Chapter 7 Structure and Synthesis of Alkenes

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Chapter 7Structure and Synthesis of Alkenes
Organic Chemistry, 5th EditionL. G. Wade, Jr.
Portions taken from Jo Blackburn Richland
College, Dallas, TX Dallas County Community
College District ã 2003, Prentice Hall And Singer
at
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Functional Group

• Pi bond is the functional group.
• More reactive than sigma bond.
• Bond dissociation energies
• CC BDE 146 kcal/mol
• C-C BDE 83 kcal/mol
• Pi bond 63 kcal/mol
  gt

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Physical Properties

• Low boiling points, increasing with mass.
• Branched alkenes have lower boiling points.
• Less dense than water.
• Slightly polar
• Pi bond is polarizable, so instantaneous
  dipole-dipole interactions occur.
• Alkyl groups are electron-donating toward the pi
  bond, so may have a small dipole moment.
  
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Polarity Examples
? 0.33 D
? 0
gt
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IUPAC Nomenclature

• Parent is longest chain containing the double
  bond.
• -ane changes to -ene. (or -diene, -triene)
• Number the chain so that the double bond has the
  lowest possible number.
• In a ring, the double bond is assumed to be
  between carbon 1 and carbon 2.
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Name These Alkenes
1-butene
2-sec-butyl-1,3-cyclohexadiene
2-methyl-2-butene
3-n-propyl-1-heptene
gt
3-methylcyclopentene
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Alkene Substituents
Common Names

• Usually used for small molecules.

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Cis-trans Isomerism

• Similar groups on same side of double bond,
  alkene is cis.
• Similar groups on opposite sides of double bond,
  alkene is trans.
• Cycloalkenes are assumed to be cis.
• Trans cycloalkenes are not stable unless the ring
  has at least 8 carbons.
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Name these
cis-1,2-dibromoethene
gt

trans-2-pentene
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E-Z Nomenclature

• Use the Cahn-Ingold-Prelog rules to assign
  priorities to groups attached to each carbon in
  the double bond.
• If high priority groups are on the same side, the
  name is Z (for zusammen).
• If high priority groups are on opposite sides,
  the name is E (for entgegen).
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• Select the higher priority group on each carbon
  in the double bond of the alkene.
• If the higher priority groups are
• on opposite sides E (entgegen opposite)
• on the same side Z (zusammen together)

CH3 gt H Cl gt CH2CH3
(E)-3-chloro-2-pentene
(Z)-3-chloro-2-pentene
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Designate each double bond as E or Z, where
applicable.
high
E ethyl gt methyl COH gt H Z methyl gt H
CH2Br gt CH3 E CH2OH gt CH3 CH2CH2gt
H Note that the second double bond is neither E
nor Z since two groups are the same
high
high
high
high
high
geraniol
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Example, E-Z
High
High
High
High
2Z
5E
(2Z, 5E)-3,7-dichloro-2,5-octadiene

gt
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Orbital Description

• Sigma bonds around C are sp2 hybridized.
• Angles are approximately 120 degrees.
• No nonbonding electrons.
• Molecule is planar around the double bond.
• Pi bond is formed by the sideways overlap of
  parallel p orbitals perpendicular to the plane of
  the molecule.
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sp2 hybrid orbitals

• To account for double bonds, a second type of
  hybrid orbital must be pictured. An sp2 hybrid
  is produced by combining one s and 2 p orbitals.
  One p orbital remains.

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2p
2p
?
2sp2
energy
2s
Unhybridized Hybridized
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sp2 hybrid orbitals

• The unhybridized p orbitals are able to overlap,
  resulting in the formation of a second bond - p
  bond.

A p bond is a sideways overlap that occurs
both above and below the plane of the
molecule Parts of the molecule are no longer
able to rotate about the bond.
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Ethene
Click on graphic to start animation
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1s orbital
p overlap
sp2 hybrids
? bond
sp2 hybrids
p overlap
? bond
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Bonding in ethene
Click on graphic to start animation
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Trigonal planar, BF3
Click on graphic to start animation
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Bond Lengths and Angles

• Hybrid orbitals have more s character.
• Pi overlap brings carbon atoms closer.
• Bond angle with pi orbitals increases.
• Angle CC-H is 121.7?
• Angle H-C-H is 116. 6? gt

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Pi Bond

• Sideways overlap of parallel p orbitals.
• No rotation is possible without breaking the pi
  bond (63 kcal/mole).
• Cis isomer cannot become trans without a chemical
  reaction occurring.

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Elements of Unsaturation

• A saturated hydrocarbon CnH2n2
• Each pi bond (and each ring) decreases the number
  of Hs by two.
• Each of these is an element of unsaturation.
• To calculate find number of Hs if it were
  saturated, subtract the actual number of Hs,
  then divide by 2.
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Heteroatoms

• Halogens take the place of hydrogens, so add
  their number to the number of Hs.
• Oxygen doesnt change the CH ratio, so ignore
  oxygen in the formula.
• Oxygen double bond?
• Nitrogen is trivalent, so it acts like half a
  carbon.

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Commercial Uses Ethylene
gt
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Commercial Uses Propylene
gt
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Other Polymers
gt
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Stability of Alkenes

• Measured by heat of hydrogenation Alkene
  H2 ? Alkane energy
• More heat released, higher energy alkene.

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Substituent Effects

• More substituted alkenes are more stable.H2CCH2
  lt R-CHCH2 lt R-CHCH-R lt R-CHCR2 lt R2CCR2
  unsub. lt monosub. lt disub. lt trisub. lt
  tetra sub.
• Alkyl group stabilizes the double bond.
• Alkene less sterically hindered.

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Disubstituted Isomers

• Stability cis lt geminal lt trans isomer
• Less stable isomer is higher in energy, has a
  more exothermic heat of hydrogenation.

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