The Structure of Benzene
Chemists first isolated benzene from coal tar distillate and found its molecular formula: C6H6. The question
of benzene's structure remained for quite some time. The problem lay with the fact that its formula implied
substantial unsaturation, yet benzene did not undergo reactions typical of alkenes. There were several
proposed structures put forward at the time (1858):
Klaus
Dewar benzene
Now known as Prismane. The Kekulé structure.
One problem with the structure of benzene was that there were three known dibromide derivatives. They could be
explained by the following three structures:
Br
BUT: closer inspection suggests that there should be two
ortho isomers:
Br
Br
Br
Br
Br
Br
Br
ortho
To resolve this, Kekulé
proposed that a rapid
interchange of the double
and single bonds in
benzene. This would make
all six carbon atoms of
benzene and the bonds
between them identical.
Br
meta
Br
para
In accordance with Kekulé's
proposition, all six carbon-carbon
bonds are the same length: 1.40Å.
This is intermediate between the
single C-C and double C=C bond
length suggested by the hypothetical
molecule 1,3,5-cyclohexatriene.
1,3,5-Cyclohexatriene
(drawn to scale)
Structure aside, we still need an explanation for the unusual reactivity (or unreactivity) of
benzene. Another puzzle is the heat of hydrogenation of benzene. Once again, the
reaction of benzene is quite different from that of alkenes.
H2
+ 28.6 kcal/mol
3H2
The addition of H2 to a double
bond is usually exothermic.
+ 85.8 kcal/mol
3H2
+ 49.8 kcal/mol
For some reason, benzene is 36
kcal/mol more stable than the
hypothetical cyclohexatriene.
Cyclohexatriene
+ 3 H2
Enthalpy
36 kcal
Benzene
+ 3 H2
85.8 kcal
49.8 kcal
Conjugated double bonds tend to be more stable than isolated double bonds. Could this
be the reason for benzene's unusual stability?
CH2
CH3
+ 3 H2
CH2
+ 80.5 kcal/mol
CH3
The Molecular Orbitals of Benzene
Ψ6
Ψ4
Ψ5
Ψ2
Ψ3
6 x 2p
Ψ2
Ψ1
The combination of the six atomic p-orbitals of the carbons of the ring leads to six π -orbitals. The lowest
energy orbital is a very stable, low-energy π -orbital that is bonding between all six carbon atoms . The
only node in this orbital is the one in the plane of the ring (which is common to all π -orbitals). With six
electrons to put in these orbitals, benzene has just enough to fill the bonding MOs. Therefore, benzene has
a filled set of low-energy molecular orbitals, just as the Noble Gases have a filled set of atomic
orbitals.
Resonance Energy
The stability of benzene can be attributed to the set of filled bonding π -type molecular orbitals that result from the
atomic p-orbitals around the ring. Molecules that share this kind of stability are often referred to as "aromatic"
molecules. Originally, this had to do with their smell, but now the term aromatic is used to refer to compounds which
have electrons that are delocalized around the ring.
The degree to which benzene is stabilized by its aromatic nature is defined by the difference between its heat of
hydrogenation relative to the hypothetical 1,3,5-cyclohexatriene. This energy, 36 kcal/mol, is called the resonance
energy. For other aromatic molecules, the resonance energy is defined in an analogous manner.
H
H
H
H
+ H2
H
In the first addition of H 2 to benzene, the resonance energy must be
sacrificed. This initial hydrogenation is endothermic (by 5.6 kcal/mol)!.
No wonder benzene is not inclined to undergo addition reactions.
H
If benzene is aromatic, then how about the following:
After a great deal of difficulty these molecules were
finally prepared. Cyclobutadiene is unstable (less stable
than expected even for a diene!). We refer to such
molecules as anti-aromatic . Cyclooctatetraene is not
planar and is considered to be non-aromatic.
How about:
H
Dihydropyrene
14 π-electrons
H
Cyclobutadiene
4 π-electrons
1,3,5,7-Cyclooctatetraene
8 π-electrons
This molecule which has a perifery of 14 π-electrons is aromatic. Clearly,
whether or not a molecule is aromatic or not depends on the number of
π -electrons. Erich Hückel laid out a rule which accurately predicts which
cyclic polyenes will be aromatic and which will not: to be aromatic, a
monocyclic compound must contain 4n+2 π -electronss in the cyclic
conjugated system, where n is an integer (0, 1, 2 etc.).
Criteria for Aromaticity
1.) The molecule must be cyclic.
2.) Each member of the ring must have an atomic p orbital orthogonal to the plane of
the ring (through which the electrons can circulate).
3.) The molecule must be sufficiently planar to allow side-on overlap of all the atomic
p-orbitals.
4.) Its gotta fit The Rüle.
N
N
N
H
O
N
If the N atom of pyrrole is sp3 hybridized, then pyrrole can't be considered aromatic.
N
N
N
H
H
H
N
N
H
H
H
H
N
N
1.58 D
+
1.81 D
+
Pyrrolidine
Pyrrole
H
H
N
H
N
+ H3O
+ H2O
+ H3O
N
N
H
+ H2O
H
N
Pyrrole
O
Furan
S
Thiophene
H
H
C
C
Cyclopentadienyl
Anion
Cyclopentadienyl
Cation
Nomenclature of Aromatic Compounds
Some benzenes are simply named by the type of substituent followed by the term "benzene".
H
CH2CH3
NO2
H
Br
C C
- H2
H
Vinylbenzene
(Styrene)
Ethylbenzene
Nitrobenzene
Bromobenzene
Many substituted benzenes have been known for over a century. They have been given trivial names that are firmly
entrenched in the language of organic chemistry. These names serve as parent names for their substituted derivatives.
H3C
CH3
NH2
OH
CH
OCH3
1.) O2
2.) H3O+
Cumene
(Isopropylbenzene)
Phenol
(not to be confused
with phenyl)
Anisole
Cl
Cl
Biphenyl (the
parent of PCBs)
CH3
Aniline (not to
be confused with
alanine,
the amino acid)
Toluene
O
C
OH
O
C
H
Cl
Cl
A PCB.
Benzoic Acid
Benzaldehyde
Substituted Benzenes
In disubstituted benzenes, the relative positions of the two substituents must be
specified. There are two ways in which this is done. IUPAC uses a numbering
system. A trivial method also persists.
CH3
CH3
CH3
CH3
para-Xylene
(intermediate in
the preparation of
poly(ethylene terephthalate)
CH3
meta-Xylene
CH3
ortho-Xylene
Polysubstituted Benzenes are named as derivatives of the parent benzene (if applicable).
Substituent positions are numbered from the dominant functional group, or follow rules
akin to those for numbering alkanes.
CH3
C
Cl
NH2
O
OH
NO2
o-Methylbenzoic acid
(2-Methylbenzoic acid)
m-Nitroaniline
(3-Nitroaniline)
Cl
p-Dichlorobenzene
(1,4-Dichlorobenzene)
OH
CH3
Cl
F
F
Cl
Cl
2,4,5-Trichlorophenol
H3C
Br
1-Bromo-2,3-difluorobenzene
CH3
1,3,5-Trimethylbenzene
(Mesitylene)
Reactions of Benzene
Just like alkenes, benzene has a substantial amount of electron density due to the π-orbitals. As a
result, benzene also undergoes reactions with electrophilic species "E+".
Alkenes undergo electrophilic addition reactions:
+ Br2
C C
H3C
Br
H
H
Benzene, however undergoes electrophilic substitution reactions:
Br
H
HC CH2
H
Br
FeBr3
+ Br2
H3C
+ HBr
The general mechanism for ALL electrophilic aromatic substitution reactions is the following two step process:
B
H
+ H—B
H
E
+
H
E
E
H
σ-complex
H
The σ-complex is a very important intermediate. It is stabilized by delocalization of the positive charge:
H
E
H
H
E
H
H
E
H
Substitution vs Addition
Br
Energy
H
H
Br
H
σ-complex
σ-complex
Reaction Progress
Br
H
Br+
H
H
Energy
H
Br+
HBr
H
H
Br
H
σ-complex
Br
σ-complex
Reaction Progress
H
Br
Br
Electrophilic Aromatic Substitution - Electrophiles E+
EAS reactions differ only in the identity of E+ and how it is generated.
Nitration
+ HNO3
H2SO4
heat
NO2
O
O
N O
N O
+ H2SO4
H
O
H
Reduction of nitroaromatics
gives anilines.
O
+ HSO4 -
N
H
O
O
In the case of nitration, E+ is NO2+.
Sulfonation
+ SO3
H2SO4
heat
SO3H
O
O
S O
S OH
+ H2SO4
+ HSO4 -
O
O
Halogenation
Br
Fe or
FeBr3
+ Br2
+ HBr
Cl
+ Cl2
Fe or
FeCl3
Br Br + FeBr3
+ HCl
δ+
Br
Br
δFeBr3
+ H2O
Alkylation of Aromatic Compounds - The Friedel-Crafts Reaction
+ RCl
H3C
CH
Cl
R
AlCl3
+ HCl
H3C δ+
CH Cl
H3C
+ AlCl3
H3C
δAlCl3
In some alkylations, this complex
may serve as the alkylating electrophile.
H3C
H + AlCl4
C
H3C
When a relatively stable carbocation
is possible, then it is likely the electrophile.
Again, the mechanism for this reaction is no different than any other Electrophilic Aromatic Substitution.
Cl
H
H3C
C
CH3
CH3
CH
CH
CH3
H
CH3 + HCl
H3C
Alternativly, carbocation electrophiles can be generated by the protonation of alkenes by strong acid:
H
H3PO4
H
Full mechanism: p 800.
H
H
+ H3PO4
H
H
H
+ H2PO4
Carbocation Rearrangement during Friedel-Crafts Alkylations
+ CH3CH2CH2Cl
CH2CH2CH3
AlCl3
CH3
CH
CH3
40%
35°C
5h
60%
PhH
PhH
H
δ+
H3C C C
H2
H
H3CH2CH2C Cl + AlCl3
Friedel-Crafts Acylation
C
AlCl3
H3C
δAlCl3
C
H + AlCl4
H3C
O
O
H3C C Cl
Cl
Rearrangements like this are
always a problem when a
more stable carbocation can
result from a hydride or alkyl
shift.
O
CH3
H3C C Cl + AlCl3
O
H3C C+ Cl
δ
δAlCl3
An acid chloride.
Mechanism
O
O
C
H3C
H
Cl
C
O
H3C C Cl + HCl
H3C C O
H3C C O
The Acylium cation, a resonance
stabilized carbocation, is the
electrophile.
CH3
The Friedel-Crafts acylation can be used to circumvent the problems of carbocation rearrangement.
O
O
H2NNH2, KOH
C
AlCl3
(Wolff-Kishner
reduction)
CH2CH3
C
Cl
H3CH2C
or
Zn/Hg amalgam, HCl
(Clemmenson reduction)
CH2CH2CH3
Reactions of Substituted Benzenes
CH3
CH3
CH3
CH3
NO2
HNO3,
Acetic Acid
+
+
NO2
Toluene
2-Nitrotoluene
Statistical
Actual
40%
60%
3-Nitrotoluene
40%
3%
NO2
4-Nitrotoluene
20%
37%
This reaction is faster than the corresponding nitration of benzene. Why? And what is
the reason for this particular product distribution? All alkyl groups and several other
substituents show this pattern of reactivity.
NO2
NO2
NO2
NO2
NO2
HNO3,
H2SO4
+
NO2
Nitrobenzene
NO2
1,3-Dinitrobenzene
93% Yield
Minor products
This reaction is much slower than the nitration of benzene. Toluene gives mainly ortho
and para products while nitrobenzene gives the meta product almost exclusively. What
is it about the substituent that directs the position of the incoming electrophile?
Cl
Cl
Cl
Cl
NO2
HNO3,
H2SO4
30%
o-Chloronitrobenzene
+
+
NO2
1%
m-Chloronitrobenzene
NO2
69%
p-Chloronitrobenzene
This reaction is
slower than the nitration of
benzene. All of the halogens follow this pattern of reactivity. What's going on?
Substituent Directing Effects
σ-Complex Intermediates
CH3
CH3
CH3
O
NO2
NO2
+
H
N+
CH3
CH3
NO2
H
+
NO2
H
+
O
CH3
CH3
O
O
CH3
NO2
NO2
H
+
NO2
H
H
CH3
CH3
+
+
N+
CH3
CH3
CH3
CH3
NO2
CH3
+
+
O N
+ O
H
NO2
+
H
NO2
H
NO2
NO2
The σ -complexes of the ortho- and para-substituted toluenes are stabilized by the methyl group. This is reflected in a
lower activation energy leading to these intermediates - they are formed faster. ANY SUBSTITUTENT CAPABLE OF
STABILIZING AN ADJACENT POSITIVE CHARGE WILL GIVE PREDOMINANTLY ORTHO- AND PARASUBSTITUTION.
More Substituent Directing Effects
O
N+
O
O-
σ-Complex Intermediates
ON+
O
NO2
+
N+
O N+ ONO2
H
O
NO2
N+
O-
O
O
N+
O-
O
O
NO2
ON+
NO2
NO2
NO2
H
+
O-
O
NO2
N+
NO2
H
NO2
H
+
O-
+
+
N+
N+
H
+
O
O
O-
O
N+
NO2
H
O
NO2
O-
NO2
NO2
NO2
N+
+
+
O
O N
+
H
NO2
+
H
NO2
H
The σ -complex intermediates for o- and p-substitution are destabilized due to unfavourable charge interactions in their
resonance structures. This is reflected in the activation energy for the formation of these complexes (the Eas are higher
than that of the m-substitution σ -complex) and they are formed more slowly. ANY SUBSTITUENT WHICH
DESTABILIZES
AN
ADJACENT
POSITIVE
CHARGE
WILL
BE
A
META-DIRECTOR.
Substituents with a lone pair of electrons on the atom attached to the ring are a special case. The lone
pair can be used very effectively to stabilize the positive charge. An additional resonance structure can
be written:
: OH
: OH
:
:
:
:
:
:O H
: OH
OH
OH
+
+
+
H
E+
H
E
H
E
Examples of this type of substituent:
:
NR2
:
NHR
O
H
: :
OR
H
E
E
O
:
NH2
OH
N C R
: :
: :
: :
:
: :
: :
:
: :
O C R
Halide substituents have three lone pairs of electrons and are capable of using them to stabilize an
adjacent carbocation. As such they are also ortho- para- directing groups. But they are different than all
other ortho- para- directing groups in one
key respect, as we
Cl
I
F
Br
shall see.
:
:
Z
:
Z=
E
Directing Groups and Reaction Rate
We have seen how substituents can be placed into two classes: those that
are ortho/para-directors and those that are meta-directors. What about the
effect of substituents on the rate of reaction?
We made the following observations:
1.) Benzenes substituted with ortho/para-directors will react more quickly
than benzene itself UNLESS THE SUBSTITUENT IS A HALIDE
(F, Cl, Br, I).
2.) Benzenes substituted with meta-directors ALWAYS REACT MORE
SLOWLY THAN BENZENE ITSELF.
Why?
Recall that these substitutions are ELECTROPHILIC, i.e. the attacking
species are attracted by the large amount of electron density circulating in the
aromatic ring.
Alkyl groups are "electron releasing" or "electron donating".
This is how they are able to stabilize adjacent positive
CH
+ 3 charges in EAS reactions and in other carbocation species.
(Recall that 3° carbocations are more stable than 2°
carbocations because they bear more alkyl substituents.)
Substituents with lone pairs on them can delocalize an electron pair into the
aromatic ring thus dramatically increasing the electron density. This is
reflected in the resonance structures for phenol.
:
:
:
:
+ : OH
+ OH
+ OH
_
+ OH
_
_
In both these cases, electron density is donated by the substituent into the
aromatic ring. Substituents that are capable of doing this are called
ELECTRON DONATING GROUPS. By donating electron density into the
aromatic ring they increase the charge density of the aromatic cloud making
it more attractive to attacking electrophiles. These groups are therefore said
to be ACTIVATING GROUPS as well as being ortho/para directors.
Meta-Directing Groups
These are groups which destabilize an adjacent positive charge, usually because the atom
attached to the aromatic ring itself carries a formal positive charge, or is positively
polarized by virtue of being attached to more electronegative atom(s). Examples (only the
destabilized resonance structure of the σ-complex is shown):
_
O
H
O
_
O
O
N+
N+
+
+
E
H
H
H
: NH2
H
N+
H
H
H 3O +
+
E+
E
H
N+
H
E
Protonation of the amino group ties up the lone pair that
could stabilize the positive charge. The N atom now bears a
+ve charge making the NH3+ group a meta-director.
In the the nitro group
there is always a +ve
formal charge on N.
_
+
H
O
H
C+
C
+
+
E
H
:O
H
:
: :
:O
H
C
E
H
E
:
:
ANY KIND of carbonyl attached to an aromatic ring will be a meta-directing group because of the
positive polarization of the carbon (as seen in the left-hand resonance structure. Delocalizing the
+ve charge (onto the O atom) does not help in this case. O is very electronegative and abhores to be
electron deficient (it has only six electrons in the right-hand resonance structure).
_
:N
N
All of the following are meta-directors:
O
C+
C
O
Amides
Esters
C
O
C
+
NR2
+
OR
C
O
OH
O
Carboxylic Acids
C
C
R
H E
H E
Cl
Aldehydes
Acid
For similar reasons, nitrile substituents
Ketones
Halides
are also meta-directors.
What about meta-directors?
These groups destabilize adjacent positive charges because they themselves
are electron deficient and/or bear formal positive charges.
_
O + O
Meta-directors are electron withdrawing groups. They suck
N
electron density away from the aromatic electron cloud
making it less attractive to electrophiles. This slows the
reaction between the electrophile and the aromatic ring as a
result.
Thus, meta-directors are said to be
DEACTIVATING.
+
What about the halides?
Well, the halides strike a unique balance. They are ortho/para-directors because
their lone pairs are capable of stabilizing an adjacent positive charge in a π -type
of interaction. At the same time, the halogens are electronegative enough to
diminish the electron density of the ring through the σ-bond..
:
: Cl :
+
Directing
influence
o,p
o,p
o,p
o,p
m
m
Halides are considered to be electron-withdrawing groups.
They are unique among substituent types in that they are
ORTHO/PARA DIRECTING AND DEACTIVATING.
Summary
Substituent
-NH2 , -NHR, -NR2 (amino and aminoalkyl),
-OH (hydroxy)
Effect on Rate
Very Strongly
Activating
-NHCOR (amide), -OR (ether),
-OCOR (ester - O end)
Strongly
Activating
R (alkyl), Aryl, Vinyl
Activating
X = F, Cl, Br, I (Halides)
-CHO (aldehyde), -COR (ketone),
-CO2H (carboxylic acid),
-CO2R (ester), -COCl (acid halide), -CN (nitrile),
-SO3H (sulfonic acid)
-NO2 (nitro)
Deactivating
Strongly
Deactivating
Very Strongly
Deactivating