Conjugated diene: multiple bonds alternate with single bonds

- colored pigments of fruits and flowers are polyenes

1,3-Butadiene(conjugated)

1,4-Pentadiene(nonconjugated)

Lycopene, a conjugated polyene(red pigment in tomato)

Ch.14 Conjugated Dienes and Ultraviolet Spectroscopy

Enone: alkene + ketone

Aromatic compound: cyclic conjugated polyene

O

H3C

CH3CH3

O

Progesterone, a conjugated enone(hormone)

Benzene, a cyclic conjugated molecule

14.1 Preparation of Conjugated Dienes

NBS

CCl4

Br

KOt-Bu

t-BuOH

- base-induced elimination of HX from allylic halide

CrO3/AlO3

cat.

600oC+ 2H2

- thermal craking: industrial preparation of 1,3-butadiene (used for polymer synthesis)

heat+ 2H2O

OH

OHAlO3

Isoprene(2-Methyl-1,3-butadiene)

- chloroprene, isoprene: precursors for polymer synthesis

Cl Chloroprene(2-Chloro-1,3-butadiene)

Stability of Conjugated Dienes- conjugated dienes: more stable than nonconjugated dienes- measured by heats of hydrogenation

∆Hohydrog

-126 kJ/mol

-253 kJ/mol

-236 kJ/mol

expected(-126 x 2 = -252)

(-126 x 2 = -252) expected

- stable 1,3-conjugated diene releases less energy on hydrogenation: (16 kJ/mol more stable than two isolated alkenes)

1. Orbital hybridization: more s character in C-C single bond

bond formed by overlap of sp2 and sp2 orbitals

bond formed by overlapof sp2 and sp3 orbitals

- sp2 orbitals has more s character than sp3 orbitals, the electrons in sp2 orbitals are closer to nucleus, and the bonds they form are somewhat shorter and stronger

14.2 Molecular Orbital Description of 1,3-Butadiene

Why conjugated dienes are stable?

2. Molecular orbital interactionsnodal plane

π-antibonding (π*) MO

π-bonding (π) MO

node

- in molecular orbitals, both electrons occupy the lower energy, bonding orbital, leading to a net lowering of energy and formation of a stable bond

Form four π-molecular orbitals in 1,3-butadiene from four atomic p-orbitals

bonding MO (no node)

bonding MO (1 node)

antibonding MO (2 node)

antibonding MO (3 node)

ψ1

ψ2

ψ3∗

ψ4∗

atomic orbitals

molecular orbitals

Delocalized: π electrons are spread out over the π framework; electron delocalization leads to lower energy and greater stability of the molecule

1,3-Butadiene(conjugated)

partial double bond

1,4-Pentadiene(nonconjugated)

- in ψ1, favorable bonding interaction between C2-C3; C2-C3 bond has certain amount of double-bond character

HClCl Cl

Markovnikov addition

14.3 Electrophilic Additions to Conjugated Dienes: Allylic Carbocations

+ HClH HCl- Cl

3o carbocation

HBrH

BrH

Br+

71%(1, 2 addition)

29%(1, 4 addition)

Electrophilic addition of conjugated diene: 1,2 and 1,4 additions

Br2Br

BrBr

Br+

55%(1, 2 addition)

45%(1, 4 addition)

Allylic carbocation: stable cation

HBr

H

secondary, allyliccarbocation

H

primary carbocation(Not formed)

H

Br-

H HBr Br

+

71%(1, 2 addition)

29%(1, 4 addition)

- more stable cation yields major product: sterically unhindered reaction

H

H

H

H

H

H

H

H

NBS, CCl4

Br

Br

+

83% 17%reaction of less hindered primary end is favored

unsymmetrical radicals

Allylic bromination of unsymmetrical alkene: radical bromination

Cyclic system

HCl

perferred path

- follow more stable 3o cation pathway

Temperature effects on the product ratio

14.4 Kinetic versus Thermodynamic Control of Reactions

HBrH

BrH

Br+

71% 29%

(1, 2 addition) (1, 4 addition)

at 0 oC

15% 85% at 40 oC

Temperature effects on the product ratio

∆GCo

∆GC

Ener

gy

Reaction progress

∆GBo

∆GB

Thermodynamic controlKinetic control

AB

C

A BC

Thermodynamic control: enough energy is supplied for reactant molecules to surmount the barriers to both products (Esupplied > ∆GC

‡, ∆GC

‡)- higher temperature- reversible, equilibrium process- C is more stable than B, thus, C is the major product- it doesn't matter that C forms more slowly than B, because the two are in equilibrium- the product of a readily reversible reaction depends only on thermodynamic stability

A BC Thermodynamic control(vigorous conditions, reversible)

Kinetic control: - low temperature- irreversible, non-equilibrium process- B forms faster than C, thus, B is the major product- it doesn't matter that C is more stable than B, because the twoare not in equilibrium- the product of an irreversible reaction depends only on relative rates

A BC Kinetic control(milds conditions, irreversible)

1,2 addition: kinetic 1,4-addition: thermodynamic

Ener

gy

Reaction progress

BrBr

1950 Novel Prize winners: Diels, Alder

14.5 The Diels-Alder Cycloaddition Reaction

O

+Benzene

heat

O

96%

Diels-Alder cycloaddition reaction: pericyclic reaction- take place in a single step by a cyclic redistribution of bonding electrons- the two reactants join together through a cyclic transition state- two new C-C bonds form at the same time

+

cyclic T.S.

Mechanism of Diels-Alder cycloaddition reaction:- concerted cyclic transition state- π orbital overlaps are important

Dienophile: electron-deficient alkenes

14.6 Characteristics of the Diels-Alder Reaction

O

OEt

OCN

O

O

O

O

O

OEtO

unreactive

1. Stereospecific reaction: the stereochemistry of the starting dienophile is maintained during the reaction

OMe

O

+ OMe

O

cis

CH3 CH3

(Z)

OMe

O

+ OMe

O

trans

H3C CH3

(E)

2. Endo rule:

R

R

1-carbon bridge

2-carbon bridge exo substituent(anti to larger bridge)

endo substituent(syn to larger bridge)

- endo product is fovored over exo product in D-A

- secondary orbital interactions

O

secondary orbitalinteraction

O

O

OO

O

O

HH

Endo product

O

O

O

OO

OH

H

Exo product

- endo selectivity

O

+

OH

CH3

CH3

CH3

CH3

HEndo

O

O

+H

H

O

O

H

H

CH3

CH3

CH3

CH3

Endo

cis-ring junction

3. ortho, para rule:

EWG+

EWG

RR

H EWGR

ortho metamajor product

EWG+

EWGH EWG

para metamajor product

R

R

R

- endo & ortho, para selectivity

O

+

OH

CH3

CH3

H Endo, ortho

H

OH3C

HHH

cis

Diene: electron-rich 1,3-conjugated dienes

s-trans s-cis

only s-cis conformation can undergo D-A reaction, but s-trans is more stable conformation

s-trans s-cis

CH3CH3

severe steric strain

fixed s-trans

- following dienes are unreactive: unable to adopt s-cisconformation

+25oC

HH

fixed s-cis diene: highly reactive

- so normally D-A reaction requires elevated temperature for less reactive substrates- Lewis acid catalyzed D-A reaction: low temperature- under high pressure conditions: decrease entropy

>

polymerization of conjugated dienes: structurally more complex- cis-trans isomer- initiator: radical, cation- 1,4-addition polymerization

14.7 Diene Polymers: Natural and Synthetic Rubbers

Incis-Polybutadiene

trans-Polybutadiene

Natural Rubber: isoprene

In Natural rubber (Z)

Gutta-percha (E)

Isoprene

gutta-percha: harder, more brittle than rubber- a variety of minor applications: covering on golf balls

chloroprene

In

Cl Cl

Neoprene (Z)

Cl Cl

Chloroprene

neoprene: expensive, good weather resistance- used for industrial gloves and hoses

natural and synthetic rubbers: soft, tacky

vulcanization: heating crude rubber with a few percent by weight of sulfur- sulfur forms bridges, or cross-links, between polymer chains- locking the chains together into immense molecules that can no longer slip over one another- much harder, improved resistance to wear and abraison- smells sulfur when tire burns

SS

SS

SS

14.8 Structure Determination in Conjugated Systems: Ultraviolet Spectroscopy

Mass Spectrometry size and formula

Infrared Spectroscopy functional groups

Ultraviolet (UV) Spectroscopy conjugated π-system

Nuclear Magnetic Resonance Spectroscopy

C-H framework

Electromagnetic Spectrum

increasing energy

wavelength (λ) in m10-8 10-6 10-4 10-3 10-2 10-1

X-Rays Ultraviolet Infrared Micro

wavesRadio wavesγ-rays

10-12 10-10

Visible

3.8 x 10-7 m 7.8 x 10-7 m

- UV range for organic molecule: 200 - 400 nm- corresponds to energy level of π-electron in unsaturated molecules- information of π-systems

14.9 Ultraviolet Spectrum of 1,3-Butadiene

ψ1

ψ2

ψ3∗

ψ4∗

hv

HOMO

LUMO

π

π∗

ground-stateelectronic

configuration

excited-stateelectronic

configuration

HOMO: highest occupied molecular orbitalLUMO: lowest unoccupied molecular orbital

π→ π* excitation: 217 nm for 1,3-butadiene

UV

electronic transition

π

π∗

200 250 300 350 400Wavelength (nm)

0

0.5

1.0

1.5λmax = 217 nm

Absorbance (A

)

π→ π* excitation: 217 nm for 1,3-butadiene

Absorbance (A):

A = logI0I

I0 = intensity of the incident lightI = intensity of the transmitted light

Molar absorptivity (ε): exact amount of UV light absorbed is expressed- physical constant- characteristic of the particular substance- typically, ε = 10,000 - 25,000

ε =A

C x l

A = AbsorbanceC = Concentration (mol/L)l = Sample path length (cm)

UV spectrum is simple and broad- λmax (wavelength at the top of the peak)

14.10 Interpreting Ultraviolet Spectra: The Effects of Conjugation

- π-conjugated systems: absorb UV frequency (200-400nm)- the greater the extent of conjugation, the smaller energy gap between HOMO and LUMO: longer wavelength required

λmax= 217 nm

O

λmax= 219 nm 254

258 290171

256 275

π→ π* transition wavelength: depends on energy gap between HOMO and LUMO, which depends on the nature of the conjugated system

β-Carotene

- some extended organic π-conjugated systems: colored (400-800 nm)

14.11 Cojugation, Color, and the Chemistry of Vision

eg) β-carotene: 11 doubles in conjugation, λmax= 455 nm (visible, blue absorption) → yellow-orange color

β-Carotene

CH2OH

Vitamin A 11-cis-RetinalCHO

16

11

15

The Chemistry of Vision

Rhodopsin

16

11

15 N-OpsinMetarhodopsin II

N-Opsin

Cis Trans

light

light sensitive receptor in the retina of the human eye:- 3 million rod cells: responsible for the seeing dim light- 100 million cone cells: responsible for the seeing bright light

The cis-trans isomerism of rhodopsin is accompanied by a change in molecular geometry, which in turn causes a nerve impulse to be sent to the brain where it is perceived as vision.

In rod cells: a light sensitive rhodopsin formed from 11-cis-retinal and the protein opsin

• Translates light into nerve signals • Allows vision under wide dynamic range (starlight to sunlight) • Encodes wavelength allowing us to see in color • Provides visual acuity

The Retina

OLED (Organic Light Emitting Display)

N

H

O

O

N

H

BuO

NNt

N

N

EuO

O

N

CH2CH3

N

CH2CH3

1. Green

2. Blue

3. Red

NO

AlO

N

ON

DTVBi

BCzVBi

Alq3Quinacridone

PBD Eu(DBM)3

Light Emitting Materials

Photolithography

Chemistry @ Work

photolithography: producing integrated circuit chips

CH3

OH OH

CH3

Novolac resin

ON2 hv

H2O

COOH

+ N2

Diazonaphthoquinone washed with dilute base

diazoquinone-novoloc system: polymer resist currently used in chip manufacturing

Chemistry @ Work Photolithography

Chapter 14

Problem Sets

20, 26, 31, 37, 43, 49, 54