I know that to find the number of hyperconjugative structures in aliphatic systems we use the formula $a + 1$ (where $a$ is the number of α-hydrogens in that compound)

However, my teacher answered to the above question as:

No, this formula is not valid for aromatic systems.

Why is it so? What is the exact formula? Is it that we don't count its original structure as a hyperconjugative structure?

  • $\begingroup$ Because of the aromaticity which occurs in cyclic system. It means that you have already two limiting structures in which hydrogen are alpha-hydrogens (respect to the double bound which is once at left and once at the right). $\endgroup$ – Alchimista May 21 '19 at 7:37
  • $\begingroup$ @Alchimista alright, but then what's the exact formula? $\endgroup$ – Ishi May 21 '19 at 7:46
  • $\begingroup$ What such a formula should be useful for? Which already is not that clear for aliphatic molecules, at least to me. One should analyse case by case and keep in mind is thinking of limiting structure, the weight of which might be even unrelated to their number. Understand what mesomers are and forget formulae to count things existing only within a precise frame. Anyway, I would say that for toluene one can draw 2 x the three structures, so 2n. $\endgroup$ – Alchimista May 21 '19 at 10:36
  • $\begingroup$ @Alchimista okay, I will keep that in mind, thankyou :) $\endgroup$ – Ishi May 21 '19 at 10:50

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