OrganicConjSystems

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on ugate ystems,,

Ultraviolet S ectrosco


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Wave Properties of Electrons

Standing wave vibrates in fixed location. Wave function, , is a mathematical description of

s ze, s ape, an or en a on. Amplitude may be positive or negative. Nod : Am litude is zero.

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Wave Interactions

Linear combination of atomic orbitals:

between different atoms is bondformation.

on the same atom is hybridization.

n rv i n f r i l

Waves that are in phase add together.

. Waves that are out of phase cancel out.

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Sigma Bonding

A bond may be formed by s-s, p-p, s-p, or

The bonding molecular orbital (MO) is

orbitals.

than the atomic orbitals.

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Bonding MO

Formation of a -bonding MO: When the 1sorbitals of two hydrogenatoms overlap in phase with each other, they interact constructively to

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orm a on ng .


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Antibonding MO

Formation of a * antibondin MO: When two 1sorbitals overla out of

Chapter 2 6

phase, they interact destructively to form an antibonding MO.


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H2: s-sOverlap

Chapter 2 7


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Cl2: p-pOverlap

When two porbitals overlap along the line betweenthe nuclei, a bonding orbital and an antibondingorbital result.

Most of the electron densit is centered alon the linebetween the nuclei.

This linear overlap is another type of sigma bonding

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.


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Carbon: sp3 hybridization


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Methane

109.5o

Each of the four C-H bonds results from head-on s overlaof a singly occupied carbon sp3 hybrid orbital with a singlyoccupied hydrogen 1s orbital. Sigma bonds are formed by

- -

singly occupied sp3

hybrid orbital of carbon.


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Carbon: sp2 hybridization

- three hybrid sp2 orbitals that lie on the same plane at 120o

- a third p orbital that is lying perpendicular to the plane


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Carbon: sp hybridization

The carbon atoms forms: two hybrid sp orbitals and two porbitals.


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Conjugated double bondsare separated by one single

bond. Example: 1,3-pentadiene.

Isolated double bonds are

separated by two or moresin le bonds. Exam le: 1 4-pentadiene.

Cumulated double bonds areC HH

on adjacent carbons.Example: 1,2-pentadiene. H CH2CH3


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Stabilities of Dienes


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Structure of 1,3-Butadiene

Most stable conformation is planar. Single bond is shorter than 1.54 . Electrons are delocalized over molecule.


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Constructing Molecular Orbitals

mo ecu ar or ta s are t e s eways over ap o porbitals.

porbitals have 2 lobes. Plus (+) and minus (-),

not electrical charge.

When lobes overlap constructively, (+ and +, or -

- .

- ,

and a node forms; antibonding MO.


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Ethylene MOs

The combination oftwo porbitals mustg ve wo mo ecu arorbitals.

is a bonding MO.

is an anti-bondingM .


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1 MO for 1,3-Butadiene

Lowest energy. All bonding

interactions. Electrons aredelocalized over

.


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2 MO for 1,3-Butadiene

2 bondinginteractions. 1 antibonding

interaction. A bonding MO.


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* 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.

Highest energy.

state.


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MO Energy Diagram

The averageenergy of

electrons islower in theconjugated

compound.


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Conformations of 1,3-Butadiene

s-trans conformer is more stable than the s-. .

.

H

HH

HH

HH

H- H

H H

-


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Allylic Cations

Carbon adjacent to C=C is allylic. .

Stability of 1 allylic 2 carbocation. a y o a y c car oca on.

H+

H+


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1,2- and 1,4-Addition to Conjugated

enes

u

produces the most stable intermediate.

For conjugated dienes, the intermediate is a.

,of which have the delocalized positive charge.


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Addition of HBr

_Br Br

_

H H H H

H3C C C CH2

Br

H3C C C CH2

Br

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


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Kinetic vs. Thermodynamic Control

Major product

at 40C

a or pro uctat -80C


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Allylic Radicals

Stabilized by resonance.

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

Substitution at the allylic position competes witha on o ou e on .

,of reagent with light, heat, or peroxides to initiatefree radical formation.


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Bromination Using NBS

N-Bromosuccinimide (NBS) provides a low,2.

-produce Br2 and prevent HBr addition.


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MOs for the Allylic System


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SN2 Reactions of Allylic Halides


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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)

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Examples of Diels-Alder Reactions

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Stereochemical Requirements

.

Dienes C1 and C4 orbitals must overlawith dienophiles porbitals to form newsigma bonds.

Both sigma bonds are on same face of thediene: synstereochemistry.

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s-cis Conformation of the Diene

When the diene is in s-trans conformation, the endorbitals are too far a art to overla with the

orbitals of the dienophile

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Cyclopentadiene: Diels Alder Reaction


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Cyclopentadiene: Diels-Alder Reaction

Cyclopentadiene

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Concerted Mechanism

4 + 2 cycloaddition concerted cyclic movement of 6 electrons

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the 2 end p orbitals of the diene with those of the dienophile

syn Stereochemistry


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synStereochemistry

-diene and dienophile

The dienophile adds to one face of the diene, and the diene

adds to one face of the dienophile

Substituents that are on the same side of the diene ordienophile will be cis in the newly formed ring

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E d R l


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Endo Rule

-on the dienophile have a secondary overlap with.

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R i ifi it


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Regiospecificity

reaction will have electron-donating and electron-withdrawin rou s1 2 or 1 4 but not 1 3.

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Pericyclic Reactions

Diels-Alder reaction is example.

Woodward and Hoffmann predicted reaction

orbital symmetry.

MOs must overlap constructively to stabilizethe transition state.

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Symmetry-Allowed Reaction

Diene contributeselectrons from itshighest energyoccup e or a(HOMO).

Dienophile receives

lowest energyunoccu ied orbital

43(LUMO).


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Forbidden Cycloaddition

[2 + 2] cycloaddition

form cyclobutene has

-of HOMO and LUMO.

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Photochemical Induction

Absorption of correct energy photon willpromote an electron to an energy level thatwas previously unoccupied.

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[2 + 2] Cycloaddition

allowed, but

thermall forbidden.

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Ul i l S


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Ultraviolet Spectroscopy

200-400 nm photons excite electrons from a * .

in energy.

A compound that has a longer chain ofconjugated double bonds absorbs light at a

longer wavelength.

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* Transition for Ethylene and Butadiene


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Transition for Ethylene and Butadiene

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Obtaining a UV Spectrum


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Obtaining a UV Spectrum

of a reference beam through solvent only

r

solution of the sample (Is).

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sor ance s t e og o t e rat o r s

Th UV S t


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The UV Spectrum

Usually shows broad peaks.

Read max from the graph.

Absorbance, A, follows Beers Law:=

,sample concentration in moles per liter, and li h l n h f h li h h in n im r .

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Sample UV Absorptions


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Sample UV Absorptions

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Woodward-Fieser Rules


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Woodward Fieser Rules

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Problem on

Diels-Alder Cycloaddition

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SOLVED PROBLEM 15-1

Use the endo rule to predict the product of the following cycloaddition


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Use the endo rule to predict the product of the following cycloaddition.

Solution

Imagine this diene to be a substituted cyclopentadiene; the endo product will be formed.

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SOLVED PROBLEM 15-1 (continued)

Solution (continued)In the imaginary reaction, we replaced the two inside hydrogens with the rest of the cyclopentadiene ring.

Now we put them back and have the actual product.

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SOLVED PROBLEM 15-2

Predict the products of the following proposed DielsAlder reactions.


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ed ct t e p oducts o t e o ow g p oposed e s de eact o s.

o ut on

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Solution (continued)

SOLVED PROBLEM 15-2 (continued)


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Solution (continued)

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Provide the structure of the major organic product


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Provide the structure of the major organic product

n e o ow ng reac on.

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