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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
Cl
BrI
pentyl chloride cyclohexyl bromide 1-methylethyl iodide
• Substitutive nomenclature
Br
I
Cl
CH3
2-bromopentane 3-iodopropane 2-chloro-5-methylheptane
4.3 IUPAC nomenclature of alcohols
OH
OHOH
1-pentanol cyclohexanol 2-propanol
OH OH
CH3
H3C OH
2-pentanol 1-methyl cyclohexanol
5-methyl-
2-heptanol
4.4 Classes of alcohols and alkyl halides
Cl OHBr
Primary (1o)
Secondary (2o)
OH ICl
Tertiary (3o)
BrCH3
(CH3)3COHCH2CH3
Cl
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
CH3CH2CH3 CH3CH2F CH3CH2OH
propane fluoroethane ethanol
b.p. -42oC -32 oC 78oC
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
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 4.6
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
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
4.9 Full SN1 mechanism showing energy changes
Figure 4.11
4.10 Carbocation structure and stability
Figure 4.8
Figure 4.15
Hyperconjugation
Figure 4.12
4.10 Relative carbocation stability
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> > >
4.11 Relative rates of reaction of R3COH with HX
Rate-determining step involves formation of carbocation
Figure 4.16
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)
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
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
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
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)
4.16 Bond Dissociation Energies (BDE)
4.17 Mechanism of Methane Chlorination
Cl Cl
Cl
CH3Cl 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
4.18 Free Radical Halogenation of Higher Alkanes
CH3CH3 + Cl2420oC
CH3CH2Cl + HCl
78%
CH3CH2CH2CH3 + Cl2 CH3CHCH2CH3
Cl
+ HCl
28% 72%
hCH3CH2CH2CH2Cl
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, 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
Br2
Brh+ HBr
76%, only product
Chlorination – early TS looks less like radical
Consider propagation steps – endothermic with Br·, exothermic with Cl·