Aromatic Substitution Chemistry
(Part of Chapter 2 and Chapter 11)
Aliphatic Compounds:
pentane
2-pentene
2-pentyne
Aromatic Compounds:
Aromatic compounds have closed loops of electrons that give rise to an aromatic
ring current. Aromatic compounds are the arch examples of delocalized bonding.
(March 4th edition)
Resonance Hybrid
Resonance or Kekule structures
All aromatics are planar, conjugated, cyclic, unsaturated molecules. (see Hückel
Rule) The structure of benzene is not represented as two distinct structures as shown
above, but is really thought of as a hybrid of the two forms. This form is the resonance
hybrid. In benzene this means that the structure is not alternating double and single
bonds but rather a bond order of 1.5 is determined.
There are other possible, higher energy, zwitterionic forms, that do not
significantly contribute to the resonance hybrid but may be taken into account if there are
specific stabilizing groups on the aromatic ring.
H
H
H
H
The basis set of p orbitals of the sp2 hybridized carbons is shown below. So are
some representative bond lengths that are pertinent.
an sp2 hybridized carbon
H
C
C
1.53 Å
C
C
1.32 Å
C
C
1.398 Å
The basis set of p-orbitals
The arrows shown in the resonance structures represent the electron movements.
There are a number of arrows that are used in Organic Chemistry, these are shown below.
Please use the correct arrows for each specific use.
Types of Arrows in use in Organic Chemistry
Resonance arrow, separates two resonance structures, do not confuse with equilibrium.
In resonance structures only electron movement occurs, no atomic positions change.
Equilibrium arrow, separates sides of a chemical equilibium. Atomic positions change
during this chemical reaction.
One way reaction arrow. Reaction is not reversible. Atomic positions change.
Retrosynthetic arrow. This arrow is used during retrosynthetic analysis. This is beyond the
scope of this course. Do not use this arrow as a reaction arrow.
Two electron movement arrow. This arrow moves two electrons from the location of the
tail to the location of the head.
One electron movement arrow. This arrow moves one electron from the location of hte tail
to the location of the head.
Arenes
Polycyclic aromatic hydrocarbons that are derivatives of benzene with additional
fused rings are called arenes.
benzene
naphthalene
anthracene
pyrene
The resonance energies of fused systems increase as the number of principal
canonical forms increases.
A couple of other arenes
benzo[a]pyrene - a carcinogen
a helicene
Note that the helicenes are not exactly planar but must be twisted somewhat,
although orbital overlap is decreased, it is still present. Recent research shows that large
arrays of fused benzene rings can be bent significantly and still retain aromatic properties.
One important aromatic property that has impact in spectroscopy is the ring
current. Electrons circulate if the ring is placed in a magnetic field in such a way as to
create an opposing magnetic field. This causes shielding and deshielding regions around
the molecule which create changes in the location of signals in NMR sepectra.
The figure on the next page shows the behaviour of electrons in a strong applied
magnetic field (B0). Notice that while the magnetic field opposes the field in the centre
of the electrons, on the periphery of the benzene the generated field augments the applied
field. Thus protons on the outside of the ring observe a higher magnetic field that the
applied field. This has implications in where the protons are observed in the NMR
spectrum (they appear “downfield” in the region around 7.2 ppm). Aromatic protons are
highly characteristic in the NMR and are easily observed in most cases.
Ring current
Lines of magentic force
B0
H
shielding
region
H
deshielding
region
a strong
applied
magnetic
field
Stability
Aromatic compounds have resonance stabilization in excess of that expected for
closely related structures with localized π electrons.
cyclohhexene
+ H2
-120 kJ
-120 kJ
cyclohexadiene
+ 2 H2
-231 kJ
-240 kJ
benzene
+ 3 H2
-208 kJ
-360 kJ
In cyclohexaediene the enthalpy of the reaction is reduced slightly from the
theoretical number due to the need to pay some energy in to account for a small amount
of additional stability due to resonance interactions.
In benzene this difference is very large due to the need to pay 152 kJ in energy to
break the additional stabilization due to aromaticity, thus reducing the energy given off in
the reaction. Further below we discuss this matter further in some detail.
Aromatic Electrophilic Substitution
Most substitutions in aromatic systems involve the attack of an electron rich arene
ring system on to an electron poor electrophyle. This is in contrast to most substitutions
in aliphatic chemistry in which the electron rich nucleophile attacks the electron deficient
carbon.
The attacking electrophile is a positive ion (or positive end of a dipole or induced
dipole). After the reaction the “leaving group” must depart without its electrons. The
most common departing group is the proton, H+.
The mechanism is known as the arenium ion mechanism, and is characterized by
the initial formation of the intermediate arenium ion, also known as the Wheland
Intermediate. In the second step the leaving group departs. There is some resemblance of
this mechanism to the attack of nucleophiles on the carbonyls of esters or amides to give
tetrahedral intermediates, except that the charges are reversed.
The electrophile can be generated in various ways, examples are shown below.
Generation of Electrophiles (E+)
Cl
Cl2 + AlCl3
Cl
Cl AlCl3
active
electrophile
Lewis
acid
chloronium
ion
O
H
HNO3 + H2SO4
O
N
H
O
H
R3C
Cl
R3C
+ AlCl3
Cl AlCl3
O
O
+ N
H
O
NO2
nitronium
ion
R3C
carbonium
ion
O
O
R
C
Cl
+ AlCl3
R
C
Cl AlCl3
R
C
O
+
AlCl4
acyllium
ion
The Arenium Ion Mechanism
E
H
H
B
H
H
E
H
- H+
E
one canonical
form of the
arenium ion
In the first step the pair of electrons from the double bond attacks the electrophile,
breaking the aromatic stabilization of the benzene ring. The arenium ion is a high energy
intermediate. The arenium ion is stabilized by resonance, as shown in a separate figure
below. In the second step, some base present in the system (usually the solvent, but it can
be other species with the ability to accept a proton) removes the proton to regenerate the
aromatic stabilization.
We often simply show the proton falling off, but to be completely correct it must
be recognized that the proton is never “free” in solution, it is always interacting with
something. Usually we will indicate the loss or gain of atoms, ions or molecules over the
arrow in the step to maintain the atom balance of the reaction.
Three canonical forms (resonance structures) of the arenium ion exist and the ion
is known as the Wheland Intermediate. The positive charge is stabilized by the resonance
interaction with the two remaining double bonds in the ring.
H
E
H
E
H
H
E
H
H
H
H
H
H
H
H
H
H
H
H
H
Wheland
Intermediate
or
arenium ion
H
H
E
Hückel Rule
Conjugated monocyclic planer polyenes with 4n+2 π e- are aromatic.
Conversely, such compounds with 4n π e- are antiaromatic.
Some Examples of Aromatic Compounds – 4n+2 π electrons
H
H
H
n=1
n=1
n=0
n=1
Some Examples of Antiaromatic Compounds – 4n π electrons
H
n=1
n=1
n=2
Cyclooctatetrane is destabilized due to antiaromaticity if it is in a planar
configuration, therefore it twists out of planarity in order to minimize the overall orbital
energy.
Orbital energies of cyclic planar conjugated systems can be estimated using
Frost’s Circle. Two examples are given below which compare both benzene and
cyclobutadiene with their isolated double bond counterparts.
Compound
Three isolated
double bonds
Frost's Circle
not
applicable
Orbital
Energies
Orbital Energy
Sketch
Total Electron
Energy
α−β
6α + 6β
α+β
α − 2β
Bezene
α−β
6α + 8β
α
α+β
α + 2β
Compound
Two isolated
double bonds
Frost's Circle
not
applicable
Orbital
Energies
Orbital Energy
Sketch
Total Electron
Energy
α−β
4α + 4β
α+β
Cylcobutadiene
α − 2β
α
α + 2β
4α + 2β
Stabilization and Destabilization in Aromatic and Antiaromatic Systems
It is observed that benzene has 2β of additional stabilization energy over that of
three isolated double bonds. This is the aromatic stabilization. In the case of
cyclobutadiene, we see that there is 2β of stabilization lost over the case of two isolated
double bonds. This is the destabilization observed in antiaromatic systems.