Chapter 14
Conjugated Dienes and
Ultraviolet Spectroscopy
Conjugated Dienes
• Multiple Bonds Alternating with Single Bonds
1,3 Butadiene
1,4 Pentadiene
H2C=CH-CH=CH2
H2C=CH-CH2-CH=CH2
CONJUGATED
NOT CONJUGATED !!!
Examples of Conjugated Dienes
Lycopene – a conjugated polyene
CH 3
C
CH 3
O
CH 3
O
Progesterone – a
conjugated enone
Benzene – a cyclic
conjugated molecule
Preparation and Stability of
Conjugated Dienes
Diene Preparation
•Based Induced Elimination of HX
H
H
Br
NBS
CCl4
Cyclohexene
+K -OC(CH )
3 3
HOC(CH3)3
3-Bromocyclohexene
1,3-Cyclohexadiene
Diene Preparation
• Thermal cracking of butane using a chromium
oxide/aluminum oxide catalyst
600 Oc
CH3CH2CH2CH3
H2C=CHCH=CH2 +2 H2
Catalyst
Acid-catalyzed double
dehydration
CH3
H3C C C C OH
H2 H2
OH
Al203
Heat
CH3
H2C C C CH2
H
+2 H20
Special Properties of Conjugated
Dienes
• Length of the central single bond is shorter than
non-conjugated similar molecule
• Comparison of 1,3-Butadiene and Butane
148 pm
H2C=CH-CH=CH2
1,3-Butadiene
Shorter
Bond
153 pm
CH3-CH2-CH2-CH3
Butane
Special Properties of Conjugated
Dienes
• Unusual stability evidenced by heats of hydrogenation
• More highly substituted alkenes are more stable than
less substituted ones
• More highly substituted alkenes release less heat on
hydrogenation because they contain less energy to start
with
Heats of Hydrogenation for Some Alkenes and Dienes
Alkene or Diene
H3C
C C
H2 H
CH2
HOhydrog
(kj/mol)
(kcal/mol)
Product
H3C
C C CH3
H2 H2
-126
-30.1
CH3
CH3
H3C
C
CH2
H3C
C
H
CH3
-119
-28.4
H2C
C
H
C
H
H3C
C C CH3
H2 H2
-236
-56.4
-229
-54.7
-253
-60.5
CH2
CH3
CH3
H2C
C
H
C
CH2
H2C
C
H
C C
H2 H
CH2
H3C
C C
H2 H
CH3
H3C
C C C CH3
H2 H2 H2
Molecular Orbital Description
of 1,3 Butadiene
Stability of Conjugated Dienes is
due to orbital hybridization
• Typical C-C single bonds result from sigma overlap of
sp3 orbitals on both carbons
CH3-CH2-CH2-CH3
Bonds formed by overlap of sp3 orbitals
• Conjugated dienes have a central C-C bond that results
from sigma overlap of sp2 orbitals on both carbons
H2C=CH-CH=CH2
Bonds formed by overlap of sp2 orbitals
Stability of conjugated dienes is
due to orbital hybridization
• Since sp2 orbitals have more s character (33%)
than sp3 orbitals (25% s), the electrons in sp2
orbitals are closer to the nucleus and the bonds
they form are shorter and stronger
• The “extra” stability of conjugated dienes result
from the greater amount of s character in the
bonds forming the C-C bond
Stability of conjugated dienes is
due to orbital hybridization
E
N
E
R
G
Y
+
-
+
-
-
+
-
+
+
-
-
+
+
+
+
+
-
+
+
-
-
-
-
-
+
+
-
-
-
-
+
+
+
+
+
+
-
-
-
-
Four isolated
p orbitals
Antibonding
(3 nodes)
Antibonding
(2 nodes)
Bonding
(1 node)
Bonding
(0 nodes)
Why is the conjugated bond stronger?
•
П electrons are “delocalized” over the entire П
framework rather than localized between two specific
nuclei.
• П certain amount of double bond character exists in a
conjugated bond over the single bond area.
Compare 1,3-Butadiene with 1,4 Pentadiene
Partial double bond character
+
+
+
+
+
+
C
C
C
C
C
C
-
-
-
-
-
-
C
+
+
C
C
-
-
1,3-Butadiene
1,4-Pentadiene
a conjugated diene
a non-conjugated diene
Electrophilic Additions to
Conjugated Dienes: Allylic
Carbocations
• Electrophilic addition to 1,3-Butadiene
yields a mixture of two products:
• 1,2 addition
• 1,4 addition
Non-conjugated alkene addition
reactions
CH3
H3C C CH2
HCl
H3C C CH3
Ether
2-Methylpropene
H2C C C C CH2
H H2 H
Cl
+
H3C C CH3
CH3
CH3
Tertiary
Carbocation
HCl
Ether
2-Chloro-2methylpropane
Cl
Cl
H3C C C C CH3
H H2 H
Conjugated diene 1,2 and 1,4
addition reactions
H
H
H
C
C
H
C
H
H
C
+
H
HBr
C
Br
C
H
C H
H
+
Br2
25oC
Br
C C C C Br
H2 H H H2
1,4-Dibromo-2-butene
(45%; 1,4 addition)
Br
H
H
H
C
C
C
H
3-Bromo-1-butene
(71%; 1,2 addition)
1,3-Butadiene
(a conjugated diene)
1,3-Butadiene
C
H
H
H2C C C CH2
H H
H
H
C H
H
1-Bromo-2-butene
(29%; 1,4 addition)
Br
+
Br
C C C CH2
H2 H H
3,4-Dibromo-1-butene
(55%; 1,2 addition)
1,4 addition products are due to
allylic carbocation intermediates
H
H
H
H
H
H
C
C
C
H
C
C
H
C
C
H
H
C
H
H
C
+
C
H
C
H
C
H
Br-
H
Secondary, allylic
H
H
+
H
H
H HBr
H
C
C
H
H
H
C
C
+
H
Br-
H
Primary, nonallylic
(NOT formed)
Kinetic versus
thermodynamic control of
reactions
• At room temperature, electrophilic addition to a
conjugated diene leads to a product mixture
where the 1,2 adduct predominates over the 1,4
adduct.
• At high temperatures, the product ratio changes
and the 1,4 adduct predominates
Br
H2C C C CH2
H H
+
HBr
H2C C C CH3
H H
1,2 adduct
+
H3C C C C Br
H H H2
1,4 adduct
At 0oC:
71%
29%
At 40oC:
15%
85%
Kinetic Versus Thermodynamic
Control
• Kinetic control dominates reactions where the
product of an irreversible reaction is the one that
forms fastest
• Thermodynamic control dominates reactions
where the product of a readily reversible
reaction depends on thermodynamic stability
A Kinetic Control Reaction
• B forms faster because it requires less energy
• C is more stable, but requires more energy
• The reaction occurs under mild conditions and is
irreversible
• No equilibrium is reached
A Thermodynamic Control Reaction
• This reaction is held under higher temperatures and equilibrium is
reached
• Since C is more stable than B, C is the major product
• The product of a readily reversible reaction depends only on
thermodynamic control
The Diels-Alder
Cycloaddition Reaction
• Conjugated diene
• Dienophile
• Diels-Alder reaction:
* Stereospecific
* Prefer Endo product to Exo product
Conjugated diene
• Contain alternating double and single bond:
• Adopt S-cis conformation :
• More stable than non-conjugated diens.
Examples of conjugated diens
• 1,3-Butadiene
H2 C
•
1,3-Pentadiene
• 1,3-Cyclopentadiene
H
H
C
C
C H2
Conjugated diene VS.
Non-conjugated diene
Non-conjugated diene
Conjugated diene
Non-conjugated diene
Conjugated diene
Non-conjugated diene
Conjugated diene
S-cis conformation of diens
H
S-cis
H
S-trans
S-cis
H
H
C
CH3
C
CH3
C
H
CH3
H3C
C
C
H
H
Severe steric strain
in s-cis form
C
H
S-trans
Dienophile
• Has carbon carbon double or triple bond
that is next to the positively polarized
carbon of a electron-withdrawing
substituent group
• Reactive and uncreative
O
+
H
C
H
C
Reactive
C
H
_
H
Propena
(Acrolein)
O
H
C
_
+
OCH2CH3
C
Reactive
C
H
H
Ethyl propenoate
(Ethyl acrylate)
_
O
H
C
Reactive
C
+
O
C
H
+
C
_
O
Maleic anhydride
_
O
Reactive
H
C
+
H
C
C
H
C
C
+
C
O
H
_
Benzoquinone
_
N
+
H
C
C
Reactive
C
H
H
Propenenitrile
(Acrylonitrile)
_
O
+
OCH3
C
Reactive
C
C
H
Methyl propynoate
O
CH2
H
C
C
C
H
H
Unreactive
CH3
N
Unreactive
O
Unreactive
Diels-Alder reaction
H
C
H
O
C
C
H
H
C
CH3
+
C
H
C
H
H
C
H
H
Conjugated
diene
O
Dienophile
Benzene
Heat
C
CH3
Stereospecific
• The stereochemistry of the starting dienophile is
maintained during the reaction, and a single
product stereoisomer results.
• Example:
H
C
CO2CH3
H
CH2
H
C
CO2CH3
+
C
H
CH2
C
H
CH3
H
CH3
H
C
CO2CH3
H
CH2
C
H
CO2CH3
+
C
H
CH2
H3C
C
H
H
CH3
CHO
+
CHO
Endo & Exo product
H
O
Endo
H
O
+
O
O
O
O
Exo
H
H
O
O
O
Diene Polymers
cis-Polybutadiene
In
1,3-Butadiene
trans-Polybutadiene
Natural rubber (Z)
In
Isoprene
Gutta-percha (E)
Cl
Cl
Cl
Cl
In
Chloroprene
Neoprene (Z)
Ultraviolet Spectrum of 1,3Butadiene
Ultraviolet Excitation of 1,3- Butadiene
When irradiated with UV energy, electrons absorb the energy and are
Promoted from a П bonding molecular orbital to an antibonding П *
Molecular orbital
Ψ4*
(lowest unoccupied molecular orbital)
E
N
E
R
G
Y
Ψ3* LUMO
П*
hv
UV irradiation
Ψ2 HOMO
Four p atomic orbitals
П
(highest occupied molecular orbital)
Ψ1
Ground State
Excited State
Ultraviolet Spectrum
• A UV spectrum is recorded by irradiating a
sample with UV light of continously
changing wavelength.
• When the wavelength corresponds to the
energy level required to excite an electron
to a higher level, energy is absorbed
• This absorption is detected and displayed
on a chart that plots wavelength versus
absorbance
Structure determination in
Conjugated systems
• Ultraviolet Spectroscopy
Energy
Near
infrared
Visible
= 200nm
Ultraviolet
X-rays
Vacuum
ultraviolet
= 400nm
Infrared
Ultraviolet Absorptions of Some Conjugated Molecules
Name
max(nm)
Structure
220
CH3
2-methyl-1,3-butandiene
H2 C
C
CH
CH2
1,3-Cyclohexandiene
1,3,5-Hexatriene
1,3,5,7-Octatetraene
256
H2 C
CH
CH
CH
CH
H2 C
CH
CH
CH
CH
2,4-Cholestadiene
258
CH2
CH
CH
CH2
290
275
3-Buten-2-one
219
CH3
H2 C
CH
C
O
Benzene
203
Naphthalene
220
Ultraviolet spectrum of
Beta-carotene
The absorption occurs in the visible region