Chapter 17 - Benzene and Aromatic compounds
Benzene is the simplest aromatic hydrocarbon (arene).
Nomenclature: we will not discuss nomenclature of benzene derivatives
in class but you are responsible for learning how to name these types of
compounds.
Benzene has 4 degrees of unsaturated hence is highly unsaturated.
However, remarkable changes in reaction chemistry occur when the atoms
of the ring are fully conjugated. Thus, whereas unsaturated hydrocarbons,
such as alkenes, alkynes, and dienes readily undergo addition reactions,
benzene does not. For example,
HBr
H2/Pt
R2C
CR2
no reaction
very slowly formed
no reaction
Diels-Alder
Clearly, benzene is not a triene!
In fact, a comparison of the energy of benzene with that of 1,3,5hexatriene (which, like benzene, contains 3 double bonds but is an acyclic
molecule) shows that the ring structure is substantially lower in energy
(by 18 kcal/mol).
The remarkable stability of benzene can be explained by the strong
stabilization resulting from its aromatic character, which involves
delocalization of electrons around the ring. Experimentally, the magnitude
of delocalization energy, otherwise referred to as resonance
stabilization (~36 kcal/mol), can be quantitatively determined by
measuring heats of hydrogenation – see page 615
Aromaticity
Aromaticity provides the benefits of high stability, etc. But what is
required for a molecule to be aromatic? There are some simple rules:
1) The molecule must be cyclic
2) This cycle must be fully conjugated
3) The cycle must be planar – maximizes orbital overlap
4) The electrons must be able to “circulate”- hence must be delocalized
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5) The conjugated cycle must contain 4n+2 π electrons (Huckel’s rule),
where n = 0,1,2,3,4,.....
If the conjugated cycle has only 4n π electrons, it is anti-aromatic, and will
either be highly reactive, or will distort in order to violate one of the other
rules (1-4).
The Structure of Benzene, C 6 H 6
- benzene has a regular hexagonal structure with uniform C-C bond
lengths of 1.39 Å (does not contain alternating single and double
bonds!)
Let’s look at the MO diagram for benzene: we start with six 2p orbitals
hence we get three bonding and three antibonding MO’s – Figure 17.9.
In drawing M.O. diagrams for benzene, the first, most obvious factor is
that while it is possible to draw a diagram where there are no nodes, it is
not possible to draw a diagram where there is only ONE node! This
restriction against having an odd number of nodes (1, 3, or 5) is due to
the cyclic nature of benzene, and the high degree of symmetry involved.
Note that there are two possibilities for MO’s with 2 or 4 nodes. These
MO energy levels have the same energy hence are called degenerate.
Finally, only one MO energy level that has 6 nodes can be drawn.
Thus, there are six MO energy levels, of which two pairs are degenerate;
since there are six 2p electrons, they occupy the 3 lowest energy,
bonding MO’s.
Note that the HOMO is comprised of two degenerate orbitals, as is the
LUMO.
The inscribed polygon (also called a Frost Circle) method can be used to
find the MO energy level diagram for planar, cyclic, fully conjugated
molecules like benzene: the appropriate polygon (same size as the ring) is
inscribed, vertex down, in a circle of radius 2β with the nonbonding line
dividing the circle exactly in half; intersections of the ring with the circle
will mark the positions of the molecular orbitals – see Page 628
We find that:
(1) For conjugated cycles with an EVEN number of atoms, there will be a
single high energy level, and a single low energy level.
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For conjugated cycles with an ODD number of atoms, there will be a single
low energy level, and a degenerate (i.e. double) high energy level – see
page 629
(2) There are as many MO energy levels as there are atoms, and the rest
come as degenerate pairs!
(3) The “nonbonding” energy level runs through the middle of the
diagram.
Other Examples of aromatic compounds:
i) pyridine, furan, pyrrole, thiophene, etc.
ii) cyclopentadienyl anion; cycloheptatrienyl cation (C7H7+, tropylium ion):
cyclopentadiene has a pKa of 15 and is 20 orders of magnitude more
acidic than pentadiene; cycloheptatrienyl cation is remarkably stable
compared to other carbocations; its salt appears to be indefinitely stable
under normal conditions
Let’s consider the MO diagram for the cyclopentadienyl anion (Cp-) –
page 629 (odd number of atoms):
1) a single low-energy level, a degenerate high-energy level.
2) 5 carbons, minus the two high-energy levels and the one low energy
level leaves TWO, which must come as a degenerate pair.
3) The nonbonding level runs through the middle of the diagram.
4) The six electrons fully occupy the set of 3 bonding MO’s and no
antibonding or nonbonding MO’s are occupied, as in benzene.
iii) Polycyclic aromatic hydrocarbons – which result from “fusing” benzene
rings together – are also aromatic. Examples include: naphthalene,
anthracene, and pyrene
It is also possible to fuse rings of different sizes together – you should
think about whether these would be aromatic or not! Think of C60, is it
aromatic?
Examples of anti-aromatic compounds:
i) Cyclobutadiene (4π π electrons), on attempted preparation, rapidly
dimerizes to give a nonconjugated molecule:
(what sort of reaction is this?)
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When a cyclobutadiene derivative that was too sterically hindered to
react was made, it was discovered that the cycle had distorted from the
square arrangement to a rectangular one– in this instance, by strongly
localizing the bonds, and violating rule 4.
What does this distortion do to help out with the half-filled shell? If we
look at the orbital energy levels, we see that the distortion removes some
of the orbital degeneracy (by lowering the molecule’s symmetry). Both
electrons populate the lower energy level:
Notice that this leaves a HOMO and LUMO that are VERY close in energy –
highly conjugated antiaromatic cycles which distort in this way are often
highly colored, because of the closeness of these two energy levels (See
discussion of UV-Vis spectroscopy, Chpt 16).
ii) Cyclooctatetraene:
This molecule contains 8 (4n where n=2) electrons, and is antiaromatic.
Let’s consider the MO diagram for cyclooctatetraene (COT) – draw frost
circe (remember, even number of carbons):
1) a single lowest energy level, a single highest energy level.
2) That leaves 6 more orbitals - arranged as degenerate pairs.
3) Filling in the 8 electrons from the bottom up leaves the nonbonding HOMO
half-filled.
In order to avoid this high energy situation and minimize both angle strain
and torsional strain, it distorts by bending (or folding) into a tub-shape:
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We’re now violating rule #3 (and 4), and thus avoiding the aromaticity
question altogether.
Is there a way to make anti-aromatic molecules aromatic? Of course simply by adding or removing electrons! For example, if cyclooctatetraene
is reduced (electrochemically, or with a metal like K), it easily picks up
two electrons. The resulting dianion is planar, and aromatic (10 πelectrons). Similarly, cyclopentadiene (non-aromatic) is very easily
deprotonated to form cyclopentadienyl anion (6 π-electrons).
Annulenes:
Aromatic systems that are not comprised of benzene rings (often called
non-benzenoid aromatics) are a fascinating area of chemistry. These
large-ring compounds are called annulenes. One of the most significant
members of this family is [18]annulene (the number in brackets denotes
the number of carbons in the ring and, usually, the number of π
electrons):
[18]annulene is a planar, fully aromatic compound. The ring is just large
enough that the hydrogens sticking into the middle of the ring do not
interact to twist the molecule out of planarity. In order to avoid these
types of interactions, we also study dehydro annulenes (like
hexadehydro[18]annulene, above). These molecules have all internal
hydrogens removed, and in this case, also form aromatic systems. Antiaromatic annulenes also exist, such as the dehydro[12]annulene shown
below. These compounds are usually easily reduced by two electrons to
form aromatic ions.
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