Chapter 15Conjugated Systems, Orbital Symmetry, and

Ultraviolet Spectroscopy

Jo BlackburnRichland College, Dallas, TX

Dallas County Community College District2003,Prentice Hall

Organic Chemistry, 5th EditionL. G. Wade, Jr.

Chaper 15 2

Definitions

• Conjugated double bonds are separated by one single bond. Example: 1,3-pentadiene.

• Isolated double bonds are separated by two or more single bonds. 1,4-pentadiene.

• Cumulated double bonds are on adjacent carbons. Example: 1,2-pentadiene. =>

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Resonance Energy

• Heat of hydrogenation for trans-1,3-pentadiene is less than expected.

H for 1-pentene is 30.0 kcal/mol and for trans-2-pentene is 27.4 kcal/mol, so expect 57.4 kcal for trans-1,3-pentadiene.

• Actual H is 53.7 kcal, so the conjugated diene is more stable.

• Difference, (57.4 – 53.7) 3.7 kcal/mol, is the resonance energy. =>

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Relative Stabilities

twice 1-pentene

more substituted

=>

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Structure of 1,3-Butadiene• Most stable conformation is planar.

• Single bond is shorter than 1.54 Å.

• Electrons are delocalized over molecule.

=>

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Constructing Molecular Orbitals

• Pi molecular orbitals are the sideways overlap of p orbitals.

• p orbitals have 2 lobes. Plus (+) and minus (-) indicate the opposite phases of the wave function, not electrical charge.

• When lobes overlap constructively, (+ and +, or - and -) a bonding MO is formed.

• When + and - lobes overlap, waves cancel out and a node forms; antibonding MO. =>

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1 MO for 1,3-Butadiene

• Lowest energy.• All bonding

interactions.• Electrons are

delocalized over four nuclei.

=>

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2 MO for 1,3-Butadiene

• 2 bonding interactions

• 1 antibonding interaction

• A bonding MO

=>

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3* MO for 1,3-Butadiene

• Antibonding MO

• Empty at ground state

• Two nodes =>

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4* MO for 1,3-Butadiene

• All antibonding interactions.

• Highest energy.

• Vacant at ground state.

=>

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MO Energy Diagram

The average energy of electrons is lower in the conjugated compound.

=>

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Conformations of 1,3-Butadiene

• s-trans conformer is more stable than the s-cis by 2.3 kcal.

• Easily interconvert at room temperature.

HH

H

H

H

H

s-trans s-cis

H

H

H

H

HH

=>

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Allylic Cations• Carbon adjacent to C=C is allylic.

• Allylic cation is stabilized by resonance.

• Stability of 1 allylic 2 carbocation.

• Stability of 2 allylic 3 carbocation.

H2C C

H

CH2+

H2C C

H

CH2

+

=>

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1,2- and 1,4-Additionto Conjugated Dienes

• Electrophilic addition to the double bond produces the most stable intermediate.

• For conjugated dienes, the intermediate is a resonance stabilized allylic cation.

• Nucleophile adds to either carbon 2 or 4, both of which have the delocalized positive charge. =>

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Addition of HBr

H2C C

H

C

H

CH2H

+

H3C C

H

C

H

CH2+

H3C C

H

C

H

CH2+

Br_

Br_

H3C C

H

C

H

CH2

Br

H3C C

H

C

H

CH2

Br

1,2-addition product 1,4-addition product

=>

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Kinetic vs. Thermodynamic Control

Major product at 40C

Major product at -80C

=>

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Allylic Radicals

• Stabilized by resonance.

• Radical stabilities: 1 < 2 < 3 < 1 allylic.

• Substitution at the allylic position competes with addition to double bond.

• To encourage substitution, use a low concentration of reagent with light, heat, or peroxides to initiate free radical formation. =>

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Allylic Bromination

BrH

HHH

H

H

H

H

H

H

+ HBr

Br Br Br Br

H

HBrH

H

H

H

Br+ Br

=>

Br2hBr2

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Bromination Using NBS• N-Bromosuccinimide (NBS) provides a

low, constant concentration of Br2.

• NBS reacts with the HBr by-product to produce Br2 and prevent HBr addition.

=>

N Br

O

O

+ HBr N H

O

O

+ Br2

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MO’s for the Allylic System

=>

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SN2 Reactions of Allylic Halides and Tosylates

=>

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Diels-Alder Reaction• Otto Diels, Kurt Alder; Nobel prize, 1950

• Produces cyclohexene ring

• Diene + alkene or alkyne with electron-withdrawing group (dienophile)

C

C

H H

H W

C

C

H

H

W

H

=>

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Examples of Diels-Alder Reactions

=>

+

OC

OCH3

C

C

CO OCH3

CC

O

OCH3

COCH3

OC

H3C

H3C

NC

CH

CHH

+

H3C

H3C

C

C

H

C N

H

H

diene dienophile Diels-Alder adduct

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Stereochemical Requirements

• Diene must be in s-cis conformation.• Diene’s C1 and C4 p orbitals must

overlap with dienophile’s p orbitals to form new sigma bonds.

• Both sigma bonds are on same face of the diene: syn stereochemistry.

=>

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Concerted Mechanism

=>

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Endo RuleThe p orbitals of the electron-withdrawing

groups on the dienophile have a secondary overlap with the p orbitals of C2 and C3 in the diene.

=>

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RegiospecificityThe 6-membered ring product of the

Diels-Alder reaction will have electron-donating and electron-withdrawing groups 1,2 or 1,4 but not 1,3.

D

C

C

H W

H H

W

D

D C

C

H W

H H D

W

WD

not

=>

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Symmetry-Allowed Reaction

• Diene contributes electrons from its highest energy occupied orbital (HOMO).

• Dienophile receives electrons in its lowest energy unoccupied orbital (LUMO).

=>

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“Forbidden” Cycloaddition

[2 + 2] cycloaddition of two ethylenes to form cyclobutene has anti-bonding overlap of HOMO and LUMO

=>

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Photochemical Induction

Absorption of correct energy photon will promote an electron to an energy level that was previously unoccupied.

=>

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[2 + 2] Cycloaddition

Photochemically allowed, but thermally forbidden.

=>

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Ultraviolet Spectroscopy

• 200-400 nm photons excite electrons from a bonding orbital to a * antibonding orbital.

• Conjugated dienes have MO’s that are closer in energy.

• A compound that has a longer chain of conjugated double bonds absorbs light at a longer wavelength. =>

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=>

* for ethylene

and butadiene

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Obtaining a UV Spectrum

• The spectrometer measures the intensity of a reference beam through solvent only (Ir) and the intensity of a beam through a solution of the sample (Is).

• Absorbance is the log of the ratio

• Graph is absorbance vs. wavelength. =>

II

s

r

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The UV Spectrum• Usually shows broad peaks.

• Read max from the graph.

• Absorbance, A, follows Beer’s Law: A = cl where is the molar absorptivity, c is the sample concentration in moles per liter, and l is the length of the light path in centimeters.

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UV Spectrum of Isoprene

=>

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Sample UV Absorptions

=>

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Woodward-Fieser Rules

=>

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End of Chapter 15