Aromatic compounds are the extra stable systems. Aromaticity is not restricted to benzene and its dervatives.

 

Properties: 

 

1. Extra Stable

 

2. Cyclic and planar molecules- The electron clouds lie in a plane

 

3. Undergo substitution rather than addition reation- Though they contain double bonds, unlike alkenes, they undergo substitution reactions.

 

 

1. Cyclic: The compound must be cyclic. Linear systems are not aromatic.

 

2. Planar: It should be planar so that there is maximum overlap of p-orbitals.

 

3. Huckel's rule: They must obey Huckel's rule which states that the number of  electrons should be (4n + 2). In other words, for n=0,1,2,3,4.... number of electrons will be 2,6,10,14....

 

4. Conjugation: Each C must be either sp² or sp hybridized. Usually alternate single double bonds are present. Please note that it is NOT necessary that double bonds be present alternately.

Conjugation helps in delocalization of the electrons. Thus an electron cloud-like thing is formed above and below the plane of the compound.

 

Now let us take examples,

 

1). 3-Carbon system-

 

  

 

It obeys Husckel's rule(has 2pi electrons), but it is still not aromatic because the pi electrons are not present in conjugation. one of the carbon is sp³ hybridized n thus doesn't have any empty free p-orbital for delocalization to take place. So it is NOT aromatic.

 

Now let us consider another 3membered compound,

 

Now, this compound follows all the rules. In this case, there are 2 electrons, it is planar, cyclic and all C are sp² hybridized. Due to the positive charge, there is an empty p orbital which allows delocalization of charge. You can draw resonating structures for this. Thus, it is aromatic.

 

2). 4-Carbon system

 

It contains 4 electrons n hence doesn't obey Huckel's rule. So it is not aromatic.

 

 

 

It also contains 4 electrons and is Not aromatic.

 

 

 

However, if we remove a proton from it, we get a system with 6 electrons. Now, it obeys all rules and becomes aromatic.

 

4). 6-Carbon system 

 

Benzene 

It obeys all rules ie., contains 6 electrons, planar, cyclic, conjugate double bonds. Thus it is Aromatic.

 

 

Now, this is a special case. Though it obeys Huckel's rule, contains 6 electrons, planar, cyclic, it is NOT aromatic. This is because one of the Carbons is sp³ hybridized which doesn't allow delocalization to take place.

 

Now if we remove one proton from the sp³ hybridized carbon, we get a cation which is known as:

 

This is Aromatic because all carbons are now sp² hybridized.