Transcript Chapter 4 - “Alcohols and Alkyl Halides”

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Chapter 4 - “Alcohols and Alkyl
Halides”
1)
2)
3)
4)
Functional groups and nomenclature
Role of intermolecular forces on properties
Synthesis of alkyl halides from alcohols
Two reaction mechanisms: SN1 and SN2
cation stability and the Hammond Postulate
5) Additional ways to prepare RX from ROH
6) Halogenation of alkanes: reactivityselectivity principle, radical stability and
chain reactions
Minto - Lectures 7-8
All sections are covered.
Potential Energy Diagram of SN1
Reaction of an Alcohol with HX
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Rate = k[alkyloxonium ion]
Mechanisms can be supported or disproved, but cannot be proven correct.
Minto - Lectures 7-8
Carbocation Stabilities
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Alkyl groups decrease the concentration of (a.k.a. delocalize)
positive charge in the carbocation by inductive effect and
hyperconjugation.
Minto - Lectures 7-8
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Delocalization of Electrons
b
a
b
b
This effect, hyperconjugation, depends upon the number of
s bonds between the a and b atoms next to a carbocation.
Ethyl (above) has 3, isopropyl cation has 6, and tert-butyl
has 9.
(Note: my naming differs slightly from the text)
Minto - Lectures 7-8
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Molecular Orbital Diagram in a
Hyperconjugation System
Minto - Lectures 7-8
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Gas Phase Stabilities of Cations
R—Cl
R• + Cl•
H° = DH°R-Cl
R•
R+ + e–
H° = IPR•
Cl–
H° = -EACl•
R+ + Cl–
H° = DH°R-Cl + IPR• -EACl•
Cl• + e–
Net Reaction: R—Cl
R
DH°RCl
CH3
84
CH3CH2
82
(CH3)2CH
81
(CH3)3C
79
All values are in kcal/mol
IPR
226
196
175
161
-EACl
-83
-83
-83
-83
H°
227
195
173
157
The gas phase stabilities of the cations differ greatly (last
column in table), largely due to ionization potentials for R•.
Polar solvents will reduce the differences, but only partially.
Minto - Lectures 7-8
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How is cation stability be related to rates of SN1
reactions?
Hammond postulate: the transition state will be more
similar to the species that it is closest to energetically
Exergonic reaction: early transition state (I)
Endergonic reaction: late transition state (III)
Minto - Lectures 7-8
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«–Thermoneutral reaction
I: early transition state; ‡ structure resembles reactant
II: mid-transition state; ‡ structure is the average
between reactant and product
III: late transition state; ‡ structure resembles product
Minto - Lectures 7-8
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Let’s apply the Hammond postulate to the nucleophilic substitution
of (CH3)2CHCH2OH by HX. According to the Postulate, the
transition state leading to the tert-C4H9 cation is lower in energy
than the barrier to the isobutyl cation, as it reflects the greater
stability of the tert-C4H9 cation. To make this comparison, it was
important that both oxonium ions are of similar stability. As the
barrier to forming tert-C4H9 cation is lower, it is formed faster. SN1
reactions occur with 3° and 2° ROH.
Minto - Lectures 7-8
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Minto - Lectures 7-8
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SN2 Mechanism for Forming RX from ROH
Minto - Lectures 7-8
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Other Methods of Forming RX
Minto - Lectures 7-8
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Using Relative Rate and Statistical Factors to Predict
Product Distributions
Cl2
Cl
Cl
+
+
h, 35 °C
We are ignoring
stereoisomers
here.
rate factor = 9 x 1 = 9
rate factor = 6 x 1 = 6
Cl
rate factor = 2 x 3.9 = 7.8
Yield = (9/35.9)x 100%
= 25%
Yield = (6/35.9)x 100%
= 17%
Yield = (7.8/35.9)x 100%
= 22%
Cl
Cl
+
rate factor = 2 x 3.9 = 7.8
rate factor = 1 x 5.3 = 5.3
Yield = (7.8/35.9)x 100%
= 22%
Yield = (5.3/35.9)x 100%
= 15%
SUM OF ALL RATE FACTORS
IS THE DENOMINATOR = 35.9
Bromination of the same alkane would lead to predicted yields of <1%, <1%, 8%, 8%, and 82%
for the analogous bromination products. This reaction is much more selective than chlorination.
These calculations actually underestimate the selectivity problem for
chlorination because multiple chlorination also competes with
monochlorination. The chlorination reaction is seldom used except in
cases where only one monochlorination product is possible.
Minto - Lectures 7-8