Organic Chemistry

1. Chapter 14Chapter 14
Delocalized Pi SystemsDelocalized Pi Systems
TrigonalTrigonal
TheThe ππ bond is ebond is e--
rich:rich: EE++
attackattack,,
R∙R∙
addadd))
The lobes of theThe lobes of the pp--
orbitals:orbitals:
PerpendicularPerpendicular to theto the
sigma frame andsigma frame and
Recall the double bondRecall the double bond

2. 2-Propenyl (Allyl)2-Propenyl (Allyl)
Question:Question: What about adding aWhat about adding a thirdthird
pp-orbital adjacent to the double bond?-orbital adjacent to the double bond?
Is there something special?Is there something special?
Or: Is there any specialOr: Is there any special
reactivity at the carbonsreactivity at the carbons
adjacent to a double bond?adjacent to a double bond?
H

3. a.a.
b.b.
c.c.
SSNN1 reactivity of allylic carbon like that1 reactivity of allylic carbon like that
of Rof RsecsecX,X, even though it is primary!even though it is primary!
ppKKaa ~ 40:~ 40: Acidic!Acidic! 5050
SSNN11
87 kcal mol87 kcal mol-1-1
:: Weak!Weak! 101 kcal mol101 kcal mol-1-1
Replacing one of the hydrogens in ethene withReplacing one of the hydrogens in ethene with
anotheranother spsp22
-hybridized carbon gives an-hybridized carbon gives an allylicallylic
system.system.
Allylic positionAllylic position
Observations:Observations:
H
L
H
B B
H
H

4. Clearly: AllylicClearly: Allylic ·· areare
stabilized.stabilized.--
++
Short notation: Dotted linesShort notation: Dotted lines
Why?Why? Resonance!Resonance!
CH2 H2C
CH2 H2C
CH2 H2C

5. MO Picture of 2-PropenylMO Picture of 2-Propenyl
(Allyl)(Allyl)
33 ppOs 3 MOsOs 3 MOs
RecallRecall: Bonds made by overlap: Bonds made by overlap
HH .. HH .. H HH H++
++HH ++ ++HH ++HH++HH
In phaseIn phase
++HH ++ --HH --HH++HH
Out ofOut of
phasephase
BondingBonding
Anti-Anti-
bondingbonding
NodeNode
Sign of the wave function,Sign of the wave function,
Not charge!Not charge!
EE

6. EE
ππ
σσ
σσ**
ππ**
CH2 CH2
What happens to this pictureWhat happens to this picture
when we interact with anotherwhen we interact with another
pp orbital?orbital?

7. 1)1) Interaction with theInteraction with the ππ
bonding orbital causesbonding orbital causes
energy splitting: theenergy splitting: the pp
orbital level moves up andorbital level moves up and
thethe ππ bonding level movesbonding level moves
down.down.
2)2) Interaction with theInteraction with the ππ
antibonding orbital causesantibonding orbital causes
the energy level of thethe energy level of the pp
orbital to move down toorbital to move down to
where it was originally andwhere it was originally and
the energy level of thethe energy level of the ππ
antibonding orbital to moveantibonding orbital to move
up. The two effects on theup. The two effects on the pp
orbital cancel each otherorbital cancel each other
out and the twoout and the two ππ orbitalsorbitals
are pushed apart.are pushed apart.
Interactions of a singly occupiedInteractions of a singly occupied pp--
orbital with each of theorbital with each of the ππ molecularmolecular
orbitalsorbitals
pp
EE
ππ
ππ**
00
ππ
ππ** upup
downdown
unchangedunchanged
EtheneEthene AllylAllyl pp OrbitalOrbital
NonbondingNonbonding
MOMO

8. H2C
H
C
CH2
pp
EE
# of e# of e
dependsdepends
onon ++,,∙∙,,--
ππ
ππ**
00
Resulting picture:Resulting picture:
AndAnd
locationlocation
atat
terminitermini

9. Reactivity of AllylicReactivity of Allylic
PositionPositionA. Radical HalogenationA. Radical Halogenation
CHCH22 CHCHCHCH33 ++ BrBr22 CHCH22 CH CHCH CH22BrBr ++ HBrHBr
Faster than addition!Faster than addition!Low conc.Low conc.
Mech.Mech.
1.1.
2.2.
BrBr22 2 Br2 Br
..hhυυ oror ΔΔ
Initiation:Initiation:
Propagation:Propagation: CHCH22 CHCHCHCH33 ++ BrBr ..
CHCH22 CH CHCH CH22
..
CHCH22 CH CHCH CH22
..
++ HBrHBr
CHCH22 CH CHCH CH22
..
CHCH22 CH CHCH CH22
..
BrBr22
CHCH22 CH CHCH CH22Br +Br + BrBr ..

10. Termination:Termination:
3.3.
BrBr .. ++
..CHCH22CH CHCH CH22 BrCHBrCH22CH CHCH CH22
BrBr .. BrBr ..++ BrBr22
CHCH22CH CHCH CH22
..22 CHCH22CH CHCH CH22CHCH22 CHCHCHCH22
Anything that traps radicals, including the “dirt”Anything that traps radicals, including the “dirt”
on the walls of the flask, contributes toon the walls of the flask, contributes to
termination.termination.

11. TracesTraces
LowLow
conc.conc.

12. Propene generates a symmetrical allylic radical,Propene generates a symmetrical allylic radical,
but this is not always the case. Forbut this is not always the case. For
unsymmetrical systems: mixtures. Ratiosunsymmetrical systems: mixtures. Ratios
depend on % radical character on each carbondepend on % radical character on each carbon
and TSs leading to products.and TSs leading to products.
Br2
Br· +

13. B. SB. SNN1: The Allylic Cation is Stabilized1: The Allylic Cation is Stabilized
CHCH33CH CHCHCH CHCH22ClCl
HH22OO
-- ClCl
--
CHCH33CH CH CHCH CH CH22
++
CHCH33CH CH CHCH CH CH22
++
CHCH33CH CHCHCH CHCH22OHOH CHCH33CHCH CHCHCH CH22
OHOH
fastfastslowslow
ThermodynamicThermodynamic
productproduct
Kinetic productKinetic product
ThermodynamicThermodynamic
KineticKinetic
CationCation
EE

14. C. SC. SNN2: The Allylic TS is Stabilized2: The Allylic TS is Stabilized
CHCH33CH CHCHCH CHCH22ClCl NaNaII CHCH33CH CHCH CH CC
ClCl
II
..
++
‡‡
CHCH33CH CHCHCH CHCH22II ++ ClCl
--
100 times faster100 times faster
thanthan Cl
....
δδ--
δδ--

15. D. Allylic OrganometallicsD. Allylic Organometallics
Li
CH3
CH3
++ RXRX
Allylic Grignard reagents:Allylic Grignard reagents:
Br MgMg++
X
Neutral analogs of allylic anions:Neutral analogs of allylic anions:
NMR shielded!NMR shielded!
X = OR, SR, NRX = OR, SR, NR22
H2C C
CH3
CH2
Li
H2C C
CH3
CH2R
MgBr
X

16. Conjugated DoubleConjugated Double
BondsBonds
What aboutWhat about
Nomenclature: Cis/trans E/Z reviewNomenclature: Cis/trans E/Z review
C C C CC C C C ??

17. Stability:Stability: Heats of hydrogenationHeats of hydrogenation
CHCH33(CH(CH22))33CH CHCH CH22 HH22++
1,5-Hexadiene1,5-Hexadiene
1,3-Butadiene1,3-Butadiene
But:But:
Resonance energy of butadiene ~ 3.5Resonance energy of butadiene ~ 3.5
ΔΔH˚H˚
-30.3-30.3
-60.5-60.5
-57.1-57.1
2 H2 H22++
2 H2 H22++

18. StructureStructure
Short! (1.54 Å normal)

19. OrbitalsOrbitals

20. NMRNMR
H H
H
H
JJ = 10= 10
δδ = 5.06= 5.06
δδ = 6.27= 6.27 (effect of 2(effect of 2ndnd
double bond)double bond)
JJ ~1-2~1-2
δδ = 5.16= 5.16
JJtranstrans = 17= 17
JJciscis = 10= 10
H H
H
H
137.2137.2
116.6116.6

21. ConjugationConjugation stabilizes thermodynamically, but it also increasesstabilizes thermodynamically, but it also increases
reactivity, for example inreactivity, for example in electrophilic additionselectrophilic additions (review Chapter 12).(review Chapter 12).
1,2-Addition1,2-Addition
(kinetic)(kinetic)
Intermediate cation is also stabilizedIntermediate cation is also stabilized
++ HClHCl
ClCl
-- 1,4-Addition1,4-Addition
(thermodynamic)(thermodynamic)
CH3
ClCl CH3
ClCl
CH3
CH3
ClCl
--
+ cis+ cis
HClAddnHClAddn

22. Kinetic vs ThermodynamicKinetic vs Thermodynamic
ControlControl

24. Extended ConjugationExtended Conjugation
Thermo 1Thermo 1
Thermo 2Thermo 2
KineticKinetic
++ HHBrBr
BrBr--
Three productsThree products
CH3
CH3
CH3
Quite reactiveQuite reactive
The more double bonds, the more sensitiveThe more double bonds, the more sensitive
(reactive) is the polyene.(reactive) is the polyene.

25. Cyclohexatriene is Special -Cyclohexatriene is Special -
BenzeneBenzene
Cyclic arrayCyclic array ofof six electronssix electrons has special stability,has special stability,
calledcalled aromaticityaromaticity (Chapter 15).(Chapter 15).
Benzene is relatively inert to HBenzene is relatively inert to H22-cat, electrophiles,-cat, electrophiles,
oxidants, in comparison with hexatriene.oxidants, in comparison with hexatriene.

26. Extended Conjugation in NaturalExtended Conjugation in Natural
and Unnatural Productsand Unnatural Products
Orange color of carrotsOrange color of carrots
BiologicalBiological
degradationdegradation
VisionVision

27. Organic ConductorsOrganic Conductors
Heeger, MacDiarmid, Shirakawa, Nobel Prize 2000

28. Light emitting diodes (LEDs)Light emitting diodes (LEDs)

29. Conjugated Systems UndergoConjugated Systems Undergo
Special Transformations:Special Transformations:
Pericyclic ReactionsPericyclic Reactions
The conjugatedThe conjugated ππ system can react as a unit,system can react as a unit,
involvinginvolving both endsboth ends. For example,. For example,
1. Cycloadditions:1. Cycloadditions: TheThe Diels-Alder reaction,Diels-Alder reaction,
a [4+2] cycloadditiona [4+2] cycloaddition
++
ΔΔ
44ππ-4C-4C
DieneDiene
22ππ-2C-2C
DienophileDienophile
20%20%
CycloadductCycloadduct
HC
HC
CH2
CH2
H2
C
C
H2
HC
CH2
HC
CH2
CH2
CH2

30. Diels-Alder reactions work best when we pair anDiels-Alder reactions work best when we pair an
e-riche-rich (push)(push) dienediene with anwith an e-poore-poor (pull)(pull) dienophiledienophile
e-poor dienee-poor diene with anwith an e-rich dienophilee-rich dienophile
or anor an
The Diels-Alder Reaction isThe Diels-Alder Reaction is
ChemoselectiveChemoselective
Depends on substituents:Depends on substituents:
e-Donating:e-Donating: Alkyl, alkoxy, alkylthioAlkyl, alkoxy, alkylthio
CHCH33,, CHCH33O,O, CHCH33CHCH22SS
HyperconjugationHyperconjugation
ResonanceResonance
Even though O is e-negative (inductive effect), resonance wins out.Even though O is e-negative (inductive effect), resonance wins out.
OCH3 OCH3 OCH3

31. e-Withdrawing:e-Withdrawing: CFCF33, CR, C N, NO, CR, C N, NO22
OO
Resonance:Resonance:
Example:Example:
Inductive:Inductive:
Does not compete with dienophileDoes not compete with dienophile
90%90%
++
ΔΔ
C
F
F
F
H2C C
CR
H
O
H2C C
CR
H
O
H2C C
CR
H
O
CR
O
CR
O

32. ++
ΔΔ
Orbital description:Orbital description:
spsp33
Mechanism: ConcertedMechanism: Concerted
spsp22
spsp22

33. ConsequencesConsequences
Stereospecific: Retention ofStereospecific: Retention of
Dienophile StereochemistryDienophile Stereochemistry
++
++
CisCis CisCis
TransTrans TransTrans
80%80%
90%90%
COCH3
CH3
O
COCH3
O
H3C
COCH3
O
CH3
COCH3
O
CH3

34. Retention of Diene StereochemistryRetention of Diene Stereochemistry
OCH3
OCH3
NC CN
CNNC
OCH3
OCH3
++
++
NC CN
CNNC
When both partners are stereochemically defined:When both partners are stereochemically defined:
“Endo rule” determines their approach.“Endo rule” determines their approach.
CN
CN
CN
CN
OCH3
OCH3
CN
CN
CN
CN
OCH3
OCH3

35. Endo/Exo AdditionEndo/Exo Addition
Substituents point
away from diene
Substituents
point toward
diene
DADA

36. o
o
i
i
A
A
Generally:Generally: ++
ExampleExample::
EndoEndo
o i
io
A
A
CH3
H3C NC H
HNC
CH3
H3C CN
CN
WalbaWalba LipshutzLipshutz
DylanDylan SegoviaSegovia

37. Alkynes as DienophilesAlkynes as Dienophiles
Generates 1,4-cyclohexadienesGenerates 1,4-cyclohexadienes
CO2CH3
CO2CH3
CO2CH3
CO2CH3
++
Can reactCan react
againagain
75%75%
CO2CH3
CO2CH3

38. ΔΔ
ΔΔ
hhυυ
ExothermicExothermic
(ring strain(ring strain
released)released) Light driven:Light driven: Can beat thermodynamicsCan beat thermodynamics..
Wavelength dependent (can go either way).Wavelength dependent (can go either way).ΔΔH °H ° = -9.7 kcal mol= -9.7 kcal mol-1-1
ExothermicExothermic
(C C better than C C, unless ring strain present)(C C better than C C, unless ring strain present)
ΔΔH °H ° = -14.7 kcal mol= -14.7 kcal mol-1-1
hhυυ
2. Electrocyclic Reactions:2. Electrocyclic Reactions:
Intramolecular ring closure and openingsIntramolecular ring closure and openings

39. Electrocyclic ReactionsElectrocyclic Reactions
are Stereospecificare Stereospecific
CH3
CH3
CH3
CH3
ΔΔ
ciscis–3,4-Dimethylcyclobutene–3,4-Dimethylcyclobutene
TransTrans
Only!Only!
Only!Only!
cis,transcis,trans–2,4-Hexadiene–2,4-Hexadiene
Trans,transTrans,trans
ΔΔ

40. Movement ofMovement of
SubstituentsSubstituents
ConrotatoryConrotatory
Conrotatory:Conrotatory:
They rotate in theThey rotate in the same directionsame direction
ΔΔ
H
CH3
H
CH3
H
CH3
CH3
H

41. ConrotatoryConrotatory
(counterclockwise)(counterclockwise)
Clockwise conrotation in principle possible butClockwise conrotation in principle possible but sterically prohibitedsterically prohibited::
ΔΔ
ΔΔ
CH3
H
H
CH3
CH3
H
H3C
H
H
CH3
H
CH3
CH3
H
H
CH3

42. hhυυ
disdis
hhυυ
disdis
Fascinatingly, hFascinatingly, hυυ goesgoes disrotatorydisrotatory
(rotation in(rotation in opposite directionsopposite directions))
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3

43. ΔΔ = dis= dis
Even more startling: The hexatriene/cyclohexadieneEven more startling: The hexatriene/cyclohexadiene
interconversion is alsointerconversion is also stereospecific,stereospecific, but follows thebut follows the
oppositeopposite rules of sense of rotation, compared torules of sense of rotation, compared to
butadiene/cyclobutene system:butadiene/cyclobutene system:

44. hhυυ = con= con

45. Orbital Symmetry: An inkling of how this might go……Orbital Symmetry: An inkling of how this might go……
Conrotatory
Disrotatory
Controls thermalControls thermal
closureclosure
Controls photo-Controls photo-
chemical closurechemical closure
hhνν-provides e to-provides e to ππ33
The orbital signs at theThe orbital signs at the
termini alternate with #termini alternate with #
of double bonds.of double bonds.

46. ElectronicElectronic
SpectroscopySpectroscopy
(Ultraviolet-Visible or(Ultraviolet-Visible or
UV)UV)
White (sun) light is composedWhite (sun) light is composed
of the visible spectrumof the visible spectrum
RememberRemember
spectroscopy:spectroscopy:
EE
Excited stateExcited state
Ground stateGround state
ΔΔEE == hhυυ == hhcc//λλ

47. UV-Vis spectroscopy requires much higher energy thanUV-Vis spectroscopy requires much higher energy than
NMR (kcals vs calories), does not need external “condition”NMR (kcals vs calories), does not need external “condition”
(magnet), built into molecule: electronic excitation from(magnet), built into molecule: electronic excitation from
bonding to antibonding levels, particularly easy forbonding to antibonding levels, particularly easy for ππ
systems, because occupiedsystems, because occupiedunoccupiedunoccupied ΔΔEE relatively small.relatively small.
NoNo ππ bond left!bond left!
Light causesLight causes
cis-transcis-trans
isomerization,isomerization,
radicalradical
reactionsreactions
SimpleSimple ππ bond, as in ethene:bond, as in ethene:

48. Spectrum of EtheneSpectrum of Ethene
Quoted asQuoted as λλmaxmax
Broad, because of rotational andBroad, because of rotational and
vibrational states. Electronicvibrational states. Electronic
spectroscopy is fast, nospectroscopy is fast, no
“averaging”“averaging”AA
171 nm171 nm
WavelengthWavelength λλ (given in(given in nmnm, units of 10, units of 10-9-9
m; not in frequencym; not in frequency
υυ = c/= c/λλ,, as we did in NMR, whereas we did in NMR, where λλ ~ 100 mm to 1m!)~ 100 mm to 1m!)

49. UV spectroscopy below 200 nm requires vacuum,UV spectroscopy below 200 nm requires vacuum,
because air absorbs. Normally (in atmosphere) one scansbecause air absorbs. Normally (in atmosphere) one scans
200-400 (UV), 400-800 nm (visible). This allows lower200-400 (UV), 400-800 nm (visible). This allows lower
energy tranisitons to be recorded, e.g. 1,3-butadiene:energy tranisitons to be recorded, e.g. 1,3-butadiene:
εε == AA//cc
EE
RelativelyRelatively
lowlow
energyenergy
Shoulder, sh
Peak heights are reported asPeak heights are reported as
εε :: Extinction coefficientExtinction coefficient,,
which iswhich is absorbanceabsorbance
normalized bynormalized by concentrationconcentration::
λmax
λλmaxmax = 222.5 nm (εε == 10,800)

50. AbsorptionAbsorption
in thein the
visiblevisible
450 nm450 nm orangeorange--redred
550 nm550 nm violetviolet
650 nm650 nm blueblue--greengreen
Color ofColor of
substancesubstance
Visible Absorption:Visible Absorption:
ColorColor
Light enters the prism from the top
right, and is refracted by the glass.
The violet is bent more than the
yellow and red, so the colors separate.
NewtonNewton

51. In extendedIn extended ππ systems many transitions aresystems many transitions are
possible, giving rise to more complex and notpossible, giving rise to more complex and not
readily interpretable spectra, but HOMO-LUMOreadily interpretable spectra, but HOMO-LUMO
gap gets smaller: Longest wavelength absorptiongap gets smaller: Longest wavelength absorption
is indicative of theis indicative of the extentextent of conjugation, e.g.of conjugation, e.g.
CH3
λλ max = 271 nmmax = 271 nm
ConjugatedConjugated
λλ max = 217 nmmax = 217 nm
UnconjugatedUnconjugated
Greater conjugation:Greater conjugation:
Smaller HOMO/LUMO gapSmaller HOMO/LUMO gap