Benzene%&%its%Deriva.ves% - Harvard Canvas

Week$4$ July$14,$2014$$

Benzene%&%its%Deriva.ves%

Reading:$Sec1on$16.1$1$

Draw$the$structures$of$the$following$molecules:$

toluene$$$$$mAchlorophenol$))))nitrobenzene$$$$$pAnitroaniline)))))oAchloroanisole)

Week$4$ July$14,$2014$$

Aroma.city:%More%than%just%conjuga.on%


In$the$19th$century,$benzene$was$quite$a$puzzle.$$In$1865,$Kekulé$proposed$the$nowA$familiar$structure$for$benzene$that$we$might$call$“1,3,5Acyclohexatriene.”$$This$structure$was$quite$controversial$because$it$suggests$that$there$should$be$four$isomeric$$dibromobenzenes,$while$in$fact$there$are$only$three.$$$$

Of$course,$we$know$that$there$is$only$$one)ortho!dibromobenzene.$$How$can$that$be?$$$

Benzene$is$an$example$of$an$aroma.c$compound.$$The$criteria$for$aroma1city$are:$$$$$$$1)$$The$molecule$(or$ion)$must$contain$a$con2guous,)planar,)cyclic)array$of$pAorbitals$$$$$$$2)$$The$array$of$pAorbitals$must$contain$4n+2$electrons$(where$n$=$0,$1,$2…)$$$$$$$$$$$$$$If$a$molecule$(or$ion)$sa1sfies$the$first$criterion,$but$contains$4n$electrons,$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$it$is$an.aroma.c,$and$especially$unstable.$

Aroma1c$compounds$are$especially%stable.$

Are$the$following$molecules$aroma2c,$an2aroma2c,$or$neither?$

BrBr

Br

Br

Br

Br

BrBr

O HN

Week$4$ July$14,$2014$$

Molecular%Orbitals%and%Aroma.city%1:%The$Hückel$Rule$&$Frost’s$Circle$


Why$4n%+%2?$$Is$there$anything$special$about$the$numbers$2,$6,$10,$14,$etc?$$To$answer$$that$ques1on,$we$need$to$look$at$the$molecular%orbitals!$

For$linear)conjugated$systems,$we$have$n$πAmolecular$orbitals,$with$up$to$nA1$nodes:$

For$cyclic)conjugated$systems,$we$s1ll$have$n$πAmolecular$orbitals,$but$where$do$the$nodes$go,$and$how$many$are$there?$$The$highest$and$lowest$energy$orbitals$look$like$what$we$might$expect$–$the$“allAbonding”$and$“allAan1bonding”$combina1ons:$

Week$4$ July$14,$2014$$



To$figure$out$what$the$other$MO’s$look$like:$$1)  Pick$your$polygon$and$draw$it$with$one$vertex$at$the$boiom.$2)  Place$an$MO$at$each$vertex.$$Note$that$we$have$pairs$of$MO’s$at$equal$energies!$3)  As$we$go$from$boiom$to$top,$we$increase$the$number$of$nodes$from$0$to$n/2.$4)  For$degenerate$pairs$of$orbitals,$nodes$are$orthogonal.$5)  Fill$with$electrons.$

Week$4$ July$14,$2014$$



OK,$but$why$is$an1aroma1c$bad?$$What’s$wrong$with$4n$electrons?$$Use$the$Frost’s$Circle$method$to$construct$the$MO$diagram$for$cyclobutadiene$to$find$out.$$$

Week$4$ July$14,$2014$$

The%Effect%of%Aroma.city%on%Reac.vity%


The%rule%is%simple:%aroma.city%=%(very!)%good,%an.aroma.city%=%bad%

Explain$the$following$observa1ons:$$1)  When$styrene$is$treated$with$Br2,$the$bromine$adds$only$to$the$terminal$alkene$and$$ $not$to$the$benzene$ring.$$$$$2)  Only$one)of$the$following$hydrocarbons$can$be$deprotonated$by$NaNH2.$

$

$

3)  Alkynes$react$with$carbenes$to$form$cyclopropenes,$but$do$NOT$react$with$mCPBA$$to$form$oxirenes.$$

BrBrBr2

NaNH2

NaNH2

NaNH2

no'reaction

no'reaction

Me Me Me Me

Me MeO

Me Me

CH2I2Zn(Cu)

mCPBAX

Week$4$ July$14,$2014$$

Electrophilic%Aroma.c%Subs.tu.on:$General$Mechanism$

Reading:$Sec1on$16.4$ 7$

When$benzene$is$treated$with$D2SO4$(D$=$deuterium,$2H),$the$protons$of$benzene$are$slowly$replaced$with$deuterium.$$Provide$a$curvedAarrow$mechanism.$

This$is$an$example$of$electrophilic)aroma2c)subs2tu2on.$$What$is$the$general$reac1on$for$a$generic$electrophile,$E+?$

HH

HH

H

H DD

D

DD

D

D2SO4

Week$4$ July$14,$2014$$

Electrophilic%Aroma.c%Subs.tu.on:$Halogena1on$


How$can$we$replace$a$hydrogen$(H+)$with$chlorine$(Cl+)$or$bromine$(Br+)?$

Week$4$ July$14,$2014$$

Electrophilic%Aroma.c%Subs.tu.on:$Nitra1on$and$Sulfona1on$

Reading:$Sec1on$16.4$ 9$

How$can$we$replace$a$hydrogen$(H+)$with$a$nitro$group$(NO2+)?$

How$can$we$replace$a$hydrogen$(H+)$with$a$sulfonic$acid$group$(SO3H+)?$

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Benzene%&%its%Deriva.ves% - Harvard Canvas