Arenes and Aromaticity

1
Chapter 6
continued
Arenes and Aromaticity
Reactions of Substituted Aromatic Compounds
2
Electrophilic Aromatic Substitution (EAS)
Substituted Benzenes
Two orientation patterns
S substituent
i.
The substituent S is an . director
Either group, or .. on atom, attached
to aromatic ring!
3
EAS Substituted Benzenes (cont.)
Two orientation patterns (cont.)
The substituent S is a director
ii.
Atom attached to benzene ring bears .
to ..atom(s)!
4
Nitration of Toluene
a. .. substitution
i. o-product

• o-Cyclohexadienyl cation intermediate includes
  resonance contributor A for A, positive charge
  on 3 C bearing alkyl group - s..
• ..

5
Nitration of Toluene (cont.)
ii. p-product

• p-Cyclohexadienyl cation intermediate includes
  resonance contributor B positive charge on 3 C
  bearing alkyl group - ..

• o-, p-Cyclohexadienyl cation intermediates
  than cyclohexadienyl cation
  intermediate of nitration of benzene

• Therefore, o, p-nitration of toluene .
  than nitration of benzene

6
Nitration of Toluene (cont.)
b. meta substitution

• m-Cyclohexadienyl cation intermediate does
  have resonance contributor with positive charge
  on 3 C bearing alkyl group

• m-Cyclohexadienyl cation intermediate about
  .cyclohexadienyl cation
  intermediate of nitration of benzene

• Therefore, m-nitration of toluene about the
  .. as
• for benzene

• As o, p-nitration than m-nitration of
  toluene, ..
• o, p- than m-products formed!

7
Nitration of Toluene Reaction Profiles

• Remember energy of intermediate cation related
  to energy of TS (. process
  ...postulate)!

• Energies of .. and -TS than that of
  ..-TS!

Energy
Reaction Coordinate
Energy

• Energy of p-TS is actually slightly lower than
  energy of o-TS
• .. and effects slightly raise energy of
  o-TS!

Reaction Coordinate
8
Nitration of Methoxybenzene (Anisole)
a. .. .. substitution
i.

• o-Cyclohexadienyl cation intermediate includes
  resonance contributor A of positive
  charge
• on ..

9
Nitration of Anisole (cont.)
ii. ..

• p-Cyclohexadienyl cation intermediate includes
  resonance contributor A .. of positive
  charge on ..

• o-, p-Cyclohexadienyl cation intermediates -
  .. delocalized positive charge - .. ..
  energy than cyclohexadienyl cation intermediate
  of nitration of benzene

• Therefore, o, p-nitration of anisole .. .
  than nitration of benzene

10
Nitration of Anisole (cont.)
iii. .. .

• .. -Cyclohexadienyl cation intermediate does
  have resonance contributor with .. on
  - group

• -Cyclohexadienyl cation intermediate ..
  energy than and . cyclohexadienyl cation
  intermediates

• o, p-Nitration .. . than m-nitration of
  anisole
• . o, p- than m-products formed!

11
Nitration of Anisole Reaction Profiles

• Energies of o-TS and p-TS .. than that of m-TS!
  

Energy
Reaction Coordinate

• Energy of p-TS . of all
• . and . effects slightly raise energy of
  o-TS!

Energy
Reaction Coordinate
12
Electrophilic Aromatic Substitution Summary

• Substituents S direct ortho, para - either alkyl
  group, or lone pair on atom of the group attached
  to benzene ring!

• Stabilize o- and p-cyclohexadienyl cation
  intermediates by inductive affect (alkyl) or
  electron donation (lone pair on atom attached to
  ring) ? lower energy of TS

• Reactions leading to o- and p-substituted
  derivatives faster than for benzene itself

? groups are ortho-, para-directing and
activating!

• Exception Cl, Br, I direct ortho, para -
  stabilization of cation by electron donation,
  but. inductive effect deactivates benzene ring
  (6.10) reaction slower than for benzene
  (complicated effect dipole, other factors - p.
  171)

? groups are ortho, para-directing and
deactivating

• p-Product is major product formed ? statistical
  effect favours o- product (21!) steric and
  electronic effects slightly raise energy of TS
  leading to o-cation intermediate

13
Nitration of Nitrobenzene
a. .. substitution

• Nitro group
• charge on atom attached to ring
• attached atom multiply bonded to electronegative
  atoms

• strongly .. (.) aryl
  ?-electrons less available for electrophile than
  in benzene.

Nitration . vigorous conditions required than
for benzene reaction much . than nitration of
benzene
14
Nitration of Nitrobenzene (cont.)
i. -product

• Nitro group strongly . positive charge on
  adjacent atom
• .. energy of cyclohexadienyl cation.

• Electrophile enters . position positive
  charge in cyclohexadienyl cation intermediate is
  on carbon atoms away from nitro group.

15
Nitration of Nitrobenzene (cont.)
ii. .-product

• o-Cyclohexadienyl cation intermediate includes
  resonance contributor A .. charge on C to
  positively charged nitrogen atom

• o-Cyclohexadienyl cation intermediate .
  energy than ..- cyclohexadienyl cation
  intermediate

16
Nitration of Nitrobenzene (cont.)
iii. ..-product

• p-Cyclohexadienyl cation intermediate includes
  resonance contributor B . charge on C
  attached to positively charged nitrogen atom

• p-Cyclohexadienyl cation intermediate .
  energy than . . cyclohexadienyl cation
  intermediate

• Therefore, -nitration .. . than o,
  p-nitration of nitrobenzene.

17
Nitration of Nitrobenzene Reaction Profiles

• Energy of m-TS . . than those of o- and p-TS!
  

Energy
Reaction Coordinate
Energy

• Energy of benzene TS lowest
• nitration of benzene .. than nitration of
  nitrobenzene!

Reaction Coordinate
18
Electrophilic Aromatic Substitution Summary
Summary Substituents S direct meta - atom
attached to ring bears positive charge and/or
multiple bonds to electro- negative atom(s)!

• All groups are electron withdrawing groups (EWG)
  (destabilize positive charge, stabilize negative
  charge)

• All raise energy of o- and p-cyclohexadienyl
  cation intermediates

• Substitution is meta because positive charge in
  resonance hybrid is on cyclohexadienyl carbon
  atoms away from atom to which group is attached.

• Electrophilic aromatic substitution slower than
  for benzene

? groups are meta directing and deactivating
Note is o, p-director and activating
is
meta directing and deactivating!
19
Electrophilic Aromatic Substitution Problems
Give sequence of reactions for following
conversions

• Orientation of substituents
• is meta
• m-directing group must be used
• -CH3 and -Br are o, p-directors,
• -COOH is m-director
• Thus, -CH3 must be converted
• to -COOH

1.
Sequence is
20
Problems (cont.)
2.

• Note orientation of substituents is para
• p-directing group must be used
• -Br is o, p-director, -COCH3 is m-director
• Thus, -Br must be introduced first.

Sequence
Note small amount of o-product is also formed
products can be separated by distillation.
21
Problems (cont.)
3.

• -OCH3 and -Br are o, p-directors
• -COCH3 is m-director.
• p-product is major product for substituted
  benzene bearing o- p-directing substituent
• thus COCH3 group must be introduced first.

Sequence
22
Problems (cont.)

• Note orientation of substituents is meta
• -NO2 and COCH3 are m-directors
• Thus, two possibilities

4.
X
a.
b.
In practice, sequence b is used not possible to
carry out Friedel-Crafts acylation (or
alkylation) of nitrobenzenes (nitro group
powerfully deactivating)
23
Problems (cont.)
5.

• -CH3 is o, p-directors, -NO2 is m-director.
• May carry out polynitration of toluene at a
  temperature higher than that used for
  mono-nitration

2,4,6-trinitrotoluene (TNT)
Actual reaction conditions

• o- and p- mononitro products form first

24
Problems (cont.)

• Then, nitration meta to the nitro groups in the
  o- and p-nitrotoluenes

5 (cont.).

• Finally, nitration of
• dinitrotoluenes

check problems in Ch. 6 (lecture plan)!