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Ch.14 Conjugated Dienes and Ultraviolet Spectroscopy

Conjugated diene: multiple bonds alternate with single bonds

1,3- 1,4-Pentadiene (conjugated) (nonconjugated)

- colored pigments of fruits and flowers are polyenes

Lycopene, a conjugated polyene (red pigment in tomato) Enone: + ketone O CH3 CH3

H3C

O

Progesterone, a conjugated enone (hormone) : cyclic conjugated polyene

Benzene, a cyclic conjugated molecule 14.1 Preparation of Conjugated Dienes - base-induced elimination of HX from allylic halide

Br NBS KOt-Bu

CCl4 t-BuOH

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

cat. CrO3/AlO3 + 2H2 600oC - , : precursors for polymer synthesis

OH AlO3 + 2H2O OH heat Isoprene (2-Methyl-1,3-butadiene)

Cl Chloroprene (2-Chloro-1,3-butadiene) Stability of Conjugated Dienes - conjugated dienes: more stable than nonconjugated dienes - measured by heats of hydrogenation o ∆H hydrog

-126 kJ/mol

-253 kJ/mol (-126 x 2 = -252) expected

-236 kJ/mol (-126 x 2 = -252) expected - stable 1,3-conjugated diene releases less energy on hydrogenation : (16 kJ/mol more stable than two isolated ) 14.2 Molecular Orbital Description of 1,3-Butadiene Why conjugated dienes are stable? 1. Orbital hybridization: more s character in C-C single bond

bond formed by overlap bond formed by overlap of sp2 and sp2 orbitals of 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 2. Molecular orbital interactions nodal plane node

π-antibonding (π*) MO

π-bonding (π) MO

- 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

∗ ψ4 antibonding MO (3 node)

∗ ψ3 antibonding MO (2 node)

atomic orbitals ψ2 bonding MO (1 node)

ψ1 bonding MO (no node)

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)

1,4-Pentadiene (nonconjugated) partial

-in ψ1, favorable bonding interaction between C2-C3 ; C2-C3 bond has certain amount of double-bond character 14.3 Electrophilic Additions to Conjugated Dienes: Allylic Carbocations

Markovnikov addition

H Cl Cl- H + HCl

3o carbocation

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

HBr + Br H H Br

71% 29% (1, 2 addition) (1, 4 addition)

Br2 + Br Br Br Br

55% 45% (1, 2 addition) (1, 4 addition) Allylic carbocation: stable cation primary carbocation (Not formed) H HBr

H H

secondary, allylic - carbocation Br

H + H Br Br

71% 29% (1, 2 addition) (1, 4 addition) - more stable cation yields major product: sterically unhindered reaction Allylic bromination of unsymmetrical alkene: radical bromination

NBS, CCl4 H H H H

H H H H unsymmetrical radicals

Br + Br 83% 17% reaction of less hindered primary end is favored Cyclic system

perferred path

HCl

- follow more stable 3o cation pathway 14.4 Kinetic versus Thermodynamic Control of Reactions

Temperature effects on the product ratio

HBr + Br H H Br (1, 2 addition) (1, 4 addition)

at 0 oC 71% 29% at 40 oC 15% 85% Temperature effects on the product ratio

C A B Thermodynamic control Kinetic control

∆GC ∆GB

Energy A ∆G o o B ∆GC B C

Reaction progress 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

C A B 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 two are not in equilibrium - the product of an irreversible reaction depends only on relative rates

C A B Kinetic control (milds conditions, irreversible) 1,2 addition: kinetic 1,4-addition: thermodynamic Energy

Br Br

Reaction progress 14.5 The Diels-Alder Reaction

1950 Novel Prize winners: Diels, Alder

O O + heat

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

+

cyclic T.S. 14.6 Characteristics of the Diels-Alder Reaction

Dienophile: electron-deficient alkenes

O O CN OEt

unreactive

O O O OEt O

O O 1. Stereospecific reaction: the stereochemistry of the starting dienophile is maintained during the reaction O O OMe + OMe CH 3 CH3 (Z) cis O O OMe + OMe

H3C CH3 (E) trans 2. Endo rule:

1- bridge

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

endo substituent (syn to larger bridge)

- endo product is fovored over exo product in D-A - secondary orbital interactions

H O H O O O O O secondary orbital Endo product interaction O O O O O O H O H

Exo product - endo selectivity

O CH O CH3 3 H H + Endo

CH3 CH3

O CH O CH3 3 H H + Endo H H O CH3 O CH3 cis-ring junction 3. ortho, para rule:

R EWG H EWG EWG +

R R ortho meta major product

EWG H R EWG EWG R +

R para meta major product - endo & ortho, para selectivity

O O CH3 H H + Endo, ortho

CH3

H H H H3C O cis

H Diene: electron-rich 1,3-conjugated dienes

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

s-trans s-cis - following dienes are unreactive: unable to adopt s-cis conformation

CH3 CH3

s-trans s-cis fixed s-trans severesteric strain fixed s-cis diene: highly reactive

>

25oC H + H

- so normally D-A reaction requires elevated temperature for less reactive substrates - Lewis catalyzed D-A reaction: low temperature - under high pressure conditions: decrease entropy 14.7 Diene : Natural and Synthetic Rubbers

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

cis- In

trans-Polybutadiene : isoprene

In Natural rubber (Z)

Isoprene

Gutta-percha (E)

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

Cl Cl Cl Cl In

Chloroprene Neoprene (Z)

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 S S

S S S S 14.8 Structure Determination in Conjugated Systems: Ultraviolet Spectroscopy

Mass Spectrometry size and formula Infrared Spectroscopy functional groups

Nuclear Magnetic C-H framework Spectroscopy Ultraviolet (UV) Spectroscopy conjugated π-system Electromagnetic Spectrum

increasing energy

Ultra Micro Radio γ-rays X-Rays Infrared violet waves waves 10-12 10-10 10-8 10-6 10-4 10-3 10-2 10-1 wavelength (λ) in m 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 HOMO: highest occupied molecular orbital LUMO: lowest unoccupied molecular orbital

∗ ψ4

∗ ψ3 π∗ LUMO hv

ψ2 HOMO π

ψ1 ground-state excited-state electronic electronic configuration configuration π → π* excitation: 217 nm for 1,3-butadiene π → π* excitation: 217 nm for 1,3-butadiene

π∗ Absorbance (A) 1.5 UV λmax = 217 nm 1.0 π

0.5 electronic transition

0 200 250 300 350 400 Wavelength (nm) Absorbance (A):

I I0 = intensity of the incident light A = log 0 I I = 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

A A = Absorbance ε = C = Concentration (mol/L) C x 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 → π* transition wavelength: depends on energy gap between HOMO and LUMO, which depends on the nature of the

290 171 λmax= 217 nm 258

O

λmax= 219 nm 256 254 275

- 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 14.11 Cojugation, Color, and the Chemistry of Vision - some extended organic π-conjugated systems: colored (400-800 nm)

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

β-Carotene The Chemistry of Vision

β-Carotene

11 6 CH2OH 1

CHO 15 Vitamin A 11-cis-Retinal 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

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

11 Cis Trans 6 1 light N-Opsin

15 N-Opsin Rhodopsin Metarhodopsin II

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. The Retina

• 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

OLED (Organic Light Emitting Display) Light Emitting Materials

H O N 1. Green N O O Al N O O N N H

Quinacridone Alq3 2. Blue

N N CH CH 2 3 BCzVBi CH2CH3

3. Red DTVBi N O Eu N O N N t Bu O

PBD Eu(DBM)3 Chemistry @ Work Photolithography

photolithography: producing integrated circuit chips Chemistry @ Work Photolithography

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

OH OH

CH3 CH3

Novolac resin

O COOH N 2 hv + N2 H2O

Diazonaphthoquinone washed with dilute base Problem Sets

Chapter 14

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