Chapter 6 Reactions of Alkenes: Addition Reactions

Chapter 6
Reactions of Alkenes:
Addition Reactions
6.1
Hydrogenation of Alkenes
Reactions of Alkenes
The characteristic reaction of alkenes
is addition to the double bond.
+ A—B
C C A C C B
H
H
H σ
H H
π
σ σ
C C + H—HH
H C C H
H
Hydrogenation of Ethylene
exothermic ΔH° = –136 kJ/mol
catalyzed by finely divided Pt, Pd, Rh, Ni
H
H
H 3 C
Example
CH 2
Problem 6.1
What three alkenes yield 2-methylbutane on
catalytic hydrogenation
H 3 C
H 2 , Pt
H 2 , Pt
H 3 C
CH 3
H
H 3 C
(73%)

Question 2
Which of the alkenes below will produce 2-
methylbutane on catalytic hydrogenation
A) 1 and 3
B) 1, 2 and 3
C) 2 and 4
D) 2, 3 and 4
6.2
Heats of Hydrogenation
can be used to measure relative stability
of isomeric alkenes
correlation with structure is same as
when heats of combustion are measured
Heats of Hydrogenation of Isomers
Heats of Hydrogenation (kJ/mol)
Ethylene 136
Monosubstituted 125-126
126
126
119
119
115
115
cis-Disubstituted
117-119
trans-Disubstituted
114-115
Terminally disubstituted 116-117
Trisubstituted 112
CH 3 CH 2 CH 2 CH 3
Tetrasubstituted 110
Question 1
Rank the following alkenes in order of
decreasing heat of hydrogenation.
Match each alkene of Problem 6.1 with its correct
heat of hydrogenation.
126 kJ/mol
Problem 6.2
highest heat of
hydrogenation;
least stable isomer
A) 1 > 3 > 2
B) 3 > 2 > 1
C) 2 > 3 > 1
D) 1 > 2 > 3
118 kJ/mol
112 kJ/mol
lowest heat of
hydrogenation;
most stable isomer

Question 4
Which alkene has the lowest heat of
hydrogenation
A) B)
6.3
Stereochemistry of Alkene
Hydrogenation
C) D)
Catalytic Hydrogenation of an Alkene
Two Spatial (stereochemical) Aspects of
Alkene Hydrogenation
(1) syn addition of both H atoms to double bond (adds
from the same side)
(2) hydrogenation is stereoselective,
corresponding to addition to less crowded face of
double bond
Two Spatial (stereochemical) Aspects of
Alkene Hydrogenation
syn Addition versus anti Addition
(1) syn addition of both H atoms to double bond
syn addition
anti addition

Example of syn-Addition
Question
CO 2 CH 3
H 2 , Pt
H
CO 2 CH 3
CO 2 CH 3
H 2 , Pt
H
CO 2 CH 3
CO 2 CH 3
H
(100%)
CO 2 CH 3
CO 2 CH 3
True (A) / False (B)
The hydrogenation product is a chiral
molecule.
H
CO 2 CH 3
Two Spatial (stereochemical) Aspects of
Alkene Hydrogenation
(1) syn addition of both H atoms to double bond
(2) hydrogenation is stereoselective,
corresponding to addition to less crowded
face of double bond
Two Spatial (stereochemical) Aspects of
Alkene Hydrogenation
(1) syn addition of both H atoms to double bond
(2) hydrogenation is stereoselective,
corresponding to addition to less crowded face
of double bond
A reaction in which a single starting material
can give two or more stereoisomeric products
but yields one of them in greater amounts than
the other (or even to the exclusion of the other)
is said to be stereoselective.
H
H 3 C
CH 3
Example of
Stereoselective
Reaction
H
H 3 C
CH 3
Example of
Stereoselective
Reaction
H 3 C
H 2 , cat
H 3 C
H 2 , cat
H 3 C
H
CH 3
H 3 C
H
CH 3
H 3 C
H
CH 3
H
H 3 C
H
Both products
correspond to
syn addition
of H 2 .
H
H
H 3 C
H
H 3 C
H
But only this
one is formed.

H
H 3 C
CH 3
Example of
Stereoselective
Reaction
H
H 3 C
CH 3
Example of
Stereoselective
Reaction
H 3 C
H 2 , cat
H 3 C
H 2 , cat
H 3 C
H
Top face of of double
bond blocked by by
this methyl group
CH 3
H 3 C
H
CH 3
H
H 3 C
H
H
H 3 C
H
H 2 adds to to
bottom face of of
double bond.
Question 14
Which one of the following terms best applies to
the hydrogenation of an alkene in the
presence of finely divided platinum (in
ethanol as solvent)
A) anti addition
B) concerted reaction
C) heterogeneous catalysis
D) endothermic reaction
6.4
Electrophilic Addition of
Hydrogen Halides to
Alkenes
Electrophilic Additions: Alkenes
When the Electrophile is a
Hydrogen Halide
δ+ δ–
C C + H—X
H C C X

Example
Mechanism
CH 3 CH 2
H
C
C
CH 2 CH 3
H
HBr
CHCl 3 , -30°C
Electrophilic addition of hydrogen halides
to alkenes proceeds by rate-determining
formation of a carbocation intermediate.
CH 3 CH 2 CH 2 CHCH
2 CH 3
Br
(76%)
Mechanism
Mechanism
Electrons flow from
the π system of the
alkene (electron rich)
toward the positively
polarized proton of
the hydrogen halide.
H
+C C
..
.. X
:
H
..
:
.. X
: –
C
C
..
: .. X
C
C
H
Markovnikov's Rule
6.5
Regioselectivity of Hydrogen Halide Addition
(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
Markovnikov's Rule
CH 3 CH 2 CH
CH 2
HBr
acetic acid
CH 3 CH 2 CHCH 3
Br
(80%)
CH 3
CH 3
C
C
H
H
HBr
acetic acid
CH 3
CH 3
C
Br
(90%)
CH 3
Example 1
Example 2
Markovnikov's Rule
Question 6
CH 3
HCl
0°C
CH 3
Cl
What is the product of the reaction of 1-
methylcyclohexene with HCl
A) B)
(100%)
C) D)
Example 3
Mechanistic Basis for Markovnikov's Rule:
Example 1
6.6
Mechanistic Basis for Markovnikov's Rule
Protonation of double bond occurs in
direction that gives more stable of two
possible carbocations.
CH 3 CH 2 CH
CH 2
HBr
acetic acid
CH 3 CH 2 CHCH 3
Br

Mechanistic Basis for Markovnikov's Rule:
Example 1 Question 3
+
CH 3 CH 2 CH 2 —CH
2
primary carbocation is less stable: not formed
HBr
+
CH 3 CH 2 CH—CH
3 + Br –
Which carbocation forms when 3-methyl-2-
pentene is protonated
A) B)
CH 3 CH 2 CH
CH 2
CH 3 CH 2 CHCH 3
Br
Mechanistic Basis for Markovnikov's Rule:
Example 3
Question 5
The reaction of 3-methyl-1-butene with HBr
produces 2-bromo-3-methylbutane and which
other alkyl halide
A) B)
H
CH 3
HCl
0°C
CH 3
Cl
C) D)
H
secondary
carbocation is
less stable:
not formed
+
H H
CH 3
H
Question 18
Which alkene reacts with HCl (electrophilic
addition) at the fastest rate
HCl
+
CH 3
Cl –
A) B)
H
CH 3
CH 3
Cl
C) D)

Rearrangements Can Occur
H 2 C
CHCH(CH
3 ) 2
6.7
Carbocation Rearrangements in Hydrogen
Halide Addition to Alkenes
HCl, 0°C0
+
CH 3 CHCH(CH
3 ) 2
H
+
CH 3 CHC(CH 3 ) 2
CH 3 CHCH(CH
3 ) 2
CH 3 CH 2 C(CH 3 ) 2
Cl (40%)
(60%)
Cl
Carbocation Rearrangements
1,2-hydride shift
a more stable
carbocation
Carbocation Rearrangements
1,2-methyl shift
a more stable
carbocation
Carbocation Rearrangements
NOTE: Carbocation does not always rearrange …
Ring Expansion
a more stable
carbocation

Markovnikov's Rule
6.8
Free-radical Addition of HBr to
Alkenes
The "peroxide effect"
CH 3 CH 2 CH
CH 2
HBr
acetic acid
CH 3 CH 2 CHCH 3
Br
(80%)
Addition of HBr to 1-Butene
Addition of HBr to 1-Butene
CH 3 CH 2 CH
CH 2
CH 3 CH 2 CH
CH 2
HBr
HBr
CH 3 CH 2 CHCH 3
Br
Carbocation product :only
CH 3 CH 2 CH 2 CH 2 Br
Free Radical product only
when peroxides added to
one in absence of peroxides
reaction mixture
Addition opposite to
Markovnikov's rule
CH 3 CH 2 CH 2 CH 2 Br
only product when
peroxides added to
reaction mixture
CH 2
Photochemical Addition of HBr
+ HBrH
hν
(60%)
CH 2 Br
H
Addition of HBr opposite to Markovnikov's rule
proceeds by a free-radical chain mechanism.
Initiation steps:
..
R Ọ ...
..
.ỌO ..
R
Mechanism
..
R O . + ..
..
. O ..
R
Addition of HBr with a regiochemistry opposite
to Markovnikov's rule also occur when
initiated with light with or without added peroxides.
..
R O . ..
..
..
+ H Br
:
..
R O..
H +
.. . Br
:
..

Propagation steps:
CH 3 CH 2 CH
CH 3 CH 2 CH
.
H
CH 2
CH 2
+
..
Br
..
:
.. . Br
:
..
..
Br:
..
.
CH 3 CH 2 CH
Mechanism
CH 2
(More stable free radical)
+
.. . Br
:
..
..
Br:
..
..
CH Br:
2 ..
CH 3 CH 2 CH 2 CH
Examples of Termination Steps
(Initiation → Propagation → Termination)
Question 7
What is the correct IUPAC name of the
compound isolated from the reaction of
2-methyl-2-pentene with HBr in the presence
of peroxides
A) 3-bromo-4-methylpentane
B) 3-bromo-2-methylpentane
C) 2-bromo-2-methylpentane
D) 2,3-dibromo-2-methylpentane