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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
• Substitutive nomenclature
Cl
Br
I
2-bromopentane
3-iodopropane
CH 3
2-chloro-5-methylheptane
4.3 IUPAC nomenclature of alcohols
OH
OH
OH
1-pentanol
OH
cyclohexanol
H 3C OH
2-propanol
OH
CH 3
2-pentanol
1-methyl cyclohexanol
5-methyl-2-heptanol
4.4 Classes of alcohols and alkyl halides
Br
Primary (1o)
Cl
OH
OH
I
Cl
Secondary (2o)
Tertiary (3o)
CH3
Br
(CH3)3COH
Cl
CH2CH3
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
CH3CH2CH3
propane
b.p. -42oC
CH3CH2F
CH3CH2OH
fluoroethane
ethanol
-32 oC
78oC
4.6 Physical properties – intermolecular forces
Figure 4.4
4.6 Physical properties – water solubility
alcohols
Figure 4.5
Alkyl halides are generally insoluble in water (useful)
4.7 Preparation of alkyl halides from alcohols and HX
R OH
+
alcohol
H X
R X
hydrogen halide
+
alkyl halide
H O H
water
solvent
OH
Br
H Br
NaBr, H 2SO 4
OH
Br
heat
H O H
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 carbocation
Figure 4.9
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
Cl
H O H
Cl
H O H
CH 3
C
H 3C
CH 3
4.9 Full SN1 mechanism showing energy changes
Figure 4.11
4.10 Carbocation structure and stability
Figure 4.8
Figure 4.15
Hyperconjugation
4.10 Relative carbocation stability
Figure 4.12
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
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)
4.13 Other methods for converting ROH to RX
Cl
SOCl2
OH
PBr3
• Convenient way to halogenate a 1o or 2o alcohol
• Avoids use of strong acids such as HCl or HBr
• Usually via SN2 mechanism
Br
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
CHCl3
+
+
Cl2
Cl2

(~400oC)

(~400oC)
CCl4
+
HCl
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)
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
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

CH2CH2CH2CH3

Cl
H

CHCH2CH3
CH3
Lower energy radical,
formed faster
4.18 Free Radical Halogenation of Higher Alkanes
Figure 4.16
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·
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