation of Higher Alkanes
420oC
CH3CH3
+
Cl2
CH3CH2Cl
+
HCl
78%
h
CH3CH2CH2CH3
+
Cl2
CH3CH2CH2CH2Cl
28%
CH3CHCH2CH3
Cl
+ HCl
72%
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.

Cl
H

CH 2CH 2CH2CH3

Cl
H

CHCH 2CH3
CH3
Lower energy radical,
formed faster
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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
h
+
HBr
Br
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·
Bromination – late TS looks a lot
like radical
Chlorination – early TS looks
less like radical
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