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Chapter 14
Conjugated Dienes and Ultraviolet Spectroscopy
Conjugated Dienes• Multiple Bonds Alternating with Single Bonds
1,3 Butadiene
H2C=CH-CH=CH2
1,4 Pentadiene
H2C=CH-CH2-CH=CH2
CONJUGATED NOT CONJUGATED !!!
C O
CH3
CH3
CH3
O
Examples of Conjugated Dienes
Lycopene – a conjugated polyene
Progesterone – a conjugated enone
Benzene – a cyclic conjugated molecule
Preparation and Stability of Conjugated Dienes
Diene Preparation
H H
Cyclohexene 3-Bromocyclohexene 1,3-Cyclohexadiene
NBS
CCl4
Br+K -OC(CH3)3
HOC(CH3)3
•Based Induced Elimination of HX
Diene Preparation
• Thermal cracking of butane using a chromium oxide/aluminum oxide catalyst
CH3CH2CH2CH3
600 Oc
CatalystH2C=CHCH=CH2 +2 H2
Acid-catalyzed double dehydration
CH3 C
CH3
OH
CH2
CH2
OH Al203
HeatCH2 C
CH3
CH
CH2 +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
H2C=CH-CH=CH2 CH3-CH2-CH2-CH3
148 pm 153 pm
1,3-Butadiene Butane
ShorterBond
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
CH3 CH2
CH
CH2
CH3 C
CH3
CH2
CH2 CH
CH
CH2
CH2 CH
CH2
CH
CH2
CH3 CH2
CH2
CH3
CH3 CH
CH3
CH3
CH3 CH2
CH2
CH3
CH3 CH2
CH
CH3
CH3
CH3 CH2
CH2
CH2
CH3
CH2 CH
C CH2
CH3
Heats of Hydrogenation for Some Alkenes and Dienes
Alkene or Diene Product (kj/mol) (kcal/mol) HO
hydrog
-126 -30.1
-119 -28.4
-236 -56.4
-229 -54.7
-253 -60.5
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
+
-
+
-
+
-
+
-
+
-
+
-
+
-
+
-
Four isolated p orbitals
+
- +
-
+
- +
-
+
-
+
- +
-
+
-
+
- +
- +
- +
- Antibonding (3 nodes)
Antibonding (2 nodes)
Bonding (1 node)
Bonding (0 nodes)
ENERGY
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
C C C C
+
-
+
-
+
-
+
-C C C C C
+
-
+
-
+
-
+
-1,3-Butadiene
a conjugated diene
1,4-Pentadiene
a non-conjugated diene
Partial double bond character
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 C+
CH3
CH3HCl
EtherCH3 C
CH3
CH2
CH3 C
CH3
CH3
Cl
2-Methylpropene Tertiary Carbocation
2-Chloro-2-methylpropane
CH2 CH
CH2
CH
CH2
HCl
EtherCH3 C
HCH2
CH
CH3
Cl Cl
Conjugated diene 1,2 and 1,4 addition reactions
CC
CCH
H
H
HH
H
+ HBrC
CC
CH
H
H
HH
H
Br
H
3-Bromo-1-butene (71%; 1,2 addition)
CC
CCH
H
H
HH
H
Br
H
1-Bromo-2-butene (29%; 1,4 addition)
CH2 CH
CH
CH2
Br2
25oC
CH2
CH
CH
CH2
Br BrCH2
CH
CH
CH2Br
Br
+
+
1,3-Butadiene (a conjugated
diene)
1,3-Butadiene1,4-Dibromo-2-butene
(45%; 1,4 addition)3,4-Dibromo-1-butene
(55%; 1,2 addition)
1,4 addition products are due to allylic carbocation intermediates
CC
CCH
H
H
HH
H
CC
C+
CH
H
H
HH
HH
C+
CC
CH
H
H
HH
HH
Br-
Secondary, allylic
CC
CC
+H
H
H
HH
H
H Br-
Primary, nonallylic (NOT formed)
HBr
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
CH2 CH
CH
CH2 + HBr CH2 CH
CH
CH3
Br
CH3 CH
CH
CH2
Br+
1,2 adduct 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-AlderCycloaddition 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
• 1,3-Pentadiene
• 1,3-Cyclopentadiene
H
C C C
H
CH2H2
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
HS-cis
S-trans
S-cis
H
CC
C
CH3
CH3C
H
C
C
H
H
H
H
H3CCH3
Severe steric strain in s-cis form 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
C
C
C
O
H
HH
H
_
+
Propena(Acrolein)
Reactive
C
C
C
O
HH
H OCH2CH3+
_
Ethyl propenoate (Ethyl acrylate)
Reactive
Maleic anhydride
Reactive
H
C
O
O
C
C
C
H
O
_
_
+
+
Benzoquinone
Reactive C
C
CC
C
C
H
H
O
O
H
H +
_
+
_
Propenenitrile (Acrylonitrile)
Reactive
C
C
C
HH
H
N+
_
O
C
C
C
H
OCH3
Methyl propynoate
Reactive
+
_
C
C
C
CH2CH3
O
HH
H
Unreactive
N
Unreactive
O
Unreactive
Diels-Alder reaction
C
C
C
CC
C
C
H
H
H
H
H
H
O
H
H H
CH3 CCH3
O
+ Benzene
Heat
Conjugated diene
Dienophile
Stereospecific
• The stereochemistry of the starting dienophile is maintained during the reaction, and a single product stereoisomer results.
• Example:
C
C
C
C
H CH2
CH2
CH3
H
H
H
+
CO2CH3 H
HCH3
CO2CH3
CH3
H
HCO2CH3
C
C
C
C
H CH2
CH2HH
HH3C
+
CO2CH3
CHO
+CHO
Endo & Exo product
O
O
O+
Endo
Exo
O
O
OH
H
O
O
OH
H
Diene Polymers
1,3-Butadiene
cis-Polybutadiene
trans-Polybutadiene
In
In
Isoprene
Natural rubber (Z)
Gutta-percha (E)
In
Chloroprene Neoprene (Z)
Cl Cl ClCl
Ultraviolet Spectrum of 1,3-Butadiene
Ultraviolet Excitation of 1,3- Butadiene
ENERGY Four p atomic orbitals
Ψ4*
Ψ3*
Ψ2
Ψ1
LUMOhv
UV irradiationHOMO
Ground State Excited State
П *
П
When irradiated with UV energy, electrons absorb the energy and arePromoted from a П bonding molecular orbital to an antibonding П *Molecular orbital
(lowest unoccupied molecular orbital)
(highest occupied molecular orbital)
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
X-raysVacuumultraviolet
Visible
Visible
Near
infrared
InfraredU
ltraviolet
Energy
= 200nm = 400nm
H2C C CH
CH3
CH2
H2C CH CH2CH CHCH
H2C CHCH CHCH CH2CHCH
H2C CH C
CH3
O
Ultraviolet Absorptions of Some Conjugated Molecules
Name Structure max(nm)
2-methyl-1,3-butandiene
1,3-Cyclohexandiene
1,3,5-Hexatriene
1,3,5,7-Octatetraene
2,4-Cholestadiene
3-Buten-2-one
Benzene
Naphthalene
220
256
258
290
275
219
203
220
Ultraviolet spectrum of Beta-carotene
The absorption occurs in the visible region