Electrophilic Additions: Alkenes Addition of Hydrogen Halides

Electrophilic Additions: Alkenes
Chapter 4
Reactions of
Alkenes
Adapted from Profs. Turro & Breslow, Columbia University and Prof. Irene Lee,
Case Western Reserve University
Addition of Hydrogen Halides
What is the product?
Carbocation formation is the rate-limiting step
Carbocation Stabilities
Which carbocation is more stable?
1
Alkyl groups decrease the concentration of positive
charge in the carbocation
Molecular Orbital Diagram in a
Hyperconjugation System
Delocalization of Electrons
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).
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.
I: early transition state
(Like reactants)
II: mid-transition state
III: later transition state
(Like products)
2
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
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,
substituents, and the halogen adds to
the carbon that has the fewer hydrogen
substituents.
substituents.
CH3CH2CH
CH2
HBr
acetic acid
CH3CH2CHCH 3
Br
(80%)
Example
Example11
Markovnikov's Rule
CH3
H
C
CH3
C
H
Markovnikov's Rule
CH3
HBr
acetic acid
CH3
C
CH3
Br
(90%)
Example
Example22
CH3
HCl
CH3
0° C
Cl
(100%)
Example
Example33
3
Mechanistic Basis for Markovnikov's Rule:
Example 1
Mechanistic Basis
for
Markovnikov's Rule
Protonation of double bond occurs in
direction that gives more stable of two
possible carbocations.
carbocations.
Mechanistic Basis for Markovnikov's Rule:
Example 1
CH3CH2CH
CH2
HBr
acetic acid
CH3CH2CHCH 3
Br
Mechanistic Basis for Markovnikov's Rule:
Example 1
+
CH3CH2CH2—CH2
primary carbocation is less stable: not formed
+
CH3CH2CH—
CH—CH3 + Br –
+
CH3CH2CH—
CH—CH3 + Br –
HBr
CH3CH2CH
HBr
CH2
CH3CH2CHCH 3
CH3CH2CH
CH2
CH3CH2CHCH 3
Br
Br
Mechanistic Basis for
Markovnikov's Rule:
Example 3
Mechanistic Basis for
Markovnikov's Rule:
Example 3
H
H
+
CH3 Cl –
HCl
H
H
CH3
HCl
CH3
0° C
Cl
CH3
CH3
Cl
4
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
Rearrangements
Rearrangementssometimes
sometimesoccur
occur
H 2C
Carbocation Rearrangements in
Hydrogen Halide Addition
to Alkenes
HCl,
HCl, 0°
0°C
H
+
CH3CHCH
CHCH(CH3)2
+
CH3CHC(CH3)2
CH3CHCH
CHCH(CH3)2
CH3CH2C(CH3)2
Cl
Rearrangement of Carbocation
1,2-hydride shift
a more stable
carbocation
CHCH
CHCH(CH3)2
(40%)
(60%)
Cl
Rearrangement of Carbocation
1,2-methyl shift
a more stable
carbocation
5
Carbocation Rearrangement
Carbocation does not always rearrange …
Ring Expansion
a more stable
carbocation
Addition of Halogens to Alkene
Addition of Water to Alkene
(alcohols)
Acid-Catalyzed Addition of Alcohol
(ethers)
Addition of Halogens
in the Presence of Water
(halohydrins)
6
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
Examples of Anti-Markovnikov
Addition of an OH Group
(A type of pericyclic reaction; important
reaction and mechanism in directing
reactions both regio- and stereoselectively.)
7
Carbene: another reactive intermediate
Reaction with an Alkene
Synthesis of Bromobutane Isomers
Addition of Radicals to Alkenes
Generation of Free Radicals
Initiation → Propagation → Termination
Using 1/2 arrows for the movement of one electron
Relative Stabilities of Alkyl Radicals
Addition of Hydrogen to Alkenes
8
Catalytic Hydrogenation of an Alkene
9