How to Identify aromatic anti-aromatic & non-aromatic compounds ?

Aromatic, Anti-aromatic & non-aromatic compounds:

Before discussing the Aromatic, Anti-aromatic & non-aromatic compounds in this session, you have to recollect the following kinds of stuff. We have already discussed this,

What is Aromaticity?

What is Anti-Aromaticity?

So Huckel’s Rule for Both Aromatic and Anti-Aromatic is the same but only differs in formula (4n+2)&(4n). Other criteria 1,2,3 are the same for both.

1. It must be Planer Molecule/ion
2. It must be a cyclic molecule/ion
3. It shouldn’t have any sp3 carbon in the ring
Aromatic Anti-aromatic & non-aromatic compound

In short words, we can simply say that the aromatic compound and anti-aromatic compounds same but difference in the number of π electrons present.

Non-aromatic compounds :

This is different from the aromatic/anti-aromatic that we have discussed. These are Non-aromatic or Not-aromatic compounds. As the name itself says non-aromatic compounds have really nothing to do with aromaticity.

For example: Look,

Is this molecule aromatic?

non-aromatic compounds example

You’d be like, how this molecule will be considered aromatic !!

So that’s the matter behind this, Non Aromatic compounds are ones that are not related to aromatic & anti-aromatic compounds.

These compounds are being already failed in those three criteria (cyclic, conjugated, planer).

In reality, if Any of the criteria like Cyclic, planer, or fully conjugated is not obeyed means that the molecule is said to be a Non-Aromatic Compound.

Sometimes, it may be molecules look aromatic or anti-aromatic based on the factors mentioned above but the case actually is different.

An excellent example of this is Cyclooctatetraene

Aromatic Anti-aromatic & non-aromatic compound

It looks like a perfect student to be named as an Anti-aromatic compound – [cyclic, planer, fully conjugated & 8 pi electrons] – [obeys 4n rule]

earlier I said that ‘the case is different.

Cyclooctatetraene is showing some extra behavior to avoid Anti-aromaticity. Actually, Cyclooctatetraene is not a complete Planer molecule. Due to strain in the ring, it alters its structure itself and losses its planarity property

cyclooctatetraene ring structure, cyclooctatetraene struture

And another great evidence for this molecule shows a non-aromatic property is,

The pi bonds present in the ring are actually not in resonance because it undergoes an Addition reaction with bromination (electrophilic addition reactions) like a normal alkene system.

Cyclooctatetraene-reacton

Another good example is [10]-Annulene, Which has 10 pi electrons and obeys the 4n+2 rule (aromatic rule). The actual molecule is not a planer in reality.

[10]-annulene structure
10-annulene

However, it is non-aromatic because it do not have space for two inner hydrogens. It cannot adopt planer property

To make 10 carbon system with aromatic property, we attach two carbon which has space issues for their hydrogens. We will get an Aromatic compound so-called Naphthalene

Aromatic Anti-aromatic & non-aromatic compound

This 10-Annulene system has planarity and it possesses Aromaticity (4n+2)

Notice: A lot of compounds can be nonaromatic because they fall out of the aromatic-antiaromatic character as soon as only one of the criteria –cyclic, planar, fully conjugated is not obeyed.

The antiaromatic, on the other side, is very specific (and the same as aromatic) – it must meet all these criteria but instead of having 4n+2 electrons, it has 4n. And this makes anti-aromatic compounds very rare because that specific combination is energetically unfavorable