Physical Properties of Alkenes

1
Physical Properties of Alkenes
2
Dipole moments

• What is direction of dipole moment?
• Does a methyl group donate electrons to the
  double bond, or does it withdraw them?

? 0 D
3
Dipole moments

• Chlorine is electronegative and attracts
  electrons.

? 0 D
4
Dipole moments

• Dipole moment of 1-chloropropene is equal to the
  sum of the dipole moments of vinyl chloride and
  propene.

5
Dipole moments

• Therefore, a methyl group donates electrons to
  the double bond.

? 1.7 D
6
Alkyl groups stabilize sp2 hybridizedcarbon by
releasing electrons
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Relative Stabilities of Alkenes
8
Double bonds are classified according tothe
number of carbons attached to them.
monosubstituted
R'
R
R
H
R'
H
H
H
disubstituted
disubstituted
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Double bonds are classified according tothe
number of carbons attached to them.
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Substituent Effects on Alkene Stability

• Electronic
• disubstituted alkenes are more stable than
  monosubstituted alkenes
• Steric
• trans alkenes are more stable than cis alkenes

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Figure 5.4 Heats of combustion of C4H8isomers.
2717 kJ/mol
6O2
2710 kJ/mol
2707 kJ/mol
2700 kJ/mol
4CO2 8H2O
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Substituent Effects on Alkene Stability
Electronic

• Alkyl groups stabilize double bonds more than H
• more highly substituted double bonds are
  morestable than less highly substituted ones.

13
Problem 5.8

• Give the structure or make a molecular model of
  the most stable C6H12 alkene.

14
Substituent Effects on Alkene Stability
Steric

• trans alkenes are more stable than cis alkenes
• cis alkenes are destabilized by van der
  Waalsstrain

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Figure 5.5 cis and trans-2-Butene
cis-2-butene
trans-2-butene
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Figure 5.5 cis and trans-2-Butene
van der Waals straindue to crowding
ofcis-methyl groups
cis-2-butene
trans-2-butene
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van der Waals Strain

• Steric effect causes a large difference in
  stabilitybetween cis and trans-(CH3)3CCHCHC(CH3)
  3
• cis is 44 kJ/mol less stable than trans

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Cycloalkenes
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Cycloalkenes

• Cyclopropene and cyclobutene have angle strain.
• Larger cycloalkenes, such as cyclopenteneand
  cyclohexene, can incorporate a double bond into
  the ring with little or no angle strain.

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Stereoisomeric cycloalkenes

• cis-cyclooctene and trans-cycloocteneare
  stereoisomers
• cis-cyclooctene is 39 kJ/ molmore stablethan
  trans-cyclooctene

cis-Cyclooctene
trans-Cyclooctene
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Stereoisomeric cycloalkenes

• trans-cyclooctene is smallest trans-cycloalkene
  that is stable at room temperature
• cis stereoisomer is more stable than trans
  through C11 cycloalkenes

trans-Cyclooctene
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Stereoisomeric cycloalkenes

• cis and trans-cyclododeceneare approximately
  equal instability

trans-Cyclododecene
cis-Cyclododecene
When there are more than 12 carbons in thering,
trans-cycloalkenes are more stable than cis.The
ring is large enough so the cycloalkene behaves
much like a noncyclic one.