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CHEM 357
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Template(14b:(Aroma0city(
Chapter(4(of(Textbook(
Chapter(11(of(E:textbook(
Benzene(
• First(discovered(by(Michael(Faraday(in(1825(
((((by(compressed(illumina0ng(gas.(
• Structure(was(not(determined(un0l(1865(by(Kekulé(“in(a(
dream”((according(to(most)(
• Reasoned(that(Benzene(does(not(undergo(alkene(addi0on(reac0ons(is(because(of(rapid(interconversion(between(
single(&(double(bonds.(
The$Stability$of$Benzene$Benzene(is(the(best(and(most(familiar(example(of(a(substance(
that(possesses(”_____________”(or(”____________”((Aroma5city(–((
(
How(can(we(measure(this(“stability”?:(
(
(
(
Heat$of$hydrogena5on:!!compare(experimental(value(with(
"expected"(value(for(hypothe0cal(“cyclohexatriene"(
∴$ΔHh°$of$benzne$expected$3x(120$kJ/mol)$=$360$kJ/mol$
Higher(in(Energy(than(
Benzene!(
((
208!kJ/mol!
231!kJ/mol!
120!kJ/mol!
360!kJ/mol!
3(x(cyclohexene(Heats$of$hydrogena5on:!(
((
208!kJ/mol!
360!kJ/mol!
3(x(cyclohexene(• (Observed(heat(of(hydrogena0on(is(152(kJ/
mol(less(than("expected”,((
• (((more(stable((
• 152(kJ/mol(is(the(
resonance(energy(
heat(of(hydrogena0on(=(208(kJ/mol(
((
heat(of(hydrogena0on(=(337(kJ/mol(
3H2(
Pt(
3H2(
Pt(
Cyclic$conjuga5on$versus$$noncyclic$conjuga5on$
Can(we(conclude(that(all(Rings(that(
have(alterna0ng(double(and(single(
bonds(like(benzene(have(this(extra(
stabiliza0on?(
Cyclobutadiene$&$Cyclooctatetraene$
Cyclooctatetraene(is(not(planar(
It(has(alterna0ng(long((146(pm)(
and(short((133(pm)(bonds(
Cyclobutadiene(stabilized(
deriva0ve(has(alterna0ng(
long((151(pm)(&((
short((133(pm)(bonds(
Both(are(__________________________.$$An(___________(substance(is(one(that(is(____________(by(cyclic(conjuga0on(
Cyclic(conjuga0on(is(necessary,(but%not%sufficient!!!
Requirements$for$Aroma5city$
not(
aroma0c(aroma0c(
not(
aroma0c(
Antiaromatic when square
Antiaromatic when planar
Conclusion:(Must(be(some(factor(in(addi0on(to(cyclic(
conjuga0on(that(determines(whether(a(molecule(is(
aroma0c(or(not(
Both undergo distortion to be more stable as non-aromatic species
Hückel's$Rule:$In(order(to(be(considered(aroma5c,(a(compound(MUST(be:(
! (Cyclic(
! (Planar(
! (Fully(conjugated(
! (&(#(of(e:s(=(4n!+(2((Among(planar,(monocyclic,(completely(conjugated(
polyenes,(only(those(with(4n$+$2(π(electrons(possess(special(stability((are(aroma0c)(
A(Structure(is(an5aroma5c(if(it(is(planar(&(fully(conjugated(with(4n(π(electrons(in(a(cyclic(fully(conjugated(system(
Benzene!(
πXElectron$Requirement$for$Aroma5city$
an0aroma0c( aroma0c(
4(π(electrons( 6(π(electrons( 8(π(electrons(
an0aroma0c(
Completely$Conjugated$Polyenes$
aroma0c(
6(π(electrons;(completely(conjugated(
not(
aroma0c(
6(π(electrons;((conjugated,(but(cycle((
is(not(complete(
(
H( H(
Which$of$the$following$are$aroma5c$?$
+ +_
H( H(
Cycloheptatrienyl$Ca5on,$Tropylium$ion$
Ionic(Covalent(
6(π(electrons(&(posi0ve(charge(delocalized(over(7(carbons(
__________,(very(stable(called(Tropylium(ion((
(
Tropylium(ion(so(stable(that(Tropylium(bromide(is(ionic(
instead(of(covalent.(
Compare$Acidi5es$of$Cyclopentadiene$and$Cycloheptatriene$
pKa(=(16(
Ka(=(10:16(
pKa(=(36(
Ka(=(10:36(
Aroma0c(anion(
6(π(electrons(
An0aroma0c(
anion(
8(π(electrons(
Cyclopentadienide$Anion$6(π(electrons(delocalized(over(5(carbons(
nega0ve(charge(
dispersed(over(5(carbons(
stabilized(anion(
pKa(=(16(Very(acidic(for(a(
hydrocarbon(due(to(
resonance(stability(of(the(
anion(
Can$you$show$the$Electron$Delocaliza5on?$
••(–(
H( H(
H(H(
H(
Ferrocene: An Organometallic Molecule
What is the hybridization of each heteroatom for each of the above heterocyles ?
A sp B sp2 C sp3
Which of the following has a lone pair not involved in resonance ?
A B C D E$((All(lone(pairs(are(involved(in(resonance.(
Which of the following molecules are aromatic ?
. _ +
A B C D