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08 alkesynth
- 1. Chapter 8
Lecture
Structure and Synthesis
of Alkenes
Alkenes are also called olefins,
meaning ―oil-forming gas.
© 2013 Pearson Education, Inc.
© 2013 Pearson Education, Inc.
- 2. Bonding in Ethylene
C=C bond is shorter than C—C bond
a) sp2 hybrid orbitals have _______ s character than the sp3
hybrid orbitals. They are __________ to the nucleus.
b) Pi overlap brings carbon atoms _______
Bonding requires two p orbitals overlapping __________
© 2013 Pearson Education, Inc. Chapter 7 2
- 3. Cis and trans isomers cannot be
interconverted without breaking the pi
bond (264 kJ/mole).
No rotation around the carbon–carbon bond is possible
giving rise to Cistrans geometric isomers
© 2013 Pearson Education, Inc. Chapter 7 3
- 4. Elements (degrees) of Unsaturation. Also
called index of hydrogen deficiency.
• Unsaturation: A structural element that decreases the number of
hydrogens in the molecule by two.
• Double bonds and rings are elements of unsaturation (can not distinguish
with calculation
Calculate the degrees of unsaturation for a compound with formula C5H8 and
propose a structure
• Reference (saturated compound CnH2n+2)
• Compound
© 2013 Pearson Education, Inc. Chapter 7 4
- 5. • For each Group VA atom increase Hs in reference by one
• For each halogens decrease Hs atoms in reference by
one
• Group of oxygen does not change the C:H ratio, ignore
oxygen in the reference formula.
Calculate the IHD for a Compound with Formula
C4H7Br.
C6H7N.
C5H4O.
© 2013 Pearson Education, Inc. Chapter 7 5
- 6. IUPAC and New IUPAC
In a ring the
double bond is
given carbon 1
and carbon 2.
© 2013 Pearson Education, Inc. Chapter 7 6
- 7. Polyenes: se di-, tri-, tetra- before the ending -ene to specify how many double
bonds are present.
Give the double bonds the lowest numbers possible.
© 2013 Pearson Education, Inc. Chapter 7 7
- 8. EZ 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).
E-1-bromo-1-chloropropene
© 2013 Pearson Education, Inc. Chapter 7 8
- 9. Heat of Hydrogenation can provide valuable data as to the stability of the
double bond. The more substituted the double bond, the ___________its
heat of hydrogenation.
© 2013 Pearson Education, Inc. Chapter 7 9
- 10. Stability: cis-2-butene<iso-butene<trans-2-butene
-120 kJ >-117 kJ > -116 kJ
Cyclopropene is highly strained.
Rings that are five-membered or larger can easily
accommodate double bonds, and these cycloalkenes react
much like straight-chain alkenes.
© 2013 Pearson Education, Inc. Chapter 7 10
- 11. • Cis isomer is ______ stable than trans in small cycloalkenes.
• Must have at least _________carbons to form a stable trans double
bond.
• For cyclodecene (and larger), the trans double bond is almost as stable
as the cis.
• All cycloalkenes are assumed to be cis unless otherwise specifically
named trans.
© 2013 Pearson Education, Inc. Chapter 7 11
- 12. Bredt’s Rule: A bicyclic compound cannot have a double
bond at a bridgehead position unless one of the rings
contains at least eight carbon atoms.
Which of the following alkenes are
stable?
© 2013 Pearson Education, Inc. Chapter 7 12
- 13. Summary stability
• How is it quantitated?
• Substitution
• Stereochemistry
• Ring size
• Cisvs trans cyclos
• Bredt’s rule
Chapter 7 13
© 2013 Pearson Education, Inc.
- 14. Alkenes have relatively low boiling points which depend on
surface area and polarizability (______________ forces
_______ dense than water _________ on 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.
• Cisalkenes have a greater dipole moment than trans
alkenes, so they will be slightly polar and have higher bp
© 2013 Pearson Education, Inc. Chapter 7 14
- 15. Alkene Synthesis
Overview
1. R-X ----E2 dehydrohalogenation (-HX) Alkene
2. R-X ----E1 dehydrohalogenation (-HX) Alkene
3. Vic dibromide---Dehalogenation (-X2) Alkene
4. Alcohol ---Dehydration (-H2O) Alkene
© 2013 Pearson Education, Inc. Chapter 7 15
- 16. 1. Dehydrohalogenationby the E2
Mechanism
• Strong_________ abstracts H+ as double bond forms
and X- ___________ from the adjacent carbon.
• __________ and hindered ______________ alkyl
halides give alkenes in good yields.Tertiary halides are
the best E2 substrates because they cannot undergo
__________substitution.
© 2013 Pearson Education, Inc. Chapter 7 16
- 17. Bulky Bases for E2 Reactions
• If the substrate is prone to substitution, a bulky base
can minimize the amount of substitution.
• Large alkyl groups on a bulky base hinder its
approach to attack a carbon atom (substitution), yet it
can easily abstract a proton (elimination).
© 2013 Pearson Education, Inc. Chapter 7 17
- 18. Hofmann Product
Bulky bases, such as potassium tert-butoxide, abstract the
least hindered H+,giving the less substituted alkene as the
major product (Hofmann product).
© 2013 Pearson Education, Inc. Chapter 7 18
- 19. E2 Reactions Are Stereospecific: The stereochemistry of the
product depends on the stereochemistry of the reactant
© 2013 Pearson Education, Inc. Chapter 7 19
- 20. Show that the dehalogenation of 2,3-dibromobutane by
iodide ion is stereospecific by showing that the two
diastereomers of the starting material give different
diastereomers of the product.
Rotating meso-2,3-dibromobutane into a conformation where the bromine atoms are anti and
coplanar, we find that the product will be trans-2-butene. A similar conformation of either
enantiomer of the (±) diastereomer shows that the product will be cis-2-butene. (Hint: Your
models will be helpful.)
Meso-2,3-dibromobutane
(R,R)-2,3-dibromobutane
© 2013 Pearson Education, Inc. Chapter 7 20
- 21. E2 Reactions of
bromocyclohexane is slow
• An anti-coplanar conformation (180°) can only be
achieved when both the hydrogen and the halogen
occupy ___________ positions.
• The chair must flip to the conformation with the axial
halide in order for the elimination to take place.
© 2013 Pearson Education, Inc. Chapter 7 21
- 22. Explain why the following deuterated 1-bromo-2-
methylcyclohexane undergoes dehydrohalogenation by the
E2 mechanism, to give only the indicated product. Two
other alkenes are not observed.
In an E2 elimination, the hydrogen atom and the leaving group must have a trans-diaxial
relationship. In this compound, only one hydrogen atom—the deuterium—is trans to the
bromine atom. When the bromine atom is axial, the adjacent deuterium is also axial, providing
a trans-diaxial arrangement.
© 2013 Pearson Education, Inc. Chapter 7 22
- 23. 2. Debrominationof Vicinal Dibromides: reduction
with iodide ion in acetone follows an an E2
mechanism
© 2013 Pearson Education, Inc. Chapter 7 23
- 24. 3. E1 reaction
• Tertiary reacts faster than secondary alkyl halides:
• Carbocationintermediate; Rearrangements are possible.
• Solvolysis reactions in solvents as water or alcohols
• Usually have SN1 products, too, since the solvent can attack
the carbocation directly.
© 2013 Pearson Education, Inc. Chapter 7 24
- 25. 4. Dehydration of Alcohols
• Use concentrated H2SO4 or H3PO4 and remove low-boiling
alkene as it forms to shift the equilibrium and increase the yield
of the reaction.
• E1 mechanism.
• Carbocation intermediate. Rearrangements can occur.
• Reaction obeys Zaitsev’s rule.
© 2013 Pearson Education, Inc. Chapter 7 25
- 26. Dehydration Mechanism: E1
Step 1:Protonation of the hydroxyl group (fast equilibrium).
Step 2: Ionization to a carbocation (slow; rate limiting).
Step 3: Deprotonation to give the alkene (fast).
© 2013 Pearson Education, Inc. Chapter 7 26
- 27. In acid-catalyzed mechanisms,
the first step is often addition of
H+, and the last step is often
loss of H+.
© 2013 Pearson Education, Inc. Chapter 7 27
- 28. Propose a mechanism for the sulfuric acid–catalyzed dehydration of t-butyl alcohol.
The first step is protonation of the hydroxyl group, which converts it to a good leaving group.
The second step is ionization of the protonated alcohol to give a carbocation.
Abstraction of a proton completes the mechanism.
© 2013 Pearson Education, Inc. Chapter 7 28
- 29. Other Industrial methods:
a) Catalytic Cracking of Alkanes
• Long-chain alkane is heated with a catalyst to
produce an alkene and shorter alkane.
• Complex mixtures are produced.
© 2013 Pearson Education, Inc. Chapter 7 29
- 30. b) Dehydrogenation of Alkanes
• Dehydrogenation is the removal of H2 from a
molecule, forming an alkene (the reverse of
hydrogenation).
• This reaction has an unfavorable enthalpy
change but a favorable entropy change.
© 2013 Pearson Education, Inc. Chapter 7 30