alkadienes and polyunsaturated hydrocarbons

August 25, 2003Lecture 4: Ragauskas

Alkadienes

and Polyunsaturated

Hydrocarbons

Solomons and Fryhle 13.6 – 13.10

Cytostatin inhibits an enzyme that is linked to cancer

Molecule of the Week

http://www.scripps.edu/newsandviews/e_20020325/cytostatin-l.gif


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"


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


Conjugated Dienes

C-C Bond Lengths

1.54 A

1.20 A Why

1.34 A Why

1.34 A

1.47 A

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?


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


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


Conjunction of P-Orbitals

Rotate C-C bond by 90o

Are the p orbitals delocalized?

Page 8a


Page 9


Page 10

See Solomons and Fryhle 13.9

A: AbsorbanceC: molar concentration of sample: extinction coeff.L” path length

max

* *

*


Page 11

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


Page 12

H Br

H

Electrophilic Attack on Conjugated Dienes: Mechanism


Page 13

Br Br

H

Br

Electrophilic Attack on Conjugated Dienes: Mechanism


Page 14

-80 oC 80% 20%


Page 15

+ 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


Page 16

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


Page 17

Problem from Solomons and Fryhle 13.23

H Cl

Me

ClMeMajor 1,4 Product is>>

Minor 1,4 Product is>>

Me

Cl

Me

WHY????


Page 18

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

alkadienes and polyunsaturated hydrocarbons

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