Alkadienes and Polyunsaturated Hydrocarbons
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Transcript of Alkadienes and Polyunsaturated Hydrocarbons
August 25, 2003Lecture 4: Ragauskas
Alkadienes
and Polyunsaturated
Hydrocarbons
Solomons and Fryhle 13.6 – 13.10Page 1
Cytostatin inhibits an enzyme that is linked to cancer
Molecule of the Week
August 25, 2003Lecture 4: Ragauskas
Examples of Differing Dienes
H2C C CH2
buta-1,3-diene
propa-1,2-diene
hexa-1,5-diene
An Unconjuated Diene SystemDienes React Independently
Conjuated Diene SystemDienes React inter-dependently
1,2-diene systems "relatively rare"
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ClCH3
Isoprene - monomer ofNatural rubber Chloroprene - moner of neoprene
On his second voyage, Columbus saw children playing with balls made from the sap of local trees. Joseph Priestly called the material the balls were made of rubber, because it would erase pencil marks by rubbing. Michael Faraday, one of the nineteenth centuries greatest scientists, determined an empirical formula for rubber of C5H8. This is the formula for the diene isoprene. Rubber is a natural polymer of
isoprene. Crude rubber, latex, collected from the rubber tree Hevea brasiliensis is an aqueous emulsion of the polymer. It is washed, dried and coagulated to give raw rubber, each chain of which consists of ca. 5000 isoprene units. All of the double bonds in natural rubber are cis-, a corresponding trans- polymer has quite different properties.
C5H8
Polymerization
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August 25, 2003Lecture 4: Ragauskas
Conjugated Dienes
C-C Bond Lengths
1.54 A
1.20 A Why
1.34 A Why
1.34 A
1.47 A
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Note A: Ao
C-C Bond
> sp3 – sp3
> sp2-sp2
> sp-sp
What About C2-C3??
s-cis conformation s-trans conformation of 1,3-butadiene
Q: At room temperature which conformation is favoured/why?
August 25, 2003Lecture 4: Ragauskas
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August 25, 2003Lecture 4: Ragauskas
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Molecular Orbital Description of Ethene
Take 2 2p orbitals>>
E
E
Ethene Ethene Cation Ethene Anion
*
E
C CHH
H H
1s
1s
1s
1s
sp2
sp2
August 25, 2003Lecture 4: Ragauskas
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Molecular Orbital Description of 1,3-Butadiene
Take 4 2p orbitals>> E
1
2
3 *
4*
Cation
LUMO: Lowest unoccupied molecular orbitalHOMO: Highest occupied molecular orbital
Anion
August 25, 2003Lecture 4: Ragauskas
Conjunction of P-Orbitals
Rotate C-C bond by 90o
Are the p orbitals delocalized?
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See Solomons and Fryhle 13.9
A: AbsorbanceC: molar concentration of sample: extinction coeff.L” path length
max
* *
*
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See Solomons and Fryhle 13.10Electrophilic Attack on Conjugated Dienes: Products
Remember from Last Week
H Br
+
Br-
Br- Br
HBr
1, 2-Addition 1, 4-Addition
H
Br
Br
Br
Br
H
Br
Br
H
Br2
One MolarEquivalent
New!
Note: Markovnikov addition
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H Br
H
Electrophilic Attack on Conjugated Dienes: Mechanism
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Br Br
H
Br
Electrophilic Attack on Conjugated Dienes: Mechanism
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-80 oC 80% 20%
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+ H-Br
H H
BrH
1,4-addition Product
Schematic Illustration of Free-Energy vs. Rxn Coordinate for 1,4 and 1-2 additionOf HBr to 1,3-butadiene
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Problem from Solomons and Fryhle 13.34 - Provide a mechanism to accounts for the formation of CICH2CH=CHCH2OCH3 and CICH2CH(OCH3)CH=CH2
upon treatment of 1,3-butadiene with CI2 in methanol.
Cl Cl
August 25, 2003Lecture 4: Ragauskas
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Problem from Solomons and Fryhle 13.23
H Cl
Me
ClMeMajor 1,4 Product is>>
Minor 1,4 Product is>>
Me
Cl
Me
WHY????
August 25, 2003Lecture 4: Ragauskas
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H-Br + R-O-O-R
hv or heat
Problem from Solomons and Fryhle 13.20
What is Rxn Mechanism and Products?
Remember
RO-OR 2 RO
RO + H-Br RO-H + Br