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Electrophilic Aromatic Substitution

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... Directing effects of substituents already on the ring. Products of Nitration ... Substituents that make the ring react faster (than benzene) with electrophiles ... – PowerPoint PPT presentation

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Title: Electrophilic Aromatic Substitution


1
Electrophilic Aromatic Substitution
  • Activating and Directing effects of substituents
    already on the ring

2
Products of Nitration
1 hr
48 hr
0.0003 hr
3
Mechanism of Electrophilic Aromatic Substitution
With a substituent group G
Depending on the nature of the substituent, the
substituent G may stabilize the carbocation
intermediate and therefore speed the reaction, or
it may raise the energy of the carbocation and
slow the reaction. Substituents that make the
ring react faster (than benzene) with
electrophiles are called activators those that
make the ring react slower (than benzene) are
called deactivators.
4
Substituent effects
  • All activators also direct incoming electrophiles
    to the ortho- and the para-positions.
  • Most deactivators direct incoming electrophiles
    to the meta position. The exceptions are the
    halogens, which are weakly deactivating yet
    ortho-para directing.

5
Classification of Substituents
6
Product Distribution in Nitration
  • (Percent )
    (Percent )
  • X ortho meta para X
    ortho meta para
  • (meta-directing Deactivators)
    (ortho- and para-directing Deactivators)
  • -N(CH3)3 2 89 11 -F 13 1
    86
  • -NO2 7 91 2 -Cl 35 1 64
  • -CO2H 22 77 2 -Br 43 1 56
  • -CN 17 81 2 -I 45 1 54
  • -CO2CH2CH3 28 72 2 (ortho- and
    para-directing Activators)
  • -COCH3 26 72 2 -CH3 63 3 34
  • -CHO 19 72 9 -OH 50 0 50

7
o,p-Activators (alkyl aryl groups)
3º, especially stable
3º, especially stable
8
o,p-Activators with a lp of electrons
o-
4 resonance forms
m-
p-
4 resonance forms
9
o,p-Deactivators (Halogens)
o-
4 resonance forms
m-
p-
4 resonance forms
10
meta-directing Deactivators
o-
Especially UNSTABLE
m-
p-
Especially UNSTABLE
11
Explanation of meta- deactivators
  • Meta directors slow the reaction by raising the
    energy of the carbocation intermediate
    because they have (in one resonance form, shown
    below) a positively charged atom attached to the
    ring. Two positively charged atoms so close
    together is very high in energy (especially
    unstable).

nitro group sulfonic acid
ketone nitrile
12
Summary of Substituent Effects
  • When the substituent is R (alkyl) or Ar (aryl),
    the resonance hybrid cation intermediate has
    three resonance forms. For attack of the
    electrophile at the ortho or para positions, one
    of these is a 3º carbocation, which is especially
    stable. This lowers the energy of the
    intermediate, thus facilitates (speeds) the
    substitution reaction at the o- and p- positions.
  • When the substituent has a lone pair of
    electrons, such as the halogens, oxygen or
    nitrogen, the resonance hybrid for attack of the
    electrophile at the ortho and para positions has
    four resonance forms. This lowers the energy of
    the those intermediates, thus facilitates
    (speeds) the substitution reaction at the o- and
    p- positions.

13
Summary of Substituent Effects
  • When the substituent has a multiple bond
    conjugated with the ring, and the second atom
    from the ring is more electronegative than the
    first, the substituent deactivates the ring and
    directs incoming electrophiles meta. It does
    this by raising the energy of the carbocation
    intermediates from ortho and para attack by an
    electrophile even more than it raises the energy
    of the intermediate resulting from meta attack.

14
Summary
15
Additivity of substituent effects in
disubstituted aromatic rings
  • Rule 1 If the directing effects of two
    substituents reinforce each other, the predicted
    product predominates.

(o,p)
(m)
16
Additivity of substituent effects
  • Rule 2 If the directing effects of two
    substituents oppose each other, the more
    activating group dominates, but mixtures often
    result.

(o,p STRONG activator)
(o,p weak activator)
17
Additivity of substituent effects
  • Rule 3 Substitution almost never occurs between
    two substituents meta to each other.

(o,p)
X (too crowded)
(o,p)
18
Additivity of substituent effects
  • Rule 4 With a bulky o,p- director and/or a
    bulky electrophile, para substitution
    predominates.

(o,p BULKY)
(HSO3 is a BULKY electrophile)
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