tema 6. reactions of aromatics compounds
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Reactions ofAromatic Compounds
TEMA 6
Qumica Orgnica 2014
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Chapter 17 2
Eectrop!iic
Aromatic "u#stitutionEectrop!iesubstitutes for a !$drogen
on the benzene ring.
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Chapter 17 3
%nusua "ta#iit$Hydrogenation of just one double
bond in benzene is endot!ermic!
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Mec!anism
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&romination of &en'ene
"e#uires a stronger ele$trophile than %r2.
&se a strong 'e(is a$id $atalyst) *e%r3.
Br Br FeBr3 Br Br FeBr 3
Br Br FeBr 3
H
H
H
H
H
H
H
H
H
H
H H
Br+ + FeBr4
_
Br
HBr+=>
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Chapter 17 +
Energ$ (iagram for
&romination
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7
C!orination
and )odination Chlorination is si,ilar to bro,ination.
&se -lCl3as the 'e(is a$id $atalyst.
odination re#uires an a$idi$ o/idizing
agent) li0e nitri$ a$id) (hi$h o/idizes the
iodine to an iodoniu, ion.
H+
HNO3 I21/2 I+
NO2 H2O+ ++ +
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*itration of &en'ene&se sulfuri$ a$id (ith nitri$ a$id to for,
the nitroniu, ion ele$trophile.
H O N
O
O
H O S O H
O
O
+ HSO4_
H O N
OH
O+
H O N
OH
O+
H2O + N
O
O
+2then for,s asig,a $o,ple/ (ith
benzene) loses Hto
for, nitrobenzene. =>
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"ufonation
6ulfur trio/ide) 63) in fu,ing sulfuri$ a$id
is the ele$trophile.
S
O
O OS
O
O OSO
O OSO
O O
+ + +
_
_ _
S
O
O O
H
SO
O
OH
+
_
S
HOO
O
benzenesulfonic ac
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1
(esufonation
-ll steps are re8ersible) so sulfoni$ a$id
group $an be re,o8ed by heating in
dilute sulfuri$ a$id.
9his pro$ess is used to pla$e deuteriu,
in pla$e of hydrogen on benzene ring.
%enzene:d+
=>
2SO4/2O
lar!e e"cess
H
H
H
H
H
H
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*itration of Touene
9oluene rea$ts 2 ti,es faster than benzene.
9he ,ethyl group is an a$ti8ator.
9he produ$t ,i/ $ontains ,ostly ortho and
para substituted ,ole$ules.
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"igma Compe+
nter,ediate
is ,ore
stable if
nitrationo$$urs at
the ortho
orpara
position.
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Energ$ (iagram
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-$ti8ating) O:) P:
;ire$ting 6ubstituents -l0yl groups stabilize the sig,a $o,ple/ by
indu$tion) donating ele$tron density through
the sig,a bond. 6ubstituents ,it! a one pair of eectrons
stabilize the sig,a $o,ple/ by resonan$e.
O#H3
H
NO2
+
O#H3
H
NO2
+
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1+
6u,,ary of
-$ti8ators
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;ea$ti8ating ?eta:
;ire$ting 6ubstituents
@le$trophili$ substitution rea$tions fornitrobenzeneare 1) ti,es slo(erthan for benzene.
9he produ$t ,i/ $ontains ,ostly themeta isomer) only s,all a,ounts of theorthoand para iso,ers.
?eta:dire$tors dea$ti8ate all positionson the ring) but the ,eta position is lessdea$ti8ated.
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1
rtho 6ubstitution
on itrobenzene
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Aara 6ubstitution
on itrobenzene
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2
?eta 6ubstitution
on itrobenzene
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6tru$ture of ?eta:
;ire$ting ;ea$ti8ators
9he ato, atta$hed to the aro,ati$ ring
(ill ha8e a partial positi8e $harge. @le$tron density is (ithdra(n indu$ti8ely
along the sig,a bond) so the ring is less
ele$tron:ri$h thanbenzene.
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6u,,ary of ;ea$ti8ators
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?ore ;ea$ti8ators
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2
.ao#en'enes
Halogens are deactivating toward
electrophilic substitution) but are ortho)
para:dire$ting!
6in$e halogens are 8ery ele$tronegati8e)
they (ithdra( ele$tron density fro, the
ring indu$ti8ely along the sig,a bond.
%ut halogens ha8e lone pairs of ele$trons
that $an stabilize the sig,a $o,ple/ by
resonan$e. =>
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2+
6ig,a Co,ple/
for %ro,obenzene
Br
%+
Br
H
%
&+'
&+'&+'
Ortho attack
+ Br
%+
Br
H %
+
&+'
&+'&+'
Para attack
rtho and para atta$0s produ$e a bro,oniu, ion
and other resonan$e stru$tures.
=>
Meta attack
Br
%+
Br
H
H
%
+
&+'
&+' o bro,oniu, ionpossible (ith ,eta atta$0.
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@nergy ;iagra,
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6u,,ary of
;ire$ting @ffe$ts
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Mutipe "u#stituents
The most strongly activating substituentThe most strongly activating substituent
will determine the position of the nextwill determine theposition of the next
substitutionsubstitution. ?ay ha8e ,i/tures.O#H3
O2N
SO3
H2SO
4
O#H3
O2N
SO3H
O#H3
O2N
SO3H
+
=>A:iso,er B
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3
*riedel:Crafts -l0ylation
6ynthesis of al0yl benzenes fro, al0yl
halides and a 'e(is a$id) usually -lCl3.
"ea$tions of al0yl halide (ith 'e(is a$idprodu$es a $arbo$ation (hi$h is the
ele$trophile.
ther sour$es of $arbo$ationsal0enes H* or al$ohols %*3.
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@/a,ples of
Carbo$ation *or,ation
#H3 #H #H3
#l
+ (l#l3
#H3#
H3# H
#l (l#l3+ _
H2# #H #H3HF
H3# #H #H3
F+
_
H3# #H #H3
OHBF3
H3# #H #H3
OH BF3+
H3# #H #H3+
+ HOBF3
_
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*or,ation of
-l0yl %enzene
#
#H3
#H3
H+
H
H
#HH3'2+
H
H
#HH3'2
B
F
F
F
OH
#H
#H3
#H3
+
HF
BF
OHF
=>
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/imitations of
riedeCrafts "ea$tion failsif benzene has a substituent
that is more deactivating than halogenmore deactivating than halogen.
Carbo$ations rearrange. "ea$tion ofbenzene (ith n:propyl $hloride and -lCl3
produ$es isopropylbenzene.
9he al0ylbenzene produ$t is ,ore rea$ti8ethan benzene) so polyal0ylation o$$urs.
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riedeCraftsriedeCrafts
Ac$ationAc$ation -$yl $hloride is used in pla$e of al0yl
$hloride.
9he a$yliu, ion inter,ediate isresonan$e stabilized and does not
rearrange li0e a $arbo$ation.
9he produ$t is a p!en$ etonethat isess reacti3ethan benzene.
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3
?e$hanis, of -$ylation
) #
O
#l (l#l3 ) #
O
(l#l3#l+ _
) #
O
(l#l3#l+ _ (l#l4 +
_ +) # O ) # O
+
#
O
)+
H
#
H
O
)
+#l (l#l3
_#
O
) +
H#l
(l#l3
=>
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3+
Cemmensen Reduction
-$ylbenzenes $an be $on8erted to
al0ylbenzenes by treat,ent (ith
a#ueous HCl and a,alga,ated zin$.
+ #H3#H2#
O
#l1'(l#l3
2'H2O
#
O
#H2#H3*n&H!'
a, H#l
#H2#H2#H3
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ArenesArenes
A$#en'enes
Aen$#en'enes A$n$#en'enes
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6ide:Chain /idation
A$#en'enes are o+idi'ed to #en'oic
a$id by hot D?n4or a2Cr27EH264.
#HH3'2
#H #H2
-.nO4$ OH
H2O$ 0ea1
#OO
#OO
_
_
=>*ot t#ut$#en'ene
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6ide:Chain Halogenation
%enzyli$ position is the ,ost rea$ti8e.
Chlorination is not as sele$ti8e as
bro,ination) results in ,i/tures.
&r2reacts on$ at t!e #en'$ic position.
=>
#H#H2#H3
Br
hBr2$
#H2#H2#H3
B lRdi l
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Benzyl Radical