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Frost Circles…a Great TrickFrost Circles…a Great TrickInscribe a polygon of the same number of sides as the ring to be examined such that one of the vertices is at the bottom of the ringThe relative energies of the MOs in the ring are given by where the vertices touch the circleThe MOs– below the horizontal line through the center of the ring
are bonding MOs– on the horizontal line are nonbonding MOs– above the horizontal line are antibonding MOs
ππ--MOs ofMOs of CyclobutadieneCyclobutadiene(square planar)(square planar)
AntibondingAntibonding
CycloCyclo--butadienebutadiene BondingBonding
4 4 ππ electrons; bonding orbital is filled; other 2electrons; bonding orbital is filled; other 2ππ electrons singly occupy two nonbondingelectrons singly occupy two nonbonding orbitalsorbitals
AntibondingAntibonding
BondingBonding
8 8 ππ electrons; 3 bondingelectrons; 3 bonding orbitalsorbitals are filled; 2are filled; 2nonbondingnonbonding orbitalsorbitals are each halfare each half--filledfilled
ππ--MOs ofMOs of CyclooctatetraeneCyclooctatetraene(square planar)(square planar)
CycloCyclo--octatetraeneoctatetraene
Requirements forRequirements for Aromaticity
cyclic conjugation is necessary, cyclic conjugation is necessary, but not sufficient but not sufficient
Aromaticity
notnotaromatic
notnotaromaticaromaticaromaticaromatic aromatic
Hückel'sHückel's RuleRulethe additional factor that influencesthe additional factor that influences
aromaticityaromaticity is the number of is the number of ππ electronselectrons
Hückel'sHückel's Rule for Rule for AromaticityAromaticity
To be Aromatic …a compound must :To be Aromatic …a compound must :
1.1. be Cyclicbe Cyclic
2.2. have one P orbital on each atom in the ringhave one P orbital on each atom in the ring
3.3. be planar or nearly so to give orbital overlapbe planar or nearly so to give orbital overlap
4.4. have a closed loop of 4n+2 pi electrons in the have a closed loop of 4n+2 pi electrons in the
cyclic arrangement of p cyclic arrangement of p orbitalsorbitals
ππ--MOs of BenzeneMOs of Benzene
AntibondingAntibonding
BenzeneBenzene
BondingBonding6 6 ππ electrons fill all of the bondingelectrons fill all of the bonding orbitalsorbitalsall all ππ antibonding orbitalsantibonding orbitals are emptyare empty
ππ--MOs ofMOs of CyclobutadieneCyclobutadiene(square planar)(square planar)
AntibondingAntibonding
CycloCyclo--butadienebutadiene BondingBonding
4 4 ππ electrons; bonding orbital is filled; other 2electrons; bonding orbital is filled; other 2ππ electrons singly occupy two nonbondingelectrons singly occupy two nonbonding orbitalsorbitals
ππ--MOs ofMOs of CyclooctatetraeneCyclooctatetraene(square planar)(square planar)
CycloCyclo--octatetraeneoctatetraene
AntibondingAntibonding
BondingBonding
8 8 ππ electrons; 3 bondingelectrons; 3 bonding orbitalsorbitals are filled; 2are filled; 2nonbondingnonbonding orbitalsorbitals are each halfare each half--filledfilled
ππ--Electron Requirement forElectron Requirement for AromaticityAromaticity
notnotaromaticaromatic aromaticaromatic notnot
aromaticaromatic
4 4 ππ electronselectrons 6 π electrons6 π electrons 8 8 ππ electronselectrons
[18][18]AnnuleneAnnuleneThe protons resonate at-3.0 and at 9.3δ!!!
18 18 ππ electrons satisfieselectrons satisfies Hückel'sHückel's rulerule
resonance energy = 418 kJ/molresonance energy = 418 kJ/mol
bond distances range between 137bond distances range between 137--143 pm143 pm
HH HHHH
HHHHHH
HeterocyclicHeterocyclic Aromatic CompoundsAromatic Compounds
PyridinePyridine
SS••••••••
NN
HH
•••• OO••••••••
NN••••
PyrrolePyrrole FuranFuran ThiopheneThiophene
HeterocyclicHeterocyclic Aromatic CompoundsAromatic Compoundsandand
Hückel'sHückel's Rule
6 6 ππ electrons in ringelectrons in ringlone pair on nitrogen is in anlone pair on nitrogen is in anspsp2 2 hybridized orbital;hybridized orbital;notnot part of part of π π system of ring
Rule
NN••••
system of ringPyridinePyridine
lone pair on nitrogen must be part lone pair on nitrogen must be part of ring of ring ππ system if ring is to havesystem if ring is to have6 6 ππ electronselectronslone pair lone pair must be inmust be in a a pp orbitalorbitalin order to overlap with ring in order to overlap with ring ππsystem
NN
HH
••••
PyrrolePyrrolesystem
FuranFuran
two lone pairs on oxygentwo lone pairs on oxygenone pair is in a one pair is in a pp orbital and is partorbital and is partof ring of ring ππ system; other is in an system; other is in an spsp22 hybridized orbital and is nothybridized orbital and is notpart of ring part of ring ππ system
OO••••••••
system
Homework Supplement…..Please name the heterocycle and showWhich electrons on the heteroatoms are included in the pi system.Where are the unshared pairs and how are they oriented relative to the plane of the ring??
N
SN
S
Aromatic IonsAromatic Ions
H
H
H
H
H
Radical
Anion
Cation
H
H
H
Radical ___
Cation ___
Anion ___
π e’s
CyclopentadieneCyclopentadiene
Hπ e’s
Radical ___
Cation ___
Anion ___
Let’s fill these for each case, radical, anion and cation
CyclopentadienideCyclopentadienide AnionAnion
6 6 ππ electronselectrons delocalizeddelocalizedover 5 carbonsover 5 carbonsnegative charge dispersednegative charge dispersedover 5 carbonsover 5 carbonsstabilized anion
HH HH
••••––HH HHstabilized anion
HH
Acidity ofAcidity of CyclopentadieneCyclopentadiene
HH HH
HH
HH HH
HH
HH
••••––HH++ ++
ppKKaa = 16= 16
KKaa = 10= 10--1616
CyclopentadieneCyclopentadiene is unusually is unusually acidic for a hydrocarbon.acidic for a hydrocarbon.Increased acidity is due to Increased acidity is due to stability ofstability of cyclopentadienidecyclopentadienideanion.anion.
HH HH
HH HH
Let’s Move Electrons once
HH HH
HH HH
HH
••••––
HH HH
HH HH
HH
HH HH
HH HH
HH
HH HH
HH HH
HH
HH HH
HH HH
HH
Compare Acidities ofCompare Acidities ofCyclopentadieneCyclopentadiene andand CycloheptatrieneCycloheptatriene
HH HH
HH HH
HH HH
HH HH
HHHHHH
HH HHHH HH
ppKKaa = 16= 16
KKaa = 10= 10--16
ppKKaa = 36= 36
KKaa = 10= 10--3616 36
nn = 0= 044n n +2 = 2 +2 = 2 ππ electrons !!electrons !!
CyclopropenylCyclopropenyl CationCation
HH HH
HH
++++
HH HH
HH
also written asalso written as
Cyclooctatetraene DianionCyclooctatetraene Dianion
HH HH
HHHH
HH HH
HHHH
••••
••••
––
––
HH HH
HHHH
HH HH
HHHH
22––alsoalsowritten aswritten as
nn = 2= 244nn +2 = 10+2 = 10 ππ electronselectrons
ElectrophilicElectrophilic Aromatic Aromatic SubstitutionSubstitution
Electrophilic aromatic substitution: a reaction in which a hydrogen atom of an aromatic ring is replaced by an electrophileWe study– several common types of electrophiles, – how they are generated, and – the mechanism by which they replace hydrogen is the same for all
Electrophilic Electrophilic Aromatic SubstitutionAromatic Substitution
EH
H+
E
H
H
H
EH
+
+
E+ H+E+
Please Please be sure be sure that you can do that you can do this and that it this and that it makes sense to makes sense to you!!you!!
NitrationNitrationNO2H2SO4
O3HN
+••
••Nitric acid
H O NO2 O SH O
+•• •••• ••
••
••
Nitronium ionO2
HO=N=O
O3 H••
+H
+H NO2 HSO4
+••OH
H OH N +
ChlorinationChlorination
Chlorination requires requires a Lewis acid Chlorination requires requires a Lewis acid catalyst, such as AlClcatalyst, such as AlCl33 or FeClor FeCl33
Step 1: formation of a chloronium ion
+ - +•• ••
••
•• ••
••
••
••
+••
••
••
•••• ••Cl
Cl
Cl
Cl
ClCl
ClFeCl
Cl Fe
Cl Cl FeCl 4-
chloronium ion
SulfonationSulfonation
H2 SO4+H SSO3 O3 H
Benzenesulfonic acid
S OO
O SO
O O-
H
+ H
Sulfonation Sulfonation can be reversed by Heating in Hcan be reversed by Heating in H22OO
TheThe FriedelFriedel--Crafts Reaction.. Circa 1877Crafts Reaction.. Circa 1877
CharlesCharles FriedelFriedel James CraftJames Craft
Making CMaking C--C bonds is….C bonds is….
FriedelFriedel--CraftsCrafts AlkylationAlkylation
CH2 CH3CH3CH2Cl
AlCl3
An ion pair containing
a carbocation
+
••••
••
••
•• -+
Cl Al
Cl
Cl
Cl
Cl
AlCl + AlCl 4-
R Cl
ClR R
A word about theA word about the FriedelFriedel--Crafts Crafts AlkylationAlkylation
1.1. Carbocation rearrangements are commonCarbocation rearrangements are common
AlCl 3CH3 CHCH2 Cl
CH3
+ + HClC(CH 3 ) 3
tert-ButylbenzeneIsobutyl chlorideB enzene
Role ofRole of AlClAlCl33Acts as a Lewis acid to promote ionizationActs as a Lewis acid to promote ionizationof the alkyl of the alkyl halideBhalideB
++ClCl••••
••••AlClAlCl33––
(CH(CH33))33CC ClCl ••••
••••
••••++ AlClAlCl33 (CH(CH33))33CC
++(CH(CH33))33CC ClCl
••••AlClAlCl33––
••••++
FriedelFriedel--CraftsCraftsAlkylationAlkylation
2. They are tough to stop!2. They are tough to stop!
Product is more reactive than the starting material
CH3CH+2Cl + etc.
AlCl3
FriedelFriedel--CraftsCrafts AlkylationAlkylation
2. 2. alkylationalkylation fails fails on benzene rings bearing one or on benzene rings bearing one or more strongly electronmore strongly electron--withdrawing groupswithdrawing groups
CH
O O
COR
O OO
CR CNH2COH
NR3+C NSO3 H NO2
CF3 CCl 3
FriedelFriedel--CraftsCrafts AcylationAcylation
OCR
ORCX AlX 3+ +H HX
An acylbenzene
An ion pair containing
an acylium ion
O
AlCl 4-R-C +
+••••Cl
O
R-C-
Al
Cl
Cl••
•• •• +ClR-C
O ClCl
Cl
Al-Cl
FriedelFriedel--CraftsCrafts AcylationAcylationAnAn acyliumacylium ion is a resonance hybrid of two major ion is a resonance hybrid of two major contributing structurescontributing structures
– F-C acylations are free of ONE major limitation of F-Calkylations; acylium ions do not rearrange
– They still do not work on deactivated Rings– They stop after one substitution
RC+
O O+
RC
HydrogenolysisHydrogenolysis
OH2CH3COCl
AlCl3 Pd/C
There are two nice tricks hidden hereThere are two nice tricks hidden here
Please be sure to remember this reaction!!!Please be sure to remember this reaction!!!
DiDi-- andand PolysubstitutionPolysubstitution
Existing groups on a benzene ring influence further substitution in both orientationorientation and rate
Orientation:– certain substituents direct preferentially to ortho & para
positions; others direct preferentially to meta positions– substituents are classified as either
orthoortho--parapara directing ordirecting ormeta directingmeta directing
DiDi-- andand PolysubstitutionPolysubstitution
Rate:Rate:–– certain substituents cause the rate of a second certain substituents cause the rate of a second
substitution to be greater than that for benzene substitution to be greater than that for benzene itself; others cause the rate to be loweritself; others cause the rate to be lower
–– substituents are classified assubstituents are classified as•• activatingactivating toward further substitution, ortoward further substitution, or•• deactivatingdeactivating
DiDi-- andand PolysubstitutionPolysubstitution
-OCH3 is ortho-para directing and activating
p-Bromo-anisole (96%)
o-Bromo-anisole
(4%)
Anisole
++
OCH 3 OCH 3 OCH 3Br
Br
Br 2CH 3 CO 2 H
HBr
DiDi-- andand PolysubstitutionPolysubstitution-NO2 is meta directing and deactivating!
m-Dinitro-benzene (93%)
Nitro-benzene
+
++
o-Dinitro-benzene
p-Dinitro-benzene
Less than 7% combined
NO 2
NO 2
NO 2NO 2
NO 2
NO 2NO 2HNO 3
H 2 SO 4
Methyl GroupMethyl Group
Toluene undergoes nitration 1000 times faster than benzene.A methyl group is an activatingactivating substituent.
CHCH33
Trifluoromethyl GroupTrifluoromethyl Group
Trifluoromethylbenzene undergoes nitration 40,000 times more slowly than benzene .The trifluoromethyl group is adeactivatingdeactivating substituent.
CFCF33
.
Relative rates of Relative rates of NitrationNitration
ClOH H NO2
1000 1.0 0.033 6x10-8
ReactivityReactivity
Real FastReal Fast
Pretty fastPretty fast
KindaKinda slowslow
Pretty slowPretty slow
SlowSlow
Real SlowReal Slow
Effect onEffect on RegioselectivityRegioselectivity
•• OrthoOrtho--parapara directorsdirectors direct an incomingelectrophile to positions ortho and/or para to themselves.
• Meta directorsMeta directors direct an incoming electrophile to positions meta to themselves.
• All meta directorsAll meta directors are deactivating
•• AllAll orthoortho--parapara directors are activating except except halogen
Theory of Directing EffectsTheory of Directing Effects
• So…what’s going on here????
• The rate of EAS is limited by the slowest step in the mechanism…duh
• For EAS, the rate-limiting step is attack of E+ on the aromatic ring to form a resonance-stabilized cation intermediate
• The more stable this cation intermediate, the faster the rate-limiting step and the faster the overall reaction
DiDi-- andand PolysubstitutionPolysubstitution
Alkyl groups, phenyl groups, and all groups in which the atom bonded to the ring has an unshared pair of electrons are ortho-paradirecting. All other groups are meta directing.
All ortho-para directing groups except the halogens are activating toward further substitution. The halogens are weakly deactivating
DiDi-- andand PolysubstitutionPolysubstitutionalkyl groups, phenyl groups, and all groups in which the atom bonded to the ring has an unshared pair of electrons are ortho-para directing. All other groups are meta directing.All ortho-para directing groups except the halogens are activating toward further substitution. The halogens are weakly deactivating
Adding a Second Adding a Second SubstiuentSubstiuent
Methoxy Methoxy is is therefore an “ois is therefore an “o--p director”p director”
Adding a Second Adding a Second SubstiuentSubstiuent
Nitro is therefore a “meta director”Nitro is therefore a “meta director”
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