10.5 Classes of Dienes

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10.5 Classes of Dienes
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Classification of Dienes

• isolated diene
• conjugated diene
• cumulated diene

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Nomenclature

• (2E,5E)-2,5-heptadiene
• (2E,4E)-2,4-heptadiene
• 3,4-heptadiene

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10.6Relative Stabilitiesof Dienes
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Heats of Hydrogenation

• 1,3-pentadiene is 26 kJ/mol more stable than
  1,4-pentadiene, but some of this stabilization
  is because it also contains a more highly
  substituted double bond

252 kJ/mol
226 kJ/mol
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Heats of Hydrogenation
126 kJ/mol
115 kJ/mol
252 kJ/mol
226 kJ/mol
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Heats of Hydrogenation
126 kJ/mol
111 kJ/mol
126 kJ/mol
115 kJ/mol
252 kJ/mol
226 kJ/mol
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Heats of Hydrogenation
126 kJ/mol
111 kJ/mol

• when terminal double bond is conjugated with
  other double bond, its heat of hydrogenation is
  15 kJ/mol less than when isolated

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Heats of Hydrogenation
126 kJ/mol
111 kJ/mol

• this extra 15 kJ/mol is known by several
  terms stabilization energy delocalization
  energy resonance energy

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Heats of Hydrogenation
Cumulated double bonds have relatively high
heats of hydrogenation

C
2H2
CH3CH2CH3
H2C
CH2
DH -295 kJ

CH3CH2CH3
H2
DH -125 kJ
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10.7Bondingin Conjugated Dienes
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Isolated diene
1,4-pentadiene
1,3-pentadiene
Conjugated diene
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Isolated diene
p bonds are independent of each other
1,3-pentadiene
Conjugated diene
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Isolated diene
p bonds are independent of each other
p orbitals overlap to give extended p bond
encompassing four carbons
Conjugated diene
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Isolated diene
less electron delocalization less stable
more electron delocalization more stable
Conjugated diene
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Conformations of Dienes
s-trans
s-cis

• s prefix designates conformation around single
  bond
• s prefix is lower case (different from
  Cahn-Ingold-Prelog S which designates
  configuration and is upper case)

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Conformations of Dienes
s-trans
s-cis

• s prefix designates conformation around single
  bond
• s prefix is lower case (different from
  Cahn-Ingold-Prelog S which designates
  configuration and is upper case)

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Conformations of Dienes
s-trans
s-cis

• Both conformations allow electron delocalization
  via overlap of p orbitals to give extended p
  system

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s-trans is more stable than s-cis

• Interconversion of conformations requires two p
  bonds to be at right angles to each other and
  prevents conjugation

12 kJ/mol
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16 kJ/mol
12 kJ/mol
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10.8Bonding in Allenes
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Cumulated Dienes

• cumulated dienes are less stable thanisolated
  and conjugated dienes
• (see Problem 10.7 on p 375)

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Structure of Allene
118.4
131 pm

• linear arrangement of carbons
• nonplanar geometry

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Structure of Allene
118.4
131 pm

• linear arrangement of carbons
• nonplanar geometry

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Bonding in Allene
sp
sp 2
sp 2
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Bonding in Allene
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Bonding in Allene
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Bonding in Allene
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Chiral Allenes

• Allenes of the type shown are chiral

A
X
Y
B
A p B X p Y
Have a stereogenic axis
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Stereogenic Axis

• analogous to difference between
• a screw with a right-hand thread and one with
  a left-hand thread a right-handed helix and a
  left-handed helix