Chapter 7 Structure and Synthesis of Alkenes

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Chapter 7Structure and Synthesis of Alkenes
Organic Chemistry, 5th EditionL. G. Wade, Jr.
Jo Blackburn Richland College, Dallas, TX Dallas
County Community College District ã 2003,
Prentice Hall
2
Common Names

• Often called olefins as a class.
• Common names usually used for small molecules.
• Examples

3
IUPAC Nomenclature

• 1 longest Chain
• longest chain contains the most double bonds.
• 2 Numbering
• Number the chain so that double bond has the
  lowest number possible. In a ring, double bond
  is between C1 and C2.

4
IUPAC Nomenclature

• 3 Name the Substituents
• Systematic alkenyl name
• Common group names

5
IUPAC Nomenclature

• 4 Organize the name
• Add a to greek prefix if more than one double
  bond.
• -ane changes to ene (or diene, -triene)

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IUPAC Nomenclature

• 5 Organize the name
• Assign Stereochemistry
• cis- vs trans- if have a hydrogen on each carbon
• Cycloalkenes are almost always cis-.
• (Z) vs (E) for all other situations

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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

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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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Example, E-Z
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Commercial Uses Ethylene
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Commercial Uses Propylene
gt
13
Other Polymers
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Functional Group

• The functional groups is a pi bond.
• Pi bond is formed by the sideways overlap of
  parallel p orbitals perpendicular to the plane of
  the molecule.
• 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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Hybridization

• The electronic structure is sp2 hybridized.
• Hybrid orbitals have more s character.
• Molecular geometry is trigonal planar.

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Bond Lengths and Angles

• 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?

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Bond Strength

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

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Polarity

• Like alkanes, alkenes are relatively non-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.

? 0.33 D
? 0
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Physical Properties

• Low boiling points, increasing with mass.
• Branched alkenes have lower boiling points.
• Less dense than water.

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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.
• Bulky substituents are more separated.

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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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Cis/Trans Stability

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

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Cycloalkene Stability

• Cis isomer more stable than trans.
• Small rings have additional ring strain.
• Must have at least 8 carbons to form a stable
  trans double bond.
• For cyclodecene (and larger) trans double bond is
  almost as stable as the cis.

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Bicycloalkene Stability

• Bredts Rule
• A bridged bicyclic compound cannot have a double
  bond at a bridgehead position unless one of the
  rings contains at least eight carbon atoms.

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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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Propose a Structure
for C5H8

• First calculate the number of elements of
  unsaturation.
• Remember
• A double bond is one element of unsaturation.
• A ring is one element of unsaturation.
• A triple bond is two elements of unsaturation.

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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.
• Nitrogen is trivalent, so it acts like half a
  carbon.

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Structure for C6H7N?

• Find number of hydrogens
• Calculate unsaturation

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Alkene SynthesisOverview

• E2 dehydrohalogenation (-HX)
• E1 dehydrohalogenation (-HX)
• E2 dehalogenation of vicinal dibromides (-X2)
• Dehydration of alcohols (-H2O)
• Industrial Catalytic Cracking
• Industrial Dehydrogenation of Alkanes

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Removing HX via E2

• Strong base .
• Tertiary and hindered secondary alkyl halides
  give good yields.
• If the alkyl halide usually forms substitution
  products, ..

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Some Bulky Bases
(CH3CH2)3N triethylamine

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Hofmann Product

• Bulky bases abstract the least hindered H
• Least substituted alkene is major product.

H
C
H
_
3
C
H
C
H
O
3
2
C
H
C
C
C
H
3
2
C
H
C
H
O
H
3
2
H
B
r
H
H
C
H
_
3
(CH
)
CO
3
3
C
H
C
C
C
H
3
2
C
H
C
H
O
H
3
2
H
B
r
H
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E2 Diastereomers
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E2 Cyclohexanes

• Leaving groups must be trans diaxial.

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Removing HX via E1

• Only with ..
• Requires .
• Rxn. proceeds ..
• Usually have

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E2 Dehalogenation

• Remove Br2 from adjacent carbons.
• Use NaI in acetone, or Zn in acetic acid.
• Mechanism

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Dehydration of Alcohols
H
O
H
O
C
C
S
O
H
O
H
O
H
H
O
H
C
C
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Industrial Methods

• Catalytic cracking of petroleum
• Long-chain alkane is heated with a catalyst to
  produce an alkene and shorter alkane.
• Complex mixtures are produced.
• Dehydrogenation of alkanes
• Hydrogen (H2) is removed with heat, catalyst.
• Reaction is endothermic, but entropy-favored.
• Neither method is suitable for lab synthesis

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End of Chapter 7