Conjugated Dienes and Ultraviolet

Spectroscopy

2

Key Words

• Conjugated Diene

• Resonance Structures

• Dienophiles • Concerted Reaction

• Pericyclic Reaction • Cycloaddition Reaction

• Bridged Bicyclic Compound

• Cyclic Compounds • Endo

• Exo

3

What are Conjugated Dienes?

• Conjugated Dienes are carbon structures which

maintain 2 double bond

separated by a single bond.

• Conjugated Dienes can

be found in many different molecules as

shown.

Examples of Conjugated Dienes

4

Conjugated and Nonconjugated Dienes

• If Di = two and ene = double bond then Diene = two double bonds.

• If double bonds are separated by only ONE single bond, they are conjugated and their orbitals interact.

• The conjugated diene 2,4-heptadiene has properties that are very different from those of the nonconjugated diene, 1,5-heptadiene

Conjugated Diene Non-Conjugated Diene

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Polyenes

• Compounds with many alternating single and double

bonds.

• Extended conjugation leads to absorption of visible light,

producing color.

• Conjugated hydrocarbons with many double bonds are

polyenes (lycopene is responsible for red color in

tomatoes)

• Extended conjugation in ketones (enones) found in

hormones such as progesterone.

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Lycopene

O

O

H

H

H

Progesterone

Benzene

Examples of Conjugated Dienes

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Preparation and Stability of Conjugated Dienes

• Typically by elimination in allylic halide

• Specific industrial processes for large scale production of

commodities by catalytic dehydrogenation and dehydration.

NBS = N-Bromosuccimide (You add a bromine (halogen))

KOC(CH3)3 is a strong base (dehydrohalogenation)

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Dehydration of Alcohols

Preparation Conjugated Dienes

Removal of hydrogens

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Stability of Dienes

• Conjugated dienes are more stable than nonconjugated dienes based on heats of hydrogenation.

• Hydrogenating 1,3-butadiene releases 15 kJ/mol less heat than 1,4-pentadiene.

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Molecular Orbital Description of 1,3-

Butadiene

• The single bond between the conjugated double bonds is shorter and stronger than sp3

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Molecular Orbital Description of

1,3-Butadiene

• The bonding -orbitals are made from 4 p

orbitals that provide greater delocalization

and lower energy than in isolated C=C

• The 4 molecular orbitals include fewer

total nodes than in the isolated case (See

Figures 14-1 and 14-2)

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13

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Molecular Orbital Description of

1,3-Butadiene • In addition, the single bond between the two double

bonds is strengthened by overlap of p orbitals

• In summary, we say electrons in 1,3-butadiene are delocalized over the bond system

– Delocalization leads to stabilization

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Electrophilic Additions to Conjugated Dienes:

Allylic Carbocations

• Review: addition of electrophile to C=C

– Markovnikov regiochemistry via more stable carbocation

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Carbocations from Conjugated Dienes

• Addition of H+ leads to delocalized secondary

allylic carbocation

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Products of Addition to Delocalized

Carbocation

• Nucleophile can add to either cationic site

• The transition states for the two possible products are

not equal in energy

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Practice Problem 14.1: Products?

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Kinetic vs. Thermodynamic Control

of Reactions

• At completion, all reactions are at equilibrium, and the relative concentrations are controlled by the differences in free energies of reactants and products (Thermodynamic Control)

• If a reaction is irreversible or if a reaction is far from equilibrium, then the relative concentrations of products depends on how fast each forms, which is controlled by the relative free energies of the transition states leading to each (Kinetic Control)

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Kinetic and Thermodynamic

Control Example • Addition to a conjugated diene at or below room

temperature normally leads to a mixture of products in which the 1,2 adduct predominates over the 1,4 adduct

• At higher temperature, product ratio changes and 1,4 adduct predominates (See Figures 14-4 and 14-5)

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23

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Conjugated Diene Dienophile

+

Product

What is the Diels-Alder Reaction?

The Diels-Alder reaction uses a conjugated diene and a

dienophile to produce cyclic and bicyclic carbon

structures.

This is also called the [4 + 2] cycloaddition reaction for

the reaction of 4 pi electrons (diene) and 2 pi electron

(dienophile).

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Properties of Conjugated Dienes

Rotation

s-trans s-cis

• Conjugated Dienes can undergo resonance

which is the movement

of a double bond from

• Conjugated Dienes can

often rotate to either

form the s-cis or s-trans (s = single)

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What are Dienophiles?

O

O

O

O

O

O

O

O

O

C

C

OO

OO

• Dienophiles are molecules which

maintains a double

bond or triple bond.

• They are normally

bound to electron

withdrawing groups or neutral groups.

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Diels-Alder Reaction

• The Diels Alder reaction uses the resonance

movement of electrons of

the conjugated diene in

the s-cis configuration

with a dienophile to create a cyclicaddition or

bridge bicyclic structure.

• This reaction works as a concerted reaction or all

in one step similar to an

SN2 reaction.

New Bond

New Bond

+

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Limitations of Diels-Alder

Reaction

• Does not react with s-trans

configuration

• Does not react well with dienophiles

with electron donating groups.

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Products of Diels-Alder Reactions

• The products of Diels-Alder reaction are cyclic or ring compounds.

• It is also possible to form Bridged Bicyclic Compound by starting with diene found inside ring structures.

+

+

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Cyclic Product

• The reaction

produces only one

product.

• If the reaction occurs

with a cis dienophile

then the product will

be a cis product.

• If the reaction occurs

with a trans

dienophile then the

product will be a

trans product.

+

H3C

H

CH3

H

+

H

H

CH3

CH3

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Bridged Bicyclic Products

• Often the attachment

to the diene moves

up creating a bridge

while the dienophile

binds beneath it.

• The diene can bind

three ways 1) without

stereoselectivity 2)

endo and 3) exo.

O

O

O

H

H

O

H

HO

O

H

H

H

H

No stereoselectivity

Endo

Exo

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Endo Product

• This is where the dienophile attaches (down) opposite the bridge or functional groups.

• Of the Diels-Alder reactions with stero selectivity the Endo product is preferred due to decreased steric strain.

O

O

O

H

H

Endo

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Exo Product

• This is where the dienophile attaches (up) same the bridge or functional groups.

• Of the Diels-Alder reactions with stero selectivity the Exo product is less favorable due to increased steric strain.

O

H

HO

O

Exo

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Diels-Alder Examples

O

O

O

O

+

EndoMajor Product

CO2Et

CO2Et

CO2Et

CO2Et

+

H

H

+ X NR

Not in the s-cis config

+

The molecules rotates into the s-config

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Easy Retrosynthesis

• Find the double bond

• Remove the double bond.

• Add double bonds to the adjacent bonds.

• Move 2 bond in both directs, remove these new bonds.

• Add a double bond to the final bond.

O

O

O

O

O

O

O

O

O

+

O

O

O

O

O

O

+

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Diels Alder Reaction

• Can create carbon carbon single bonds by reacting

conjugated diene and a dienophile to produce cyclic and

bicyclic carbon structures.

• Reacts with electron withdrawing dienophiles or neutral

groups.

• Works with conjugated dienes in the s-cis configuration.

• The Diels-Alder reaction is stereoselective giving cis

and trans configuration to the product.

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Regiochemistry of the Diels-Alder

Reaction

• Reactants align to produce endo (rather than exo) product

– endo and exo indicate relative stereochemistry in bicyclic structures

– Substituent on one bridge is exo if it is anti (trans) to the larger of the other two bridges and endo if it is syn (cis) to the larger of the other two bridges

– If the two bridges are equal, the product with the substituent endo to the new double bond is formed.

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Conformations of Dienes in the

Diels-Alder Reaction

• The relative positions of the two double bonds in the diene are the “cis” or “trans” two each other about the single bond (being in a plane maximizes overlap)

• These conformations are called s-cis and s-trans (“s” stands for “single bond”)

• Dienes react in the s-cis conformation in the Diels-Alder reaction

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Practice Problem 14.2:

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Solution:

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Problem 14.7 (p. 478):

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Reaction Mechanism:

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Solution:

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Unreactive Dienes

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Reactive Diene: cyclopentadiene

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Experiment 49:

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Problem 14.33:

Diels-Alder Products?

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Problem 14.40:

Diels-Alder Reactants?

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Problem 14.45:

Structure of Product?

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First Diels-Alder Reaction:

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Second Diels-Alder Reaction:

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Diene Polymers: Natural and

Synthetic Rubber

• Conjugated dienes can be polymerized

• The initiator for the reaction can be a radical, or an acid

• Polymerization: 1,4 addition of growing chain to conjugated diene monomer

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Natural Rubber

• A material from latex, in plant sap

• In rubber, the repeating unit has 5 carbons and Z stereochemistry of all C=C double bonds

• Gutta-Percha is natural material with E in all C=C

• They are head-to-tail polymers of isoprene (2-methyl-1,3-butadiene)

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Vulcanization

• Natural and synthetic rubbers are too soft to be

used in products

• Charles Goodyear discovered heating with small

amount of sulfur produces strong material

• Sulfur forms bridges between hydrocarbon

chains (cross-links)

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Vulcanization:

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Synthetic Rubber

• Chemical polymerization of isoprene does not

produce rubber (stereochemistry is not

controlled)

• Synthetic alternatives include neoprene,

polymer of 2-chloro-1,3-butadiene

• This resists weathering and solvents better

than rubber

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Neoprene:

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Structure Determination in Conjugated

Systems: UV Spectroscopy

• Conjugated compounds can absorb light in the ultraviolet

region of the spectrum

• The region from 2 x 10-7m to 4 x 10-7m (200 to 400 nm) is

most useful in organic chemistry

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Structure Determination in Conjugated

Systems: UV Spectroscopy

• The electrons in the highest occupied molecular orbital (HOMO)

undergo a transition to the lowest unoccupied molecular orbital

(LUMO)

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Structure Determination in Conjugated

Systems: UV Spectroscopy

• A plot of absorbance (log of the ratio of the intensity of light in over light transmitted) against wavelength in this region is an ultraviolet spectrum – see 1,3-butadiene below

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Ultraviolet Spectrum of 1,3-

Butadiene

• Example: 1,4-butadiene has four molecular orbitals with the lowest two occupied

• Electronic transition is from HOMO to LUMO at 217 nm (peak is broad because of combination with stretching, bending)

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Quantitative Use of UV Spectra

• Absorbance for a particular compound in a specific solvent at a specified wavelength is directly proportional to its concentration

• You can follow changes in concentration with time by recording absorbance at the wavelength (kinetic experiment)

• Beers’ law: absorbance (A) = ecl – “e” is molar absorptivity (extinction coefficient – “c” is concentration in mol/L

– “l” is path of light through sample in cm

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Interpreting UV Spectra: Effect of

Conjugation

• max: wavelength where UV

absorbance for a compound is

greatest

• Energy difference between HOMO

and LUMO decreases as the extent of

conjugation increases

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Interpreting UV Spectra: Effect of

Conjugation

• max increases as conjugation

increases (lower energy)

– 1,3-butadiene: 217 nm

– 1,3,5-hexatriene: 258 nm

• Substituents on system increase

max

• See Table 14-2 for examples

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Conjugation, Color and the Chemistry of

Vision

• Visible region is about 400 to 800 nm

• Extended systems of conjugation absorb in visible region

• b-Carotene, 11 double bonds in conjugation – max = 455 nm

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Conjugation, Color and the Chemistry of

Vision

b-Carotene is converted to Vitamin A, which is converted to 11-cis-retinal:

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Conjugation, Color and the Chemistry of

Vision

• 11-cis-retinal is converted to rhodopsin in the rod cells of the retina.

• Visual pigments are responsible for absorbing light in eye and triggering nerves to send signal to brain