Download Ch04-04-alkenes-2

Chapter 4
Reactions of
Alkenes
Adapted from Profs. Turro & Breslow, Columbia University and Prof. Irene Lee,
Case Western Reserve University
Electrophilic Additions: Alkenes
Addition of Hydrogen Halides
What is the product?
Carbocation formation is the rate-limiting step
Which carbocation is more stable?
Carbocation Stabilities
Alkyl groups decrease the concentration of positive
charge in the carbocation
Delocalization of Electrons
Molecular Orbital Diagram in a
Hyperconjugation System
Hammond postulate: the transition state will be more
similar to the species that it is closer to energetically
Exergonic reaction: early transition state
resembles reactants (I).
Endergonic reaction: late transition state
resembles products (II).
I: early transition state
(Like reactants)
II: mid-transition state
III: later transition state
(Like products)
Markovnikov’s Rule
The electrophile adds to the sp2 carbon that is bonded to
the greater number of hydrogens
In a regioselective reaction, one constitutional isomer is the
major or the only product.
Explained by the intermediates, for example:
tert-butyl cation is formed faster and it is more stable
than isobutyl.
Regioselectivity of
Hydrogen Halide Addition:
Markovnikov's Rule
Markovnikov's Rule
When an unsymmetrically substituted
alkene reacts with a hydrogen halide,
the hydrogen adds to the carbon that
has the greater number of hydrogen
substituents, and the halogen adds to
the carbon that has the fewer hydrogen
substituents.
Markovnikov's Rule
CH3CH2CH
CH2
HBr
acetic acid
CH3CH2CHCH3
Br
(80%)
Example 1
Markovnikov's Rule
CH3
C
CH3
CH3
H
HBr
C
H
acetic acid
CH3
C
CH3
Br
(90%)
Example 2
Markovnikov's Rule
CH3
HCl
CH3
0°C
Cl
(100%)
Example 3
Mechanistic Basis
for
Markovnikov's Rule
Protonation of double bond occurs in
direction that gives more stable of two
possible carbocations.
Mechanistic Basis for Markovnikov's Rule:
Example 1
CH3CH2CH
CH2
HBr
CH3CH2CHCH3
acetic acid
Br
Mechanistic Basis for Markovnikov's Rule:
Example 1
+
CH3CH2CH—CH3 + Br –
HBr
CH3CH2CH
CH2
CH3CH2CHCH3
Br
Mechanistic Basis for Markovnikov's Rule:
Example 1
+
CH3CH2CH2—CH2
primary carbocation is less stable: not formed
+
CH3CH2CH—CH3 + Br –
HBr
CH3CH2CH
CH2
CH3CH2CHCH3
Br
Mechanistic Basis for
Markovnikov's Rule:
Example 3
H
CH3
HCl
CH3
0°C
Cl
Mechanistic Basis for
Markovnikov's Rule:
Example 3
H
H
+
CH3
Cl –
HCl
H
CH3
CH3
Cl
H
secondary
carbocation is
less stable:
not formed
H
+
Mechanistic Basis for
Markovnikov's Rule:
Example 3
CH3
H
H
+
CH3
Cl –
HCl
H
CH3
CH3
Cl
Carbocation Rearrangements in
Hydrogen Halide Addition
to Alkenes
Rearrangements sometimes occur
H2C
CHCH(CH3)2
HCl, 0°C
H
+
CH3CHCH(CH3)2
+
CH3CHC(CH3)2
CH3CHCH(CH3)2
CH3CH2C(CH3)2
Cl
(40%)
(60%)
Cl
Rearrangement of Carbocation
1,2-hydride shift
a more stable
carbocation
Rearrangement of Carbocation
1,2-methyl shift
a more stable
carbocation
Carbocation Rearrangement
Ring Expansion
a more stable
carbocation
Carbocation does not always rearrange …
Addition of Halogens to Alkene
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are needed to see this picture.
Addition of Water to Alkene
(alcohols)
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and a
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decompressor
Graphics decompressor
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are
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see this picture.
Graphics decompressor
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Acid-Catalyzed Addition of Alcohol
(ethers)
Addition of Halogens
in the Presence of Water
(halohydrins)
Oxymercuration and Mercuration
of Alkene
(alcohols w/o carbocation rearrangement)
Addition of Borane
Hydroboration–Oxidation
Anti-Markovnikov’s rule in product formation
(less substituted alcohols)
Vs. Markovnikov’s rule in product formation
(more substituted alcohols)
Formation of Alkyl Boranes
Anti-Markovnikov
Addition
Boron adds to least hindered carbon
Anti-Markovnikov
Addition
Markovnikov
Addition
Boron adds to least hindered carbon
and is replaced w/ -OH by oxidation
Formation of the most
stable carbocation
(A type of pericyclic reaction; important
reaction and mechanism in directing
reactions both regio- and stereoselectively.)
Examples of Anti-Markovnikov
Addition of an OH Group
Carbene: another reactive intermediate
Reaction with an Alkene
Synthesis of Bromobutane Isomers
Generation of Free Radicals
Using 1/2 arrows for the movement of one electron
Addition of Radicals to Alkenes
Initiation  Propagation  Termination
Relative Stabilities of Alkyl Radicals
Addition of Hydrogen to Alkenes
Catalytic Hydrogenation of an Alkene