Nobel Prize in Chemistry, 2001

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Nobel Prize in Chemistry, 2001
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Alkenes
Alkenes possess the double bond.
Double Bond Energy 146 kcal/mol ?
Bond Energy 83 kcal/mol ? Bond
Energy 63 kcal/mol
Conclusion ? bond is weaker than the ? bond
by 20 kcal/mol The ? bond
is much more reactive!
Compare C-C single bond 1.54 Å
1.33 Å
116.6o
1.08 Å
121.7o
The CC double bond is shorter than the C-C
single bond.
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Alkene Nomenclature

• Find the parent hydrocarbon
• Find the longest continuous chain of carbon
  atoms that contains the double bond
• and name it accordingly using the ene
  ending.

Named as octene (not decene)
2. Number the atoms in the main chain a.
Begin at the end nearer the double bond. b.
If the double bond is equidistant from the ends,
begin at the end nearer the first branch
point.
The above molecule would be numbered as
5
3. Identify and number the substituents as you
have done with alkanes.
4. Write the name as a single word. a. Use
prefixes such as mono, di, tri, tetra, etc. to
indicate multiple substituents. b. List
substituents alphabetically, ignoring prefixes.
c. Use dashes to separate substitutents, use
commas to separate numbers. d. Indicate the
position of the double bond by giving the number
of the first alkene carbon and placing that
number immediately before the parent names.
e. If more than one double bond exists, indicate
the position of both and use the suffix -diene.
3-butyl-2-octene
Name these alkenes
Answer 1,3-pentadiene
Answer 4,5-dimethyl-2-heptene
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Cycloalkene Nomenclature
Name cycloalkenes in a similar manner. However,
since there is no chain end, the double \ bond
must always be between carbons 1 and 2. Make
sure the substitutents have the lowest numbers
possible. It is not necessary to indicate the
position of the double bond.
4-isopropylcyclohexene
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1,3-cyclopentadiene
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1
2
3
Name this one.
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4
2
3,7-dimethyl-1,3,5-cycloheptatriene
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1
6
7
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Stereochemistry and E/Z Nomenclature
We know that some alkenes can exist as cis/trans
stereoisomers (geometric isomers).
trans-2-butene
cis-2-butene
What about something like this? Which is cis and
which is trans?
Answer Who knows?????? We need a more general
method for describing the
geometry about the double bond.
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E/Z Method for Double Bond Geometry
E entgegen meaning opposite Z zusammen
meaning together or the same side
In order to use this system of nomenclature, you
must first prioritize the substituents according
to the wonderful Cahn-Ingold-Prelog rules.

• Consider the sp2 carbons separately and identify
  the two substitutents
• attached to each carbon.

Left side Br and Cl Right side H
and CH3
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2. Use the Cahn-Ingold-Prelog rules to prioritize
the two substituents on each side. a. The
highest priority group has an atom with the
highest atomic number directly bonded to
the sp2 carbon.
Left side Br AN 35 Cl AN
17
Right Side H AN 1 CH3
AN 6
Br gt Cl
CH3 gt H
The above molecule has the two highest priority
groups on the opposite side and therefore this is
the E isomer
(E)-1-bromo-1-chloro-1-propene
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b. If the two directly bonded atoms have the same
AN, then look at the next atoms bonded these
atoms.
Right side Directly attached atoms
are both carbon. 2nd Atoms H, H, H
AN 1, 1, 1 C, H, H AN
12, 1, 1
Left side Directly attached atoms
are both carbon. 2nd
Atoms Cl, H, H AN 17, 1, 1
H, H, H An 1, 1, 1
CH2Cl gt CH3
CH2CH3 gt CH3
This is a E isomer.
(E)-1-chloro-2,3-dimethyl-2-pentene
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• If an atom is doubly bonded to another atom, then
  it is treated as if it were singly
• bonded to two of these atoms.

Which has highest priority?
1st atoms are both C (tie)
vs
2nd atoms H, C, C
becomes
2nd atoms H, H, C
gt
d. In the case of isotopes, use the mass number
since they have the same atomic number
1H vs. 2H (D) deuterium
AN 1 for both atoms, but deuterium has greater
mass number, therefore
2H gt 1H
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Alkene Stability
Not all alkenes are created equal! Some are more
stable than others. Two considerations
1. The degree of substitution.
Unsubstituted Monosubstituted
Disubstituted
Trisubstituted Tetrasubstituted
Least Stable
Most Stable
Steric Interaction
2. Stereochemistry
trans-2-butene
cis-2-butene
More Stable Less Stable
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How do we know about this order of
stability? Answer Heats of Hydrogenation
Measure by Calorimetry
The larger the heat of hydrogenation, the less
stable the alkene
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Calculating Degrees of Unsaturation
Degrees of Unsaturation (DU) number of rings
and/or multiple bonds in a molecule.
4 DU 2 DU
4 Du
How to calculate DU 2 C 2 - H
2
C6H6 DU 2 (6) 2 6 4
2
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Simple enough! But what about when you have a
heteroatom in the molecule Heteroatom any
atom besides C and H.

• Halogens (X) such as F, Cl, Br, and I.
• Count as a hydrogen atom, that is, add
  the number of halogens to the
• number of hydrogens.
• For example

Formula C5H11Br ? C5H12 DU 2(5) 2
12 0 2

• Oxygen or Sulfur
• Ignore!

Formula C6H10O ? C6H10 DU 2
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3. Nitrogen Subtract the number of hydrogens
from the number of hydrogens
Formula C4H5N ? C4H4 DU 3
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Electrophilic Addition of HX to Alkenes
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What happens when an asymmetric alkene reacts
with HBr?
a
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b
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Which pathway is favored. Answer Pathway a
is favored since a more stable carbocation is
formed.
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Carbocation Rearrangements
If a carbocation can rearrange to a more stable
carbocation, then it probably will. Otherwise,
it will not.
20 less stable 30 more
stable
Primary consideration There must be a viable
mechanism by which
rearrangement can take place.
1. 1,2 Hydrogen Shift (1,2 H)
1,2 H
Both bonding electrons go with the hydrogen,
leaving a charge on the carbon left behind.
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2. 1,2 methyl shift (1,2 CH3)
1,2 CH3
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What is the mechanism of this rearrangement?
Answer 1,2 H
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In what types of reactions do we observe
carbocation rearrangements? Answer In reactions
where there are carbocation intermediates.
Write a mechanism to account for both products.
Which product results from carbocation
rearrangement?
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Transition States and the Hammond Postulate
The Hammond postulate states that the transition
state of an exothermic reaction most closely
resembles the reactants in energy and
structure. Conversely, the transition state of
an endothermic reaction most closely resembles
the products in energy and structure.
This is most conveniently illustrated with a
reaction energy diagram.
Exothermic
Endothermic


E
E
Reaction Progress
Reaction Progress
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In the exothermic reaction, the transition state
(green arrow) is closer to the the reactant (red
arrow) along the reaction coordinate. Meaning
The transition state structure most closely
resembles the reactant.
How can we illustrate this in a structure
drawing? Consider the reaction A-B C
A B-C
Exothermic The transition state structure shows
that B is less than half transferred to C.
A---B--------C (not
A-------B---C )