Organic chemistry

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

Aromatic, Anti-aromatic & non-aromatic compounds :

Before discussing about the Aromatic, Anti-aromatic & non-aromatic compounds in this session, you have to recollect the some following stuffs.We have already discussed about,

What is Aromaticity ?

What is Anti-Aromaticity ?

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

1.It must be Planer Molecule/ion
2.It must be 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 number of π electrons present.

Non-aromatic compounds :

This is different from 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 is aromatic ?

non-aromatic compounds example

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

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

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

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

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

An Excellent example for this is Cyclooctatetraene

Aromatic Anti-aromatic & non-aromatic compound

It looks like a perfect student to be named as 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 behaviour to avoid Anti-aromaticity. Actually the Cyclooctatetraene is not a complete Planer molecule. Due to strain in the ring, it alter its structure itself and the losses its planarity property

cyclooctatetraene ring structure, cyclooctatetraene struture

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

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

Cyclooctatetraene-reacton

An another good example is [10]-Annulene, Which has 10 pi electrons and obey the 4n+2 rule (aromatic rule). The actually molecule not 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, if we attach those two carbon which has space issues for their hydrogens. We will get Aromatic compound so called Naphthalene

Aromatic Anti-aromatic & non-aromatic compound

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

Notice :  That 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 same like 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

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