Lecture Note
Chapter 20: Dienes, Conjugated Systems, And Pericyclic Reactions
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University:
Rutgers University Newark -
Course:
CHEM 336 | Organic Chemistry || Academic year:
2024
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Views:
282
Pages:
79
Author:
Mahitab S.
CHAPTER 20
DIENES, CONJUGATED
SYSTEMS,
AND PERICYCLIC
REACTIONS
©2018 Cengage Learning. All Rights Reserved. Chapter Outline
•
•
•
•
•
•
(20.1)
Stability of conjugated dienes
(20.2)
Electrophilic addition to conjugated dienes
(20.3)
UV-visible spectroscopy (not required)
(20.4)
Pericyclic reaction theory
(20.5)
The Diels-Alder reaction
(20.6)
Sigmatropic shifts (Claisen and Cope
rearrangements)
©2018 Cengage Learning. All Rights Reserved. Dienes
• Compounds that contain two carbon-carbon double bonds
• Types
• Unconjugated dienes: Double bonds are
separated by two or more single bonds
• Conjugated dienes: Double bonds are
separated by one single bond
• Cumulated dienes: Two double bonds share
an sp hybridized carbon
• 2p orbitals of the two double bonds do not
overlap and are not conjugated because of
the geometry of this carbon
©2018 Cengage Learning. All Rights Reserved. Example 20.1 - Conjugation
• Which of these molecules contain conjugated double
bonds?
Unconjugatefne
• Solution
Conjugated
Diene
conjugated
Diene
mmmm
Diene
• Compounds (b) and (c) contain conjugated double bonds
• The double bonds in compounds (a) and (d) are unconjugated
©2018 Cengage Learning. All Rights Reserved. Stability of Conjugated Dienes
heatsofHydrogenationisused todeterminethe Stabilityof conjugateddieneandunconjugatediene
• Relative stabilities of conjugated and unconjugated dienes can be
compared using the heats of hydrogenation
• The most stable olefins have the smallest heat of hydrogenation
(low energy level)
Thet
smallestheat
lowenergy
b
stableolefins
Most
stable
more
lessstable
©2018 Cengage Learning. All Rights Reserved. Conjugated Dienes – Stability of 1,3-Butadiene
• Predicted heat of hydrogenation of 1,3-butadiene is −254 kJ
(−30.3 kcal)/mol
• Observed heat of hydrogenation of 1,3-butadiene is −237 kJ
(−56.5 kcal)/mol
• Conjugation of the double bonds in 1,3-butadiene gives an
extra stability to the molecule of approximately 17 kJ (4.1 kcal)/
mol
©2018 Cengage Learning. All Rights Reserved. Figure 20.1 - Conjugation of Double Bonds in Butadiene
Gives Additional Stability of Approximately 17 kJ/mol
©2018 Cengage Learning. All Rights Reserved. Figure 20.1 - Structure of 1,3-Butadiene: Molecular Orbital
Model
•Additional stability of conjugated
dienes vs. unconjugated dienes
arises from delocalization of electron
density
•In conjugated diene, 4 π electrons
are delocalized over 4 parallel 2p
orbitals
•In unconjugated diene, each pair of
π electrons is localized between 2
carbons
4
delocalizedover parallel
tielectrons
4
2porbitals
©2018 Cengage Learning. All Rights Reserved. Stability of Conjugated Systems
• Compounds containing conjugated double bonds (any), not just
those in dienes, are more stable than isomeric compounds
containing unconjugated double bonds
conjugateddiene
b
moststable
• Additional stability of dienes relative to unconjugated dienes
arises from the delocalization of electron density in the
conjugated diene
©2018 Cengage Learning. All Rights Reserved. Example 20.2 - Stability of Conjugated Systems
• Using data from Table 20.1, estimate the extra stability
that results from the conjugation of double bonds in
trans-1,3-pentadiene (CH2=CH-CH=CH-CH3)
• Solution (next slide)
©2018 Cengage Learning. All Rights Reserved. Example 20.2 - Stability of Conjugated Systems
• Compare the sum of heats of hydrogenation of 1-pentene and trans-2butene with the heat of hydrogenation of trans-1,3-pentadiene (CH2=CHCH2-CH2-CH3, CH3-CH=CH=CH3)
• Conjugation of double bonds in trans-1,3-pentadiene imparts an added
stability of approximately 15 kJ (3.6 kcal)/mol
NII
©2018 Cengage Learning. All Rights Reserved. Example
• Which of the following is not conjugated?
1.
2.
conjugateddiene
unconjugated
Diene
3.
conjugated
Diene
4.
5.
Conjugated
conjugated
Diene
©2018 Cengage Learning. All Rights Reserved.
Diene Example
• How many π (pi) bonding molecular orbitals are present
in 1,3,5-hexatriene ? (CH2=CH-CH=CH-CH=CH2)
1.
2.
3.
4.
5.
2
3
4
5
6
©2018 Cengage Learning. All Rights Reserved. Example
• Arrange the following alkenes in order of least stable to
most stable
Unconjugated
1.
2.
3.
4.
5.
conjugated
A
Chapter 20: Dienes, Conjugated Systems, And Pericyclic Reactions
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