Annulenes are completed conjugated monocyclic non-benzenoid hydrocarbons, with a general formula –
CnHn ,when n is an even number
CnHn+1 , when n is an odd number.
They are named as [n]annulene , where n is the number of carbon atoms in the ring.
e.g.- Annulene with 12 carbon atoms will be written as annulene.
Annulenes can be aromatic , non-aromatic or anti-aromatic. As seen in post 120 , annulene i.e cyclobutadiene is anti-aromatic (We will discuss ‘anti-aromaticity‘ concept in the upcoming posts). We also know that annulene i.e benzene is aromatic.
annulene is NOT aromatic. Why?
1) It has 8π electrons, which is NOT a Hückel’s number.
2) X-ray structure determination shows that this molecule has a ‘tub’ conformation (spatial arrangements of atoms) , in which all atoms are NOT in the same plane. 4 C atoms are in the plane and 4 are above that plane as shown in the figure below. Thus, we do not have continuous overlapping of p-orbitals – the structure is NOT planar and thus, the π electrons are NOT delocalised.
For a normal polyene, the bond lengths are different for single and double bonds as there is no resonance. If there is resonance, then we get intermediate bond lengths – somewhere between that of a single and double bond.
For this molecule the calculated bond lengths were as follows –
C-C bond length = 1.462 Å
C=C bond length = 1.33 Å
As the bond lengths for single and double bonds are different, we can conclude that there is no resonance in this molecule. Thus, there is no conjugation between single and double bonds.
What about annulenes?
The following structures are all  annulenes. They all have 10 carbon atoms in their structure –
In structure (1) , the two internal H‘s would be in very close proximity to each other if the structure was planar. This is create a lot of steric strain. Thus ,this kind of closeness between two H‘s or any two groups is not favored. To avoid this, one ring bends and the structure is no longer planar. A pre-requisite to a molecule being aromatic is that it has to be planar. In this case, as the structure is not planar , the structure (1) is not aromatic.
Structures (2) and (3) and (4) are called the heart, boat and twist conformers respectively. These are not planar too and thus not aromatic. Structure (5) cannot exist as a planar structure as well , so it is NOT aromatic too. (In later posts we will study the E and Z isomers in detail. For now, just know that there are 5 double bonds and so there are 5 E/Z types of notation shown in the figure above).
Structure (6) is a bridgehead compound. The bridge head carbon atom is above the plane of the ring and is attached to the ring at the points of ring fusion. As (-CH2-)group is called a methylene group, this bridge can also be referred to as a methylene bridge. This bridge keeps the two rings in the same plane – not allowing them to bend. Thus, this structure is planar. It has 10 π(4n+2, n= 2) electrons and conjugated double bonds. Thus, this molecule is aromatic.
We will continue our discussion on annulenes in the next post too. Till then ,
Be a perpetual student of life and keep learning….
Good day !
References and further reading –
1.Advanced Organic Chemistry. PART A- Structure and mechanisms by Carey, Francis A., Sundberg, Richard J.