This document provides an outline and overview of conjugated systems and conjugated dienes. It discusses the structure of conjugated dienes and factors that influence their stability, including orbital overlap and delocalization of pi electrons. Reaction types are also summarized, including electrophilic addition, the Diels-Alder reaction, and factors that influence reaction rates and stereochemistry. UV-Vis spectroscopy is described as a technique to analyze compounds with conjugated double bonds.

1. CHEM 2425
Chapter 14
CONJUGATED SYSTEMS

2. © 2016 Pearson Education, Inc.
Isolated and Conjugated Dienes

3.  Benzene
 Commonly found in nature as pigments, hormones
 Isoprene
CONJUGATED SYSTEMS

4. I. Structure
OUTLINE

5.  Consider molecules with multiple double bonds…
 Example: pentadienes
cumulated (allenes) conjugated isolated
separated by: 0 single bonds 1 single bond 2 or more single bonds
 Which is most stable?
 Why?
 Long answer = molecular orbital theory
 Short answer = orbital overlap
I. STRUCTURE

6. 1. Unusually strong s bond from greater s-character of C atoms
1,3-BUTADIENE STABILITY

7. 2. Overlap of p atomic orbitals
 In its most stable conformation, the p orbitals are parallel
 Allows for p electron delocalization
 Single bond has partial C=C character, which also contributes to
stability
1,3-BUTADIENE STABILITY

8.  Conformations of 1,3-butadiene:
 s-cis (single cis) = double bonds on same side of single bond
 s-trans (single trans) = double bonds on opposite sides of single bond
 The two conformations easily interconvert at room temperature
 Which conformation is more stable?
STRUCTURE, CONT.

9.  Mild steric hindrance in s-cis conformation
CONFORMATIONS OF CONJUGATED
DIENES

10. Butadiene
• Which conformation appears to be more
stable?
a. s-cis
b. s-trans
c. neither

11. Estimating the Relative Stability
of Resonance Structures
 The more covalent bonds a structure
has, the more stable it is
(more stable) (less stable)
O O
(more stable) (less stable)

12.  Structures in which all of the atoms
have a complete valence shell of
electrons (i.e., the noble gas structure)
are especially stable and make large
contributions to the hybrid
O O
this carbon has
6 electrons
this carbon has
8 electrons

13.  Charge separation decreases stability
(more stable) (less stable)
OMe OMe

14. Alkadienes and Polyunsaturated
Hydrocarbons
1,3-Butadiene
(2E,4E)-2,4-Hexadiene
1,3-Cyclohexadiene
1
2
3
4
1
2
3
4
5
6
1
2 3
4
5
6
 Alkadienes (“Dienes”)

15.  Alkatrienes (“Trienes”)
1
2
3
4
5
6
7
8
(2E,4E,6E)-Octa-2,4,6-triene

16. The Stability of Conjugated
Dienes
 Conjugated alkadienes are
thermodynamically more stable than
isomeric isolated alkadienes
2 + 2 H2 2 2 x (-127)=-254
H o
(kJmol-1
)
=-239
Difference 15
+ 2 H2

18. I. Reactions
OUTLINE

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Reactions of Isolated Dienes

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The Mechanism

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Double Bonds can have
Different Reactivities

22. Electrophilic Attack on Conjugated
Dienes: 1,4 Addition
Cl
H
Cl
H
1
2
3
4
H Cl
25o
C
+
(78%)
(1,2-Addition)
(22%)
(1,4-Addition)

23. (a)
Cl
H
 Mechanism
Cl H + H
(a)
H
(b)
H
X
H
+
+
Cl
(b)
Cl
H
(a)
(b)

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Reactions of Conjugated Dienes

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1,2-Addition and 1,4-Addition

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Mechanism for the
Reaction of a Conjugated Diene

27. A. Kinetic Control versus
Thermodynamic Control of a
Chemical Reaction
+
HBr
Br
Br
+
(80%)
-80o
C
(20%)
(80%)
40o
C
Br
Br
+
(20%)

28. Br
Br
40o
C, HBr
1,2-Addition
product
1,4-Addition
product

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The Diels–Alder Reaction
forms a Six-Membered Ring

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The Mechanism

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Faster if there is an Electron Withdrawing
Group on the Dienophile

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The Electron Withdrawing Group makes the
Electrophile a better Electrophile

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Another Diels–Alder Reaction

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Alkynes can also be Dienophiles
The cyclic product has two double bonds.

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The Stereochemistry of the
Diels–Alder Reaction
The product will be a racemic mixture.

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How to Determine the Reactants
of a Diels–Alder Reaction

38. The Diels–Alder Reaction:
A 1,4-Cycloaddition Reaction
of Dienes
[4p+2p]
+
(diene) (dienophile) (adduct)

39. O
O
O
O
O
O
1,3-Butadiene
(diene)
Maleic
anhydride
(dienophile)
Adduct
(100%)
+
benzene
100o
C
 e.g.

40. A. Factors Favoring the Diels–Alder
Reaction
EDG
EWG
EDG
EWG
+
Type A
● Type A and Type B are normal Diels-Alder
reactions
+
Type B
EDG
EWG EWG
EDG

41. EWG
EDG
EWG
EDG
+
Type C
● Type C and Type D are Inverse Demand
Diels-Alder reactions
+
Type D
EWG
EDG EDG
EWG

42.  Relative rate
Diene D.A. cycloadduct
+
30o
C
O
O
O
OMe
> >
Diene
t1/2 20 min. 70 min. 4 h.

43.  Relative rate
Dienophile D.A. cycloadduct
+
20o
C
> >
Dienophile
t1/2 0.002 sec. 20 min. 28 h.
NC CN
NC CN
CN
CN
CN

44.  Steric effects
> >
Dienophile:
Relative rate: 1 0.14 0.007
COOEt COOEt COOEt

45. B. Stereochemistry of the
Diels–Alder Reaction
O
O
OMe
OMe
H
H
OMe
O
OMe
O
H
H
+
Dimethyl maleate
(a cis-dienophile)
Dimethyl cyclohex-4-ene-
cis-1,2-dicarboxylate
1. The Diels–Alder reaction is stereospecific:
The reaction is a syn addition, and the
configuration of the dienophile is retained
in the product

46. O
OMe
H
OMe
O
OMe
O
H
H
+
Dimethyl fumarate
(a trans -dienophile)
Dimethyl cyclohex-
4-ene-trans -1,2-
dicarboxylate
H
MeO
O

47. 2. The diene, of necessity, reacts in the
s-cis rather than in the s-trans
conformation
s-cis Configuration s-trans Configuration
R
O
+
O
R
Highly strained
X

48.  e.g.
COOMe COOMe
heat
+
(diene locked
in s-cis
conformation)
COOMe
+ No Reaction
(diene locked
in s-trans
conformation)
heat

49.  Cyclic dienes in which the double bonds are
held in the s-cis conformation are usually
highly reactive in the Diels–Alder reaction
 Relative rate
Diene D.A. cycloadduct
+
30o
C
O
O
O
> >
Diene
t1/2 11 sec. 130 sec. 4 h.

50. 3. The Diels–Alder reaction occurs primarily in
an endo rather than an exo fashion when
the reaction is kinetically controlled
H H
H H
R
H
H
R
longest bridge
R is exo
R is endo

51.  Alder-Endo Rule
● If a dienophile contains activating
groups with p bonds they will prefer
an ENDO orientation in the
transition state
X
X
X
X
H
H

52.  e.g.
O
O O
O
O
O
H
H
+

100% endo

53.  Stereospecific reaction
X
X
X
X
+
X X
X
+
X
(i)

54.  Stereospecific reaction
+
+
(ii) Y
Y
Y
Y
Y
Y
Y
Y

55.  Examples
CN
CN
+
Me
NC
NC
CN
CN
CN
CN
Me
(A)
D.A.
CN
+
NC
Me
Me
NC
CN
CN
CN
CN
CN
Me
Me
(B)
D.A.

56.  Diene A reacts 103 times faster than
diene B even though diene B has two
electron-donating methyl groups
Me
Me
H
Me
Me
(s-cis) (s-trans)

57.  Examples
+
(C)
O
O
O
O
H
H
O
O
D.A.
+
(D)
O
O
O
O
H
H
O
O
D.A.

58.  Examples
+
(E)
O
O
O
D.A.
No Reaction
● Rate of Diene C > Diene D (27 times),
but Diene D >> Diene E
● In Diene C, tBu group  electron
donating group  increase rate
● In Diene E, 2 tBu group  steric effect,
cannot adopt s-cis conformation

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Ultraviolet and Visible Spectroscopy
UV/Vis spectroscopy provides information
about compounds with
conjugated double bonds.

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An Electronic Transition
Only organic compounds with π electrons
can produce UV/Vis spectra.
A UV spectrum is obtained when
UV light (180 to 400 nm) is absorbed.
A visible spectrum is obtained when
visible light (400 to 780 nm) is absorbed.

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A UV Spectrum

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UV/Vis Absorption Bands are Broad
UV/Vis absorption bands are broad
because an electronic state has vibrational sublevels.

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Chromophore
All these compounds have the same UV spectrum.
A chromophore is that part of a molecule that is responsible
for a UV/Vis spectrum.

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The Beer–Lambert Law
A = ε c l
A = absorbance of the sample
c = concentration of substance in solution
l = length of the light path in cm
ε = molar absorptivity of the sample
(characteristic of the compound)

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Cells Used for Taking UV/Vis Spectra

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The More Conjugated Double Bonds,
the Longer the Wavelength

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The More Conjugated Double Bonds,
the Longer the Wavelength

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Conjugation Makes the
Electronic Transition Easier

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Colored Compounds Absorb
Visible Light (> 400 nm)
β-Carotene is found in carrots, apricots, and flamingo feathers.
Lycopene is found in tomatoes, watermelon, and pink grapefruit.

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Auxochrome
An auxochrome is a substituent that
alters the position and intensity of the absorption.

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Common Dyes

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Anthocyanins
Responsible for the red, purple, and blue colors
of many flowers and fruits.

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UV/Vis Spectroscopy Can Be Used
to Measure the Rate of a Reaction

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UV/Vis Spectroscopy Can Be Used
to Measure the Rate of a Reaction

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UV/Vis Spectroscopy Can Be Used
to Determine a pKa Value
The phenolate ion absorbs at 287 nm, but phenol does not.

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UV/Vis Spectroscopy Can Be Used
to Determine the Melting Temperature of DNA
The temperature increases with
increasing numbers of G-C base pairs.