conjugated systems.ppt

20-20-11

Chapter 20 Chapter 20 Conjugated Conjugated

SystemsSystems

20-20-22

Conjugated Dienes Heats of HydrogentaionConjugated Dienes Heats of Hydrogentaion

• From heats of hydrogenation, we can compare relative stabilities of conjugated and unconjugated dienes.

H0

-237 (-56.5)1,3-Butadiene

-126 (-30.1)

-127 (-30.3)

kJ (kcal)/molName

1-Pentene

1-Butene

trans-1,3-Pentadiene

1,4-Pentadiene

trans-2-Butene -115 (-27.6)

cis-2-Butene -120 (-28.6)

-226 (-54.1)

-254 (-60.8)

StructuralFormula

Longer chain has little

effect.Steric Effects

Number of

substituents.

Conjugation

stabilizes.

20-20-33

Conjugated Dienes ButadieneConjugated Dienes Butadiene

• Conjugation of the double bonds in 1,3-butadiene gives an extra stability of approximately 17 kJ (4.1 kcal)/mol .

2H2+ catalyst H0= 2(-127 kJ/mol)

= -254 kJ/mol)2 2

2H2+ H0 = -237 kJ/molcatalyst

If double bonds independent:

2H2+ catalyst H0= 2(-127 kJ/mol)

= -254 kJ/mol)2 2

2H2+ H0 = -237 kJ/molcatalyst

Experimental data does not agree: Conjugation is important and stabilizing.

20-20-44

Conjugated Dienes ButadieneConjugated Dienes Butadiene

Conjugation of double bonds in butadiene gives the molecule an additional stability of approximately 17 kJ/mol.

20-20-55

Conjugated SystemsConjugated Systems

• Systems containing conjugated double bonds, not just those of dienes, are more stable than those containing unconjugated double bonds.

3-Cyclohexenone(less stable)

2-Cyclohexenone(more stable)

O O

20-20-66

Structure of Butadiene MOsStructure of Butadiene MOs

Combination of four parallel 2p atomic orbitals gives two -bonding MOs (this screen) and two -antibonding MOs (the next screen).

20-20-77

Structure of Butadiene MOsStructure of Butadiene MOs

the two -antibonding MOs of butadiene (higher in energy).

20-20-88

How do we form the orbitals of the pi system…How do we form the orbitals of the pi system…

First count up how many p orbitals contribute to the pi system. We will get the same number of pi molecular orbitals.

Three overlapping p orbitals. We will get three molecular orbitals.

If atomic orbitals overlap with each other they are bonding, nonbonding or antibonding

Anti-bonding, destabilizing.Higher Energy

pi type anti-bond sigma type anti-bonding

If atoms are directly attached to each other the interactions is strongly bonding or antibonding. Bonding, stabilizing the system. Lower energy.

But now a particular, simple case: distant atomic orbitals, on atoms not directly attached to each other. Their interaction is weak and does not affect the energy of the system. Non bonding

non-bonded

pi type bond sigma type bonding

or

or

or or

Molecular orbitals are combinations of atomic orbitals.

They may be bonding, antibonding or nonbonding molecular orbitals depending on how the atomic orbitals in them interact.

All bonding interactions.

Only one weak, antibonding (non-bonding) interaction.

Two antibonding interactions.

Example: Allylic radical

Allylic Radical: Molecular Orbital vs ResonanceAllylic Radical: Molecular Orbital vs Resonance

Note that the odd electron is located on the

terminal carbons.

Molecular Orbital. We have three pi electrons (two in the pi bond and the unpaired electron). Put them into the molecular orbitals.

Resonance ResultAgain the odd, unpaired electron is only on the terminal carbon atoms.

But how do we construct the molecular orbitals of the pi system? How do we know what the molecular orbitals look like?

Key Ideas:

For our linear pi systems different molecular orbitals are formed by introducing additional antibonding interactions. Lowest energy orbital has no antibonding,

next higher has one, etc.

0 antibonding interactions

1 weak antibonding Interaction, “non-bonding”

2 antibonding interactions

Antibonding interactions are symmetrically placed.

This would be wrong.

Another example: hexa-1,3,5-triene

Three pi bonds, six pi electrons.Each atom is sp2 hybridized.

Have to form bonding and antibonding combinations of the atomic orbitals to get the pi molecular orbitals.

Expect six molecular orbitals.

# molecular orbitals = # atomic orbitalsStart with all the orbitals bonding and create additional orbitals. The number of antibonding interactions increases as we generate a new higher energy molecular orbital.

20-20-1414

1,2- and 1,4-Addition1,2- and 1,4-Addition

Addition of one mol of HBr to butadiene at -78°C gives a mixture of two constitutional isomers.

• We account for these products by the following two-step mechanism.

1-Bromo-2-butene10%

(1,4-addition)

-78°C

+

+

3-Bromo-1-butene90%

(1,2-addition)

CH2=CH-CH=CH2 HBr

CH2=CH-CH-CH2 CH2-CH=CH-CH2

1,3-ButadieneH HBr Br

20-20-1515


• The key intermediate is a resonance-stabilized allylic carbocation.

CH2=CH-CH=CH2 H-Br

CH2=CH-CH-CH2 CH2-CH=CH-CH2

+

+ +

CH2=CH-CH-CH2

Br H BrCH2-CH=CH-CH2

Br Br

(1,4-Addition)(1,2-Addition)

_ _

H H

H

20-20-1616


Addition of one mole of Br2 to butadiene at -15°C also gives a mixture of two constitutional isomers.

• We account for the formation of these 1,2- and 1,4-addition products by a similar mechanism.

-15°C

3,4-Dibromo-1-butene(54%)

(1,2-addition)

1,4-Dibromo-2-butene(46%)

(1,4-addition)

+

+

1,3-Butadiene

CH2=CH-CH=CH2 Br2

CH2-CH=CH-CH2CH2-CH-CH=CH2

Br Br Br Br

20-20-1717

Experimental InformationExperimental Information

• For addition of HBr at -78°C and Br2 at -15°C, the 1,2-addition products predominate; at higher temperatures (40° to 60°C), the 1,4-addition products predominate.

• If the products of the low temperature addition are warmed to the higher temperature, the product composition becomes identical to the higher temperature distribution. The same result can be accomplished using a Lewis acid catalyst, such as FeBr3 or ZnBr2.

• If either pure 1,2- or pure 1,4- addition product is dissolved in an inert solvent at the higher temperature and a Lewis acid catalyst added, an equilibrium mixture of 1,2- and 1,4-product forms. The same equilibrium mixture is obtained regardless of which isomer is used as the starting material.

20-20-1818


We interpret these results using the concepts of kinetic and thermodynamic control of reactions.

Kinetic control:Kinetic control: The distribution of products is determined by their relative rates of formation.• In addition of HBr and Br2 to a conjugated diene, 1,2-

addition occurs faster than 1,4-addition.

CH2=CH-CH-CH3 CH2-CH=CH-CH3a 2° allylic carbocation(greater contribution)

a 1° allylic carbocation(lesser contribution)

++

20-20-1919


Thermodynamic control:Thermodynamic control: The distribution of products is determined by their relative stabilities.• In addition of HBr and Br2 to a butadiene, the 1,4-

addition product is more stable than the 1,2-addition product.

BrCH2C C

H

H CH2Br

BrCH2CHCH=CH2

Br

3,4-Dibromo-1-butene (less stable alkene)

+

(E)-1,4-Dibromo-2-butene (more stable alkene)

20-20-2020


Kinetic vs thermodynamic control. A plot of Gibbs free energy versus reaction coordinate for Step 2 of addition of HBr to butadiene.

20-20-2222

UV-Visible SpectroscopyUV-Visible Spectroscopy

Absorption of radiation in these regions give us information about conjugation of carbon-carbon and carbon-oxygen double bonds and their substitution.

Region ofSpectrum

Wavelength (nm)

kcal/mol

near ultraviolet

visible

200-400

400-700

71.5 - 143

40.9 - 71.5

Energy kJ/mol

299-598

171-299

20-20-2323


• Typically, UV-visible spectra consist of one or a small number of broad absorptions.

20-20-2424


Beer-Lambert law: The relationship between absorbance, concentration, and length of the sample cell (cuvette):

• A = absorbanceabsorbance (unitless): A measure of the extent to which a compound absorbs radiation of a particular wavelength.

• = molar absorptivitymolar absorptivity (M-1cm-1): A characteristic property of a compound; values range from zero to 106 M-1cm-1.

• I = length of the sample tube (cm)

Beer-Lambert Law: A = c l

IIoAbsorbance (A) = log

20-20-2525


• The visible spectrum of -carotene (the orange pigment in carrots) dissolved in hexane shows intense absorption maxima at 463 nm and 494 nm, both in the blue-green region.

max 463 (log 5.10); 494 (log 4.77)

-carotene

20-20-2626


• A to * transition in excitation of ethylene.

20-20-2727


• A to * transition in excitation of 1,3-butadiene

20-20-2828


• Wavelengths and energies required for to * transitions of ethylene and three conjugated polyenes

724 (173)

552 (132)

448 (107)

385 (92)290

268

217

165

maxStructural FormulaName

(3E,5E)-1,3,5,7-Octatetraene

(3E)-1,3,5-Hexatriene

1,3-Butadiene

Ethylene

(nm)Energy

[kJ (kcal)/mol]

20-20-2929


Absorption of UV-Vis radiation results in promotion of electrons from a lower-energy, occupied MO to a higher-energy,unoccupied MO.• The energy of this radiation is sufficient to promote

electrons in a pi- bonding () MO to a pi-antibonding (*) MO.

• Electrons in sigma bonding MOs are lower in energy and the UV radiation energy is no longer sufficient to promote the electrons to the empty anti-bonding MOs.

• Following are three examples of conjugated systems.

1,3-Butadiene 3-Buten-2-one Benzaldehyde

O H

O

20-20-3030


UV-Visible spectroscopy of carbonyls.• Simple aldehydes and ketones show only weak

absorption in the UV due to an n to * electronic transition of the carbonyl group.

• If the carbonyl group is conjugated with one or more carbon-carbon double bonds, intense absorption occurs due to a to * transition.

O O O

2-Pentanone 3-Penten-2-one Acetophenonemax 180 nm ( 900) max 224 nm ( 12,590) max 246 nm ( 9,800)

20-20-3131

Diels Alder Reaction/Symmetry Controlled Reactions

Quick Review of formation of chemical bond.

HO- + H+ H - O - H

Electron donor

Electron acceptor

Note the overlap of the hybrid (donor) and the s orbital which allows bond formation.

HO- + H+ H - O H

For this arrangement there is no overlap. No donation of electrons; no bond formation.

20-20-3232

Diels Alder Reaction of butadiene and ethylene to yield cyclohexene.We will analyze in terms of the pi electrons of the two systems interacting. The pi electrons from the highest occupied pi orbital of one molecule will donate into an lowest energy pi empty of the other. Works in both directions: A donates into B, B donates into A.

new bonds

A B

A

B

HOMOdonor

HOMOdonor

LUMOacceptor

LUMOacceptor

B HOMO donates into A LUMO

A HOMO donates into B LUMO Note

the overlap leading to bond formation

Note the overlap leading to bond formation

20-20-3333

Try it in another reaction: ethylene + ethylene cyclobutane new bonds

A B

A B

LUMO

HOMO

LUMO

HOMO

Equal bonding and antibonding interaction, no overlap, no bond formation, no reaction

conjugated systems.ppt

Documents