Soluções para Reações de Compostos Aromáticos

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Solutions for Reactions of Aromatic Compounds
CH
SO3
H H
SO3
HC
H
C
SO3
H
4 delocalization of the positive charge on the ring
O S
O
Ar
O
ArS
O
O
O
SO
O
Ar
O
delocalization of the negative charge on the sulfonate group ("Ar" is the general abbreviation for 
an aromatic or aryl group, in this 
case, benzene; "R" is the general 
abbreviation for an aliphatic or alkyl 
group. In cases where the identity of 
the R group does not matter, it has 
been used to represent alkyl or aryl 
groups.)
CH2CH3
CH2CH3
Br Br FeBr3
Br FeBr3Br
CH2CH3
CH
Br
H
H
Br
CH
CH2CH3 CH2CH3
HC
Br
H
H
Br
HC
CH2CH3
C
CH2CH3
Br
H
H
Br
C
H
CH2CH3
5
(a) The key to electrophilic aromatic substitution lies in the stability of the sigma complex. When the 
electrophile bonds at ortho or para positions of ethylbenzene, the positive charge is shared by the 3° carbon 
with the ethyl group. Bonding of the electrophile at the meta position lends no particular advantage because 
the positive charge in the sigma complex is never adjacent to, and therefore never stabilized by, the ethyl 
group. (Unshared electron pairs on halogen not shown here.)
ortho
meta
3°—good
CH2CH3
Br Br FeBr3
CH2CH3
HC
Br H HBr
C
CH2CH3 CH2CH3
CH
Br H
para
3°—good
CH3
CH3
NO2 H
NO2
C
CH3
CH3
CH3
HC
NO2
H
CH3
H
NO2C
CH3
CH3
(b) Electrophilic attack on p-xylene gives an intermediate in which only one of the three resonance forms is 
stabilized by a substituent (see the solution to Problem 3). m-Xylene, however, is stabilized in two of its 
three resonance forms. A more stable intermediate gives a faster reaction.
m-xylene
3°—good3°—good
386