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Template 14b: Aroma0city Chapter 4 of Textbook Chapter 11 of E:textbook

Template 14b CHEM357

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Page 1: Template 14b CHEM357

Template(14b:(Aroma0city(

Chapter(4(of(Textbook(

Chapter(11(of(E:textbook(

Page 2: Template 14b CHEM357

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.(

Page 3: Template 14b CHEM357

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$

Page 4: Template 14b CHEM357

Higher(in(Energy(than(

Benzene!(

((

208!kJ/mol!

231!kJ/mol!

120!kJ/mol!

360!kJ/mol!

3(x(cyclohexene(Heats$of$hydrogena5on:!(

Page 5: Template 14b CHEM357

((

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(

Page 6: Template 14b CHEM357

heat(of(hydrogena0on(=(208(kJ/mol(

((

heat(of(hydrogena0on(=(337(kJ/mol(

3H2(

Pt(

3H2(

Pt(

Cyclic$conjuga5on$versus$$noncyclic$conjuga5on$

Page 7: Template 14b CHEM357

Can(we(conclude(that(all(Rings(that(

have(alterna0ng(double(and(single(

bonds(like(benzene(have(this(extra(

stabiliza0on?(

Page 8: Template 14b CHEM357

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(

Page 9: Template 14b CHEM357

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

Page 10: Template 14b CHEM357

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!(

Page 11: Template 14b CHEM357

πXElectron$Requirement$for$Aroma5city$

an0aroma0c( aroma0c(

4(π(electrons( 6(π(electrons( 8(π(electrons(

an0aroma0c(

Page 12: Template 14b CHEM357

Completely$Conjugated$Polyenes$

aroma0c(

6(π(electrons;(completely(conjugated(

not(

aroma0c(

6(π(electrons;((conjugated,(but(cycle((

is(not(complete(

(

H( H(

Page 13: Template 14b CHEM357

Which$of$the$following$are$aroma5c$?$

+ +_

H( H(

Page 14: Template 14b CHEM357

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.(

Page 15: Template 14b CHEM357

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(

Page 16: Template 14b CHEM357

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(

Page 17: Template 14b CHEM357

Can$you$show$the$Electron$Delocaliza5on?$

••(–(

H( H(

H(H(

H(

Page 18: Template 14b CHEM357

Ferrocene: An Organometallic Molecule

Page 19: Template 14b CHEM357

What is the hybridization of each heteroatom for each of the above heterocyles ?

A sp B sp2 C sp3

Page 20: Template 14b CHEM357

Which of the following has a lone pair not involved in resonance ?

A B C D E$((All(lone(pairs(are(involved(in(resonance.(

uliaqat
Sticky Note
http://www.physicsforums.com/showthread.php?t=451652
Page 21: Template 14b CHEM357

Which of the following molecules are aromatic ?

. _ +

A B C D