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Organic Chemistry IITA: Robert Spencer

Ch. 12: Dienes and the Allyl System: 2p Orbitals in Conjugation

Conjugated Dienes

A diene is conjugated when its coplanar double bonds are separated by a single bond. Conjugated dienes

are generally stable due to the delocalization of pi electrons along sp

2

hybridized orbitals.

= 1,3-Butadiene: Conjugated

= 1,4-Pentadiene: Isolated

= Propadiene (Allene): CumulatedC C CH

H H

H

1,3-Butadiene Resonance

1,3-Butadiene Molecular OrbitalsElectron delocalization always leads to lower energy and greater stability. Due to an interaction betweenthe pi orbitals of 1,3-butadiene's double bonds, the single bond is stronger and shorter than a typicalsingle bond.

double-bondcharacter

LowestEnergy

HighestEnergy

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Allenes in the Isomerization of Acetylenes

1. NaNH2 / NH3

2. H2O

Mechanism:

H3C C C CH3

NH2

H3C C C CH2

H3C C C CH2

NH3C C C

H

H3C H

H

NH2

C C C

H

H3C

H

C C C

H

H3C

H

NH3

H3C

H2C C C H

pKa = 26NH2

NH2H3C

H2C C C

H2O

H3C

H2C C C H

Electrophilic Additions to Conjugated Dienes

H Cl

12

3

4

+

Cl Cl

Cl

1,2-Addition

Cl

1,4-Addition

Reaction Temp: 25oC

-80oC

40%

80%

60%

20%

Kinetic Product Thermodynamic Product

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The 1,4-product is more stable because its double bond ismore substituted; however, this product requires crossinga higher energy transition state.

Kinetic control f avors the product of 1,2-addition. Thismight be explained by the location of the chloride ionimmediately following protonation.

Cl

H

Many other electrophiles can also add to conjugated dienes:

Br Br

Br

Br

Br

Br

Low Temp(Kinetic)

Br

Br

High Temp(Thermodynamic)

Br

Br Br

Br

The Allyl System

Molecular Orbitals:

AntibondingCarbanion LUMO

NonbondingCarbocation LUMO

Bonding

Energy

Effects of delocalization:

1. Allylic Halides (SN1): Resonance stabilization of allyl

cation increases reactivity of allyl halides.

2. Allyl Halides (SN2): Transition state lies at lower energy,

which leads to an accelerated reaction.

3. Allyl Radicals: Resonance favors the formation of an allylic radical (i.e. NBS Mechanism).

4. Allyl Anion: Proton at allylic position is easier to remove (more acidic) due to resonance.

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Diels-Alder Reaction of Dienes

General Mechanism:

Diene Dienophile

s-trans s-cis

Unreactive Dienes:

Diene: Require s-cis conformation

In this exothermic reaction, a conjugated diene reactswith an alkene or alkyne (dienophile) to form a six-membered ring.

Dienes are generally more reactive when substituted with electron-donating groups, whiledienophiles are more reactive with electron-withdrawing groups.

EDG EWG

EDG: OR, OH, NR2 EWG: CN, NO2, COR

Stereoselectivity

CN

CN

CN

CN

cis cis

CN

CN

trans trans

CN

NC

Bicyclic Products

+

CN

CN

H

endo(major)

+

H

CN

exo(minor)

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Molecular Orbital Analysis

The Diels-Alder reaction relies on HOMO-LUMO interactions:

LUMOEthene

HOMOButadieneLUMO

Butadiene

HOMOEthene

sigmabond

Product

This HOMO-LUMOinteraction also works

Example: Explain why this reaction is thermally forbidden

Energy

HOMO

LUMOHOMOEthene

LUMOEthene

sigmabond

no bond

In general, concerted cycloaddition reactions involving 4n + 2 pi electrons are thermally allowed andphotochemically f orbidden. Those involving 4n pi electrons are thermally f orbidden and photochemicallyallowed.

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Reverse Diels-Alder

General Mechanism:

+heat

heat

Example:

The Cope Rearrangement

General Mechanism:

R

heat

R

Examples:

H

H

heat

Transition State:

R

O

heat

H

O