Ch 11 : Aromaticity

Chapter 11 : Arenes and Aromaticity

Aromaticity Criteria

Qu: How can we recognise systems that are aromatic ?
Ans: By applying the criteria for aromaticity outlined below.

Based on the properties of aromatic compounds, there are FOUR criteria that are applied to the π system (not the molecule as a whole) that need to be met in order for the "special" aromatic stabilisation to be observed:

Cyclic : linear or acyclic π systems are not aromatic, all the atoms in the ring must be involved in the π system (i.e. no sp³ atoms).
Planar : if the ring is planar (i.e. flat) then this means there is good overlap / interaction between the adjacent "p" orbitals....not always as easy to consider as it might seem!
Conjugated : there needs to one "p" orbital from each atom in the ring, so each atom must be either sp² or sp hybridised.
The Huckel Rule..... 4n+2 π electrons in the cyclic conjugated π system (n = 0, 1, 2, 3 etc.) This is equivalent to an odd number of π-electrons pairs).

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

Study Tips:
One way to think about the first 3 criteria is to compare the p orbitals of the structure to "fence posts" surrounding a field. The posts need to be regularly spaced (i.e. no gaps), all around the field with all the posts standing upright in order to have an effective fence.

The Huckel rule....just think of "n" as an integer 0, 1, 2, 3... etc. it's just a number to feed into the simple formula to generate a set of numbers that correspond to the numbers of π electrons in aromatic systems : Those numbers are 2, 6, 10, 14 etc. π electrons (note that these numbers equate to 1, 3, 5, 7 etc. π electron pairs). So, if a cyclic, conjugated, planar π system contains one of these numbers of π electrons, then it's an aromatic system. For example, benzene is a 6 π electron system. "n" does not really relate to a simple structural property in the molecule. A simpler way to think of the Huckel rule is that it requires an odd number of π-electrons pairs.

The most important and well known aromatic system is benzene. It consists of a set of 6 sp² C arranged in a ring. This creates a set of 6 p orbitals, no gaps in the π system so we have a cyclic, conjugated π system. The planarity of the ring means that the interaction (orbital overlap) of the adjacent p orbitals is optimal. The 3 C=C in benzene mean that we have 3 pairs of π electrons = 6 π electrons = a 4n+2 number (a Huckel number) where n=1.

Benzene

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

Each atom in the cyclic, conjugated system must contribute a p orbital to the π system.
Each atom in the cyclic, conjugaterd system can only contribute one p orbital and hence a maximum of one pair of electrons (i.e. only one p orbital can be involved on any particular atom).
Each atom can only be "counted" once (afterall, there is one p orbital in the valence shell of the appropriate orientation).
Each p orbital can contribute 0, 1 or 2 electrons... (0 if the p orbital is empty, 1 if it is involved in a π bond or 2 if it is a lone pair).
Refining "your "predicted" hybridisation for heteroatoms (esp. O, N and S) will allow lone pairs to be part of the π system.
Carbocations and boron atoms (C⁺ & B) are sp² hybridised and have empty "p" orbitals. This allows them to be part of the conjugated π system but they contribute NO electrons.

What about the terms "non-aromatic" and "anti-aromatic" ?

If a system satisfies all of the first three criteria (i.e. it has a cyclic, planar and conjugated π system), but has 4n π electrons in the cyclic conjugated π system (n = 0, 1, 2, 3 etc. which is equivalent to an even number of π-electrons pairs), then the system will be anti-aromatic. Note that anti-aromatic compounds are rare because they tend to be highly unstable.

If a system fails to satisfy at least one of the first three criteria (has a cyclic, planar and conjugated π system), then the system will be non-aromatic.
Similarly, systems with odd numbers of π electrons in the cyclic conjugated π system (i.e. not 4n+2 or 4n systems) are typically non-aromatic.