I. Extended Pi SystemsA. Linear Multiple Conjugated -bonds

1) 1,3,5-Hexadiene H2C=CH—CH=CH—CH=CH2

2) Thermodynamically stable because of - interactions and resonance

3) Kinetically reactive

a) Low Ea for electrophilic additions

b) Carbocation is highly delocalized

H2C CH CH CH CH CH2Br2

BrCH2 CH CH CH CH CH2

BrCH2 CH CH CH CH CH2

BrCH2 CH CH CH CH CH2

Br-

BrCH2 CH CH CH CH CH2

Br

BrCH2 CH CH CH CH CH2

Br

BrCH2 CH CH CH CH CH2

Br

+

+

1,2

1,4

1,6

c) Highly conjugated molecules are often highly reactive

B. Cyclic Extended systems are unreactive

1) The simplest cyclic conjugated system is benzene

2) Benzene is very unreactive because it has 2 resonance forms without going to the radical, cation, or anion

3) Inert enough to use as a solvent for other organic reactions

4) Benzene chemistry is the subject of Chapter 15

-carotene

H

H

H

H

H

H

H2, PdSlow

No Rxn

No Rxn

No Rxn

KMnO4

H+, H2O Br2

II. Diels Alder Cycloaddition ReactionA. Dienes and Alkenes react to give cyclohexadienes

1) Cycloadditions are the last new major category of reaction we will learn

2) The reaction works best with e- rich dienes and

e- poor alkenes (dienophiles)

3) Electron-Poor Alkenes

a) Substitute the alkene with e- withdrawing (electronegative) groups

b) Induction = removing e- density through -bonds with electronegative groups (halides, haloalkyl groups)

c) Electron withdrawing groups can also work through resonance

+200 oC

20%

CF3

H2C

H

R

O

H2C

H

R

O

H2C

H

R

O

C

N

H2C

H

C

N

H2C

H

C

N

H2C

H

CarbonylsNitriles

4) Diene’s substituted with electron donating groups (alkyl groups) are

electron rich

5) Sample Reactions

B. The concerted Diels-Alder Reaction Mechanism

1. Concerted mechanisms happen all in one step (like SN2)

H3C

H3C

CH3

CH3

H3C

H3C

+

O

H H

O O

OMe+OMe

O

90%94%

I- + CH3Cl ICH3 + Cl-

2. Diels-Alder Mechanism:

a) T.S. stabilized like benzene

b) 3 weak -bonds broken, 1 weak -bond and 2 strong -bonds formed

C. Diels-Alder Reactions are Stereospecific

1. Stereochemistry at the dienophile is retained

2. Stereochemistry of diene is retained

D. The Endo Rule

OMe

O

O

OMe+

O

OMe

O

OMe

cistrans

O

OMe

O

OMe+ OMe

O

O

OMe

CH3

CH3

NC CN

CNNC

+

CN

CN

CNCN

CH3

CH3

cis trans

CN

CN

CNCN

CH3

CH3

+

NC CN

CNNCCH3

CH3

trans, trans cis, trans

HH

+

H

CO2CH3H

CO2CH3CO2CH3

CO2CH3

HH

1) Exo Addition puts two ester groups near the bridgehead CH2

2) Endo Addition puts two ester groups away from the bridgehead CH2

i. Endo Cycloaddition is preferred in making a bicyclic

ii. Attraction of the -systems of the diene and dienophile explains

iii. Diels-Alder Stereochemistry:

HH

H

CO2CH3H

CO2CH3

CO2CH3

CO2CH3

HH

HH

H

CH3O2C H

CH3O2C

CO2CH3

CO2CH3

HH

HH

H

C H

CO

MeO

O

MeO

i

i

ex

ex

o

o

en

en

exen

en ex

+

o

i

o

i

III. Electrocyclic Reactions = ring from a single -systemA. Heat or Light can drive the formation of a ring from a -system

1) Pericyclic Reaction = reaction with a cyclic transition state

a) Diels-Alder Reactions

b) Electrocyclic Reactions

2) Cyclization is preferred for trienes (H = -14.5 kcal/mol)

3) Ring cleavage is preferred for dienes (H = -9.7 kcal/mol)

4) Addition of heat () forces the reaction to the most stable product (thermodynmic control)

5) Addition of light (hv) forces the reaction to the least stable product (kinetic control)

hv

hv

B. Electrocyclic Reactions are Concerted and Stereospecific

1) Concerted Mechanisms

2) Stereospecificity

a) Cyclobutene thermal ring opening is conrotatory

i. sp3—sp3 -bonds rehybridize to sp2 + p orbital for double bond

ii. New p-orbitals must rotate to become planar with -bond

iii. Cylcobutene with heat rotates same direction (clockwise)

iv. Line-structure depictions

b) Butadiene light activated ring closing is disrotatory

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

cis, transtrans, trans

c) Hexatriene thermal ring closing is disrotatory

d) Cyclohexadiene light activated ring opening is conrotatory

e) Summary:

CH3

CH3

CH3

CH3

CH3

CH3

hvCH3

CH3

IV. Polymerization of Conjugated Dienes1) 1,2-Polymerization

2) 1,4-Polymerization

3) Products are still unsaturated

a) Cross-linked polymers polymerized the unsaturation

b) Increased Elasticity: cross-links cause polymer to snap back after deformation

H2C CH CH CH2

CH CH2 CH CH2

CH

CH2

CH

CH2

n

H2C CH CH CH2 CH2 CH CH CH2

Cross-Linking

V. UV-Visible SpectroscopyA. UV-Visible Light

1) UV = 200-400 nm

2) Visible UV = 400-800 nm

3) Transition caused is moving an e- from one MO to higher one

hc

hE

B. UV-Vis Spectrometry

1. Sample usually dissolved in solvent having no absorption itself

EtOH, MeOH, cyclohexane

2. Spectrometer Schematic

C. Organic Molecules and UV-Vis Spectroscopy

1. -bond MO’s are separated by large energy gaps (overlap is very good for bonding MO, very bad for antibonding MO)

2. -bond MO’s are more closely spaced

3. * and n* transitions occur with UV and Visible light energies

4) Molar Extinction Coefficient ( = A/C)

a) A = absorbance

b) C = Molar concentration

c) Units of are L/mol

d) Since concentration is figured in, is the same for any solution of a particular molecule (can identify an unknown)

5) Wavelength () of absorption is indicative of kind of bond absorbing

a) is taken at max (the highest peak)

b) More double bonds lowers the energy of absoption = longer

c) Fewer double bonds increases the energy = shorter (Table 14-2)

d) Absorption above 400 nm make a compound visible

i. Dyes often have conjugated systems

ii. -carotene is bright orange due to conjugation

iii. White light contains all wavelengths (colors) of light

iv. We see the colors that are not absorbed

R O Y G B I VIncreasing Energy of Colors

-carotene absorbs here, so we see orange