1. Extended Pi Systems Linear Multiple Conjugated p-bonds
1,3,5-Hexadiene H2C=CH—CH=CH—CH=CH2
Thermodynamically stable because of p-p interactions and resonance
Kinetically reactive
Low Ea for electrophilic additions
Carbocation is highly delocalized

2. Highly conjugated molecules are often highly reactive
Cyclic Extended p systems are unreactive
The simplest cyclic conjugated system is benzene
Benzene is very unreactive because it has 2 resonance forms without going to the radical, cation, or anion
Inert enough to use as a solvent for other organic reactions
Benzene chemistry is the subject of Chapter 15
b-carotene

3. Diels Alder Cycloaddition Reaction
Dienes and Alkenes react to give cyclohexadienes
Cycloadditions are the last new major category of reaction we will learn
The reaction works best with e- rich dienes and
e- poor alkenes (dienophiles)
Electron-Poor Alkenes
Substitute the alkene with e- withdrawing (electronegative) groups
Induction = removing e- density through s-bonds with electronegative groups (halides, haloalkyl groups)
Electron withdrawing groups can also work through resonance
20%
Nitriles
Carbonyls

5. Diene’s substituted with electron donating groups (alkyl groups) are
electron rich
Sample Reactions
The concerted Diels-Alder Reaction Mechanism
1. Concerted mechanisms happen all in one step (like SN2)

6. Diels-Alder Mechanism:
T.S. stabilized like benzene
3 weak p-bonds broken, 1 weak p-bond and 2 strong s-bonds formed

7. Diels-Alder Reactions are Stereospecific
Stereochemistry at the dienophile is retained
Stereochemistry of diene is retained
The Endo Rule

8. Exo Addition puts two ester groups near the bridgehead CH2
Endo Addition puts two ester groups away from the bridgehead CH2
Endo Cycloaddition is preferred in making a bicyclic
Attraction of the p-systems of the diene and dienophile explains
Diels-Alder Stereochemistry:

9. Electrocyclic Reactions = ring from a single p-system
Heat or Light can drive the formation of a ring from a p-system
Pericyclic Reaction = reaction with a cyclic transition state
Diels-Alder Reactions
Electrocyclic Reactions
Cyclization is preferred for trienes (DH = kcal/mol)
Ring cleavage is preferred for dienes (DH = -9.7 kcal/mol)
Addition of heat (D) forces the reaction to the most stable product (thermodynmic control)
Addition of light (hv) forces the reaction to the least stable product (kinetic control)

10. Electrocyclic Reactions are Concerted and Stereospecific
Concerted Mechanisms
Stereospecificity
Cyclobutene thermal ring opening is conrotatory
sp3—sp3 s-bonds rehybridize to sp2 + p orbital for double bond
New p-orbitals must rotate to become planar with p-bond
Cylcobutene with heat rotates same direction (clockwise)

11. Line-structure depictions
b) Butadiene light activated ring closing is disrotatory
trans, trans
cis, trans

12. Hexatriene thermal ring closing is disrotatory
Cyclohexadiene light activated ring opening is conrotatory
Summary:

13. Polymerization of Conjugated Dienes
Products are still unsaturated
Cross-linked polymers polymerized the unsaturation
Increased Elasticity: cross-links cause polymer to snap back after deformation

14. UV-Visible Spectroscopy
UV-Visible Light
UV l = nm
Visible UV l = nm
Transition caused is moving an e- from one MO to higher one

15. UV-Vis Spectrometry
Sample usually dissolved in solvent having no absorption itself
EtOH, MeOH, cyclohexane
Spectrometer Schematic
Organic Molecules and UV-Vis Spectroscopy
s-bond MO’s are separated by large energy gaps (overlap is very good for bonding MO, very bad for antibonding MO)
p-bond MO’s are more closely spaced
pp* and np* transitions occur with UV and Visible light energies

18. Molar Extinction Coefficient (e = A/C)
A = absorbance
C = Molar concentration
Units of e are L/mol
Since concentration is figured in, e is the same for any solution of a particular molecule (can identify an unknown)
Wavelength (l) of absorption is indicative of kind of bond absorbing
e is taken at lmax (the highest peak)
More double bonds lowers the energy of absoption = longer l
Fewer double bonds increases the energy = shorter l (Table 14-2)
Absorption above 400 nm make a compound visible
Dyes often have conjugated p systems
b-carotene is bright orange due to conjugation
White light contains all wavelengths (colors) of light
We see the colors that are not absorbed
R O Y G B I V
Increasing Energy of Colors
b-carotene absorbs here, so we see orange