“There is no exception to the rule, that every rule has an exception.”- James Thurber.
After learning all about aromaticity and antiaromaticity, we now proceed further to study what are the exceptions to these rules . There are many compounds which show aromatic behaviour but don’t necessarily follow the rules we set for aromaticity. We have already studied one such class of compounds – the fullerenes , which are not planar, yet aromatic. We shall learn some more exceptions in this post.
In 1959, it was Saul Winstein ,a Canadian chemist , who discovered a new class of compounds while studying tris- homocyclopropenyl cation. These compounds are referred to as homoaromatic compounds.
These compounds are aromatic however they defy the condition of aromatic compounds where they need to be planar.
Generally, the carbon atoms in aromatic compounds are sp2 hybridized. Thus, they have a triagonal planar geometry and continuous p-orbital overlap. However, in homoaromatic compounds there is one sp3 hybridized carbon atom , which disrupts the continuous overlap of p-orbitals. In spite of this discontinuity of p-orbital overlap, these compounds show aromatic behaviour. In short, these compounds form a stabilized cyclic conjugated system by bypassing that one sp3 atom !
Examples of Homoaromatic compounds.
Example 1 – Cyclooctatrienyl ion or homotropylium cation(C6H9+).
The best example to study homoaromaticity is the cyclooctatrienyl ion or homotropylium cation(C6H9+) –
When cyclooctatetraene is dissolved in sulphuric acid, a proton adds to one of the double bonds to form cyclooctatrienyl cation (As one double bond is less in the cation, the ‘-tetraene ‘ becomes ‘-trienyl’ ).
The cation can be represented as follows –
The sp3 carbon atom is not in the plane of the ring-
In the NMR spectrum of this cation, the Ha and Hb protons show sharply different chemical shifts.
Ha proton → on the side of the ring, just above the ring →absorbs 5.8ppm upfield of Hb → this indicates existence of a ring current.
In order for the orbitals to overlap effectively , the sp3 carbon atom is forced to lie almost vertical above the plane of the aromatic ring. As these two protons have different signals it is understood that they are not the same and thus there is a substantial barrier for the conformational process(will learn more about conformers when we study stereochemistry) that interchanges Ha with Hb. Normally conformational change involves energy on the order of 2-4 kcal/mol. However, the ΔG for this process is 22.3kcal/mol (this is a lot of energy for conformational change !).This means that, due to the energy barrier, the two protons cannot be interchanged easily or we can conclude that only if the molecule gets 22.3kcal/mol energy , it can exchange Ha with Hb .
Example 2 – Cyclobutenyl cation.
In the above molecule, the methylene(-CH2) carbon atom , which is above the plane of the ring , is sp3 hybridized. This cation can be synthesized by treating 3-acetoxycyclobutene with a superacid (an acid stronger than conc. H2SO4).
As seen above, even in this molecule the two methylene H‘s are NOT interchangeable. Ha is on the side of the aromatic ring and Hb is not. The energy barrier for conformational change here is 8.4 kcal/mol (still very high compared to normal 2-3kcal.mol value).
Other systems which do not strictly follow the definition of aromaticity, but are aromatic in nature are –
- Möbius aromatic systems – These have 4n π electrons and yet they are aromatic. Here large cyclic [4n]annulenes are stabilised by twisting of the p- orbitals in a mobius strip. The twisting results in one point where there is a phase reversal i.e a node is formed.
Hückel Orbital Array
Mobius Orbital Array
4n+2 = Aromatic
4n = Antiaromatic
4n = Aromatic
4n+2 = Antiaromatic
- Fullerenes – Spherically aromatic structures , which are not planar.
- Cyclopropane – This molecule displays aromaticity in σ orbitals rather than π bonds.
- Ferrocene – This inorganic complex exhibits aromaticity in 3 dimension. It has two cyclopentadienyl rings on opposite sides of an central iron atom.With this post we end our discussion on the topic aromaticity. In the next post we will try to solve some problems on aromaticity. Till then,
Be a perpetual student of life and keep learning….
Good day !
References and further reading –
- Advanced organic Chemistry by Carey & Sundberg 4th edition.