Chapter 14

Conjugated Dienes and Ultraviolet Spectroscopy

Conjugated Dienes• Multiple Bonds Alternating with Single Bonds

1,3 Butadiene

H2C=CH-CH=CH2

1,4 Pentadiene

H2C=CH-CH2-CH=CH2

CONJUGATED NOT CONJUGATED !!!

C O

CH3

CH3

CH3

O

Examples of Conjugated Dienes

Lycopene – a conjugated polyene

Progesterone – a conjugated enone

Benzene – a cyclic conjugated molecule

Preparation and Stability of Conjugated Dienes

Diene Preparation

H H

Cyclohexene 3-Bromocyclohexene 1,3-Cyclohexadiene

NBS

CCl4

Br+K -OC(CH3)3

HOC(CH3)3

•Based Induced Elimination of HX

Diene Preparation

• Thermal cracking of butane using a chromium oxide/aluminum oxide catalyst

CH3CH2CH2CH3

600 Oc

CatalystH2C=CHCH=CH2 +2 H2

Acid-catalyzed double dehydration

CH3 C

CH3

OH

CH2

CH2

OH Al203

HeatCH2 C

CH3

CH

CH2 +2 H20

Special Properties of Conjugated Dienes

• Length of the central single bond is shorter than non-conjugated similar molecule

• Comparison of 1,3-Butadiene and Butane

H2C=CH-CH=CH2 CH3-CH2-CH2-CH3

148 pm 153 pm

1,3-Butadiene Butane

ShorterBond

Special Properties of Conjugated Dienes

• Unusual stability evidenced by heats of hydrogenation• More highly substituted alkenes are more stable than less

substituted ones• More highly substituted alkenes release less heat on

hydrogenation because they contain less energy to start with

CH3 CH2

CH

CH2

CH3 C

CH3

CH2

CH2 CH

CH

CH2

CH2 CH

CH2

CH

CH2

CH3 CH2

CH2

CH3

CH3 CH

CH3

CH3

CH3 CH2

CH2

CH3

CH3 CH2

CH

CH3

CH3

CH3 CH2

CH2

CH2

CH3

CH2 CH

C CH2

CH3

Heats of Hydrogenation for Some Alkenes and Dienes

Alkene or Diene Product (kj/mol) (kcal/mol) HO

hydrog

-126 -30.1

-119 -28.4

-236 -56.4

-229 -54.7

-253 -60.5

Molecular Orbital Description of 1,3 Butadiene

Stability of Conjugated Dienes is due to orbital hybridization

• Typical C-C single bonds result from sigma overlap of sp3 orbitals on both carbons

CH3-CH2-CH2-CH3

Bonds formed by overlap of sp3 orbitals

• Conjugated dienes have a central C-C bond that results from sigma overlap of sp2 orbitals on both carbons

H2C=CH-CH=CH2

Bonds formed by overlap of sp2 orbitals

Stability of conjugated dienes is due to orbital hybridization

• Since sp2 orbitals have more s character (33%) than sp3 orbitals (25% s), the electrons in sp2 orbitals are closer to the nucleus and the bonds they form are shorter and stronger

• The “extra” stability of conjugated dienes result from the greater amount of s character in the bonds forming the C-C bond

Stability of conjugated dienes is due to orbital hybridization

+

-

+

-

+

-

+

-

+

-

+

-

+

-

+

-

Four isolated p orbitals

+

- +

-

+

- +

-

+

-

+

- +

-

+

-

+

- +

- +

- +

- Antibonding (3 nodes)

Antibonding (2 nodes)

Bonding (1 node)

Bonding (0 nodes)

ENERGY

Why is the conjugated bond stronger?

• П electrons are “delocalized” over the entire П framework rather than localized between two specific nuclei.

• П certain amount of double bond character exists in a conjugated bond over the single bond area.

Compare 1,3-Butadiene with 1,4 Pentadiene

C C C C

+

-

+

-

+

-

+

-C C C C C

+

-

+

-

+

-

+

-1,3-Butadiene

a conjugated diene

1,4-Pentadiene

a non-conjugated diene

Partial double bond character

Electrophilic Additions to Conjugated Dienes: Allylic

Carbocations

• Electrophilic addition to 1,3-Butadiene yields a mixture of two products:

• 1,2 addition

• 1,4 addition

Non-conjugated alkene addition reactions

CH3 C+

CH3

CH3HCl

EtherCH3 C

CH3

CH2

CH3 C

CH3

CH3

Cl

2-Methylpropene Tertiary Carbocation

2-Chloro-2-methylpropane

CH2 CH

CH2

CH

CH2

HCl

EtherCH3 C

HCH2

CH

CH3

Cl Cl

Conjugated diene 1,2 and 1,4 addition reactions

CC

CCH

H

H

HH

H

+ HBrC

CC

CH

H

H

HH

H

Br

H

3-Bromo-1-butene (71%; 1,2 addition)

CC

CCH

H

H

HH

H

Br

H

1-Bromo-2-butene (29%; 1,4 addition)

CH2 CH

CH

CH2

Br2

25oC

CH2

CH

CH

CH2

Br BrCH2

CH

CH

CH2Br

Br

+

+

1,3-Butadiene (a conjugated

diene)

1,3-Butadiene1,4-Dibromo-2-butene

(45%; 1,4 addition)3,4-Dibromo-1-butene

(55%; 1,2 addition)

1,4 addition products are due to allylic carbocation intermediates

CC

CCH

H

H

HH

H

CC

C+

CH

H

H

HH

HH

C+

CC

CH

H

H

HH

HH

Br-

Secondary, allylic

CC

CC

+H

H

H

HH

H

H Br-

Primary, nonallylic (NOT formed)

HBr

Kinetic versus thermodynamic control of

reactions

• At room temperature, electrophilic addition to a conjugated diene leads to a product mixture where the 1,2 adduct predominates over the 1,4 adduct.

• At high temperatures, the product ratio changes and the 1,4 adduct predominates

CH2 CH

CH

CH2 + HBr CH2 CH

CH

CH3

Br

CH3 CH

CH

CH2

Br+

1,2 adduct 1,4 adduct

At 0oC: 71% 29%

At 40oC: 15% 85%

Kinetic Versus Thermodynamic Control

• Kinetic control dominates reactions where the product of an irreversible reaction is the one that forms fastest

• Thermodynamic control dominates reactions where the product of a readily reversible reaction depends on thermodynamic stability

A Kinetic Control Reaction

• B forms faster because it requires less energy• C is more stable, but requires more energy• The reaction occurs under mild conditions and is

irreversible• No equilibrium is reached

A Thermodynamic Control Reaction

• This reaction is held under higher temperatures and equilibrium is reached

• Since C is more stable than B, C is the major product• The product of a readily reversible reaction depends only on

thermodynamic control

The Diels-AlderCycloaddition Reaction

• Conjugated diene

• Dienophile

• Diels-Alder reaction:

* Stereospecific

* Prefer Endo product to Exo product

Conjugated diene• Contain alternating double and single bond:

• Adopt S-cis conformation :

• More stable than non-conjugated diens.

Examples of conjugated diens• 1,3-Butadiene

• 1,3-Pentadiene

• 1,3-Cyclopentadiene

H

C C C

H

CH2H2

Conjugated diene VS. Non-conjugated diene

Non-conjugated diene

Conjugated diene

Non-conjugated diene

Conjugated diene

Non-conjugated diene

Conjugated diene

S-cis conformation of diens

H

HS-cis

S-trans

S-cis

H

CC

C

CH3

CH3C

H

C

C

H

H

H

H

H3CCH3

Severe steric strain in s-cis form S-trans

Dienophile

• Has carbon carbon double or triple bond that is next to the positively polarized carbon of a electron-withdrawing substituent group

• Reactive and uncreative

C

C

C

O

H

HH

H

_

+

Propena(Acrolein)

Reactive

C

C

C

O

HH

H OCH2CH3+

_

Ethyl propenoate (Ethyl acrylate)

Reactive

Maleic anhydride

Reactive

H

C

O

O

C

C

C

H

O

_

_

+

+

Benzoquinone

Reactive C

C

CC

C

C

H

H

O

O

H

H +

_

+

_

Propenenitrile (Acrylonitrile)

Reactive

C

C

C

HH

H

N+

_

O

C

C

C

H

OCH3

Methyl propynoate

Reactive

+

_

C

C

C

CH2CH3

O

HH

H

Unreactive

N

Unreactive

O

Unreactive

Diels-Alder reaction

C

C

C

CC

C

C

H

H

H

H

H

H

O

H

H H

CH3 CCH3

O

+ Benzene

Heat

Conjugated diene

Dienophile

Stereospecific

• The stereochemistry of the starting dienophile is maintained during the reaction, and a single product stereoisomer results.

• Example:

C

C

C

C

H CH2

CH2

CH3

H

H

H

+

CO2CH3 H

HCH3

CO2CH3

CH3

H

HCO2CH3

C

C

C

C

H CH2

CH2HH

HH3C

+

CO2CH3

CHO

+CHO

Endo & Exo product

O

O

O+

Endo

Exo

O

O

OH

H

O

O

OH

H

Diene Polymers

1,3-Butadiene

cis-Polybutadiene

trans-Polybutadiene

In

In

Isoprene

Natural rubber (Z)

Gutta-percha (E)

In

Chloroprene Neoprene (Z)

Cl Cl ClCl

Ultraviolet Spectrum of 1,3-Butadiene

Ultraviolet Excitation of 1,3- Butadiene

ENERGY Four p atomic orbitals

Ψ4*

Ψ3*

Ψ2

Ψ1

LUMOhv

UV irradiationHOMO

Ground State Excited State

П *

П

When irradiated with UV energy, electrons absorb the energy and arePromoted from a П bonding molecular orbital to an antibonding П *Molecular orbital

(lowest unoccupied molecular orbital)

(highest occupied molecular orbital)

Ultraviolet Spectrum

• A UV spectrum is recorded by irradiating a sample with UV light of continously changing wavelength.

• When the wavelength corresponds to the energy level required to excite an electron to a higher level, energy is absorbed

• This absorption is detected and displayed on a chart that plots wavelength versus absorbance

Structure determination in Conjugated systems

• Ultraviolet Spectroscopy

X-raysVacuumultraviolet

Visible

Visible

Near

infrared

InfraredU

ltraviolet

Energy

= 200nm = 400nm

H2C C CH

CH3

CH2

H2C CH CH2CH CHCH

H2C CHCH CHCH CH2CHCH

H2C CH C

CH3

O

Ultraviolet Absorptions of Some Conjugated Molecules

Name Structure max(nm)

2-methyl-1,3-butandiene

1,3-Cyclohexandiene

1,3,5-Hexatriene

1,3,5,7-Octatetraene

2,4-Cholestadiene

3-Buten-2-one

Benzene

Naphthalene

220

256

258

290

275

219

203

220

Ultraviolet spectrum of Beta-carotene

The absorption occurs in the visible region