In the early days of chemistry when Every scientist was after the compound which today is called the mother compound of the organic chemistry BENZENE...

The fact that benzene has six carbons in its molecule suggested many compoundes with chains. But not one of them suggested the one that we know today. Thats because a six carbon ring is highly concerned about the angle between the bonds. This highly stabilizes or destabilizes a compound..

There were so many structures suggested but no one cud match all the properties untill the one that we today know fiinally proposed..

So this structure brought with it a very strange and a new theory with it called RESONANCE. This effect actually stabilizes a compound to much larger extend than any other effect that you will have to study in your syllabus for JEE. I dont know if holds true after JEE or not but yeah till you give your JEE this is true.

For a compound to be blessed by this effect it has to follow some rules which are:

1. The compund should be planar

2. The copmpund should possess conjugated double or sometimes triple bond(pie electrons to be more general)

3. The conjugated pie electrons should be confined within a cycle

If a compound follows all these rules then and only then it is resonance stabilized, else its not.

Keep in mind the first rule. thats what JEE asks its questions upon if its really about the basics of resonance..

best of luck

Hi

Resonance energy( Reason for stability)

Resonance hybrids are always more stable than any of the canonical structures would be, if they existed. The delocalization of the electrons lowers the orbital energies, imparting this stability. The resonance in benzene gives rise to the property of aromaticity. The gain in stability of the resonance hybrid over the most stable of the (non-existent) canonical structures is called the resonance energy.

A canonical structure that is lower in energy makes a relatively greater contribution to the resonance hybrid, or the actual picture of the molecule. In fact, resonance energy, and consequently stability, increase with the number of canonical structures possible, especially when these (non-existent) structures are equal in energy.

Resonance energy of a conjugated system can be 'measured' by heat of hydrogenation of the molecule. Consider the example of benzene. The energy required to hydrogenate an isolated π-bond is around 28.6 kcal/mol (120 kJ/mol). Thus, according to the VB picture of benzene (having three π-bonds), the complete hydrogenation of benzene should require 85.8 kcal/mol (360 kJ/mol). However, the experimental heat of hydrogenation of benzene is around 49.8 kcal/mol (210 kJ/mol). The difference of 36 kcal/mol (150 kJ/mol) can be looked upon as a measure of resonance energy. One must bear in mind again that resonance structures have no physical existence. So, even though the term 'resonance energy' is quite meaningless, it offers an insight into how different the VB picture of a molecule is from the actual molecule itself. The resonance energy can be used to calculate electronegativities on the Pauling scale.