Question

among these aromatic compounds the correct order of resonance energy per ring is:

A.$I>II>III$
B.$III>II>I$
C.$III>I>II$
D.$II>I>III$

Answer 1

Hint: The resonance energy is defined as the difference between the electronic energy level of a real molecule and hypothetical kekule structure. It is a measure of extra stability of the conjugated system compared to the corresponding number of isolated double bonds. But resonance energy per ring in a fused system may differ than the expected.

Complete Step by step answer: The resonance energy in aromatic compounds is due to the double bond which are in conjugation with each other and when this conjugation occurs the pi electrons move, some amount of energy is released. More the resonance energy evolved, less will be the aromatic system’s energy and hence more stable is the system.
The resonance energy of a benzene ring is $36KJmo{l}^{-1}$ . Resonance energy of naphthalene is $61KJmo{l}^{-1}$ and at last resonance energy of anthracene is $84KJmo{l}^{-1}$ .
Now when we talk about the resonance energy of naphthalene it is $61KJmo{l}^{-1}$ and that of benzene is $36KJmo{l}^{-1}$ . now subtract resonance energy of benzene from naphthalene we get only $25KJmo{l}^{-1}$ , it means the pi electron distribution is not equal over both the benzene ring and also you can clearly see from the structure of naphthalene, one double bond is shared between the two fused ring. So per ring energy has been reduced
Similarly in case of anthracene, Resonance energy of benzene is $36KJmo{l}^{-1}$ , if we gave 3 benzene ring then it should have $(3\times 36)KJmo{l}^{-1}=108KJmo{l}^{-1}$ energy. But this is not the case. It only has $84KJmo{l}^{-1}$ energy. So per ring the energy has decreased.
We can say as the number of rings increases the resonance energy per ring decreases.
Hence the resonance energy per ring for benzene is maximum and then for naphthalene and at last anthracene.

The correct option will be A. benzene > naphthalene > anthracene.

Note: As the energy increases the stability of the system decreases and as a result of this that system becomes more reactive. Therefore the polycyclic fused aromatic compounds are less stable and more reactive than benzene.