Aromaticity

Chapter 11 : Arenes and Aromaticity

Aromaticity is a property associated with the extra stability of certain types of p systems.  Fundamentally it arises from having the maximum number of electrons in the p bonding  molecular orbitals. As we will see, it is not restricted to benzene, substituted benzenes, 6-membered rings or just hydrocarbons.

The property of aromaticity is generally about a significant extra stability associated with a resonance delocalized structure.
This extra stability moderates the reactivity of these systems compared to alkenes and means aromatics can be considered as their own functional group.

The presence of aromatic systems can also influence the reactivity of functional groups directly attached to the aromatic system particularly in the benzylic position. Therefore it is important that you are able to recognise the aromatic systems.

Aromaticity

Aromaticity is a property associated with the extra stability of certain types of p systems due to the nature of the molecular orbitals. It is not restricted to substituted benzenes, 6-membered rings or just hydrocarbons.

Based on the properties of compounds, there are FOUR criteria about the p system that need to be met in order for the "special" aromatic stabilisation to be observed:

  1. Conjugated (there needs to one "p" orbital from each atom in the cycle, so each atom must be either sp2 or sp hybridised)
  2. Cyclic (Linear systems are not aromatic)
  3. Planar (so there is good overlap / interaction between the "p" orbitals.... not always easy to consider)
  4. The Huckel Rule..... 4n+2 p electrons ( this is equivalent to an odd number of p-electrons pairs) in the cyclic conjugated system.
Study Tip : One way to think about the first 3 criteria is to compare the p orbitals of the system to fence posts surrounding a field. The posts need to be regularly spaced, all around the field with all the posts standing upright in order to have an effective fence.

In order for a compound to be aromatic, all FOUR of these criteria must be met.

The most important and well known aromatic system is benzene:

Benzene
6 pi electron system so 4n+2 where n = 1
benzene pi system

To aid in counting the electrons the following factors may help:

  1. Each atom in the conjugated, cyclic system can only contribute a maximum of one pair of electrons (or one orbital)
  2. "Refining" your "predicted" hybridisation will allow lone pairs to be part of the p system.
  3. Carbocations and boron atoms (C+ & B) are sp2 hybridised and have empty "p" orbitals. This allows them to be part of the conjugated system but they contribute NO electrons.

Resonance Energy

The resonance energy of a compound is a measure of the extra stability of the conjugated system compared to the corresponding number of isolated double bonds.  This can be calculated from experimental measurements.
 

calculating the resonance energy from thermodynamic data The diagram shows the experimental heats of hydrogenation, DHh, for three molecules, benzene, 1,3-cyclohexadiene and cyclohexene. These are related in that under appropriate conditions they can all be reduced to the same product, cyclohexane.

The DHh for "cyclohexatriene", a hypothetical molecule in which the double bonds are assumed to be isolated from each other, is calculated to be 3 times the value for cyclohexene. This value reflects the energy we could expect to be released from 3 isolated C=C.

By comparing this value with the experimental value for benzene, we can conclude that benzene is 152 kJ or 36 kcal / mol more stable than the hypothetical system. This is the resonance energy for benzene.

What is the resonance energy of 1,3-cyclohexadiene ?

res. energy = (2 x 120) - 231 = 9 kJ / mol

 
In principle, resonance energies can be calculated for any p systems. The following table contains data on a selection of systems, and some comments about them in relation to benzene or about their aromaticity.
Resonance energies

Polyaromatic Hydrocarbons

Larger systems of benzene rings fused together are known. These are the polyaromatic hydrocarbons (sometimes referred PAHs). A collection of images of some common systems are shown. The planarity of these extended p systems can be seen when the CHIME images are rotated. Test out the Huckel rule on each of them and check by holding the mouse over the line drawing.
The chemistry of these systems is similar to that of the parent benzene, but they are usually more reactive than benzene. Evidence for this can be seen in the resonance energy data which shows that their resonance energies are less than the same number of isolated benzenes.

* Purine is a precursor to the DNA bases Adenine and Guanine.

Heteroaromatics

Aromatic compounds which contain heteroatoms (e.g. O, N, S) are part of the cyclic conjugated p system are called heteroaromatics.

The involvement of the heteroatom in the cyclic system requires that it provides a p-orbital to be part of the conjugated p system.
This can either be as part of a p bond in the ring, or a lone pair in a p-orbital to satisfy the criteria for aromaticity.
This implies that the heteroatom be sp2 hybridized.

Several of the simpler and more common heteroaromatics are shown below. In each case consider role of  heteroatom in the p system.  Is it part of a p bond or does it contribute a pair of electrons to the p system ?
Position your mouse over the line drawing to check.

* Purine is a precursor to the DNA bases Adenine and Guanine.

What about Other Neutral Hydrocarbons ?

The following table contains two hydrocarbons that have been notably absent so far even though both are cyclic and have alternating C=C and C-C, an contain one less or one more C=C than benzene.
Neither are aromatic since they fail to satisfy the 4n + 2  p electron Huckel rule. Infact they are examples of 4n p electron systems (an even number of  p electron pairs).
Such systems are quite unlike benzene: e.g. they lack the aromatic stability, the CC bonds are different lengths and cyclooctatetraene is not planar.
 

Compound Name
Line Drawing
3D Model
Resonance Energy
Cyclobutadiene
4 pi electron system, a 4n system
Cyclooctatetraene
8 pi electron system so 4n system
20 kJ/mol
5 kcal/mol

Implications

Aromaticity typically has a profound effect on the properties of systems. Here are a few examples that illustrate some important possibilities:
 
 

Electrophilic aromatic susbtitution Aromatics tend to undergo substitution and retain the aromatic unit rather than lose it as would be the case if electrophilic addition occurred as is the case with alkenes.
keto-enolo tautomerism The ketone form of the tautomerization is usually preferred. Not so here where the stability of the aromatic product makes the enol of the phenol form more favorable. 
Cyclopentadienyl anion Hydrocarbons are not usually very acidic (pKa > 50). But cyclopentadiene has a much lower pKa due to the aromatic stability of its aromatic conjugate base.
What other systems have a pKa = 16 ? Water (15.7), methanol and ethanol (16) ie. R-OH
Basicity of pyridine Pyridine is like benzene but a N has replaced one CH. The N atom is weakly basic since the lone pair is in an sp2 hybrid orbital.
Is the conjugate acid aromatic ? Yes, it is still a cyclic, planar, conjugated, 6 pi electron system.
Basicity of pyrrole Pyrrole is a much weaker base than pyridine (see above). This is because the lone pair on the N atom is already involved in the aromatic array of p electrons. Protonation results in loss of aromaticity and is therefore unfavorable.