arrange benzene, naphthalene and anthracene in increasing order of reactivity giving reasons.
Answers
The resonance energy for phenanthrene is 92 Kcal/mol, that for anthracene is 84 Kcal/mol and for naphthalene and benzene rings are 61 and 36 Kcal/mol respectively.
Stability can be compared only for isomeric or related compounds or at best for unsaturated hydrocarbons it is compared only when they give same hydrogenated products. However, for polycyclic aromatic hydrocarbons, stability can be said to be proportional to resonance energy per benzene rings.
Here resonance energy per benzene ring decreases from 36 Kcal/mol for benzene to 30.5 Kcal/mol for naphthalene, 30.3 Kcal/mol for phenanthene and 28Kcal/mol for anthracene.
Hence, order of stability (or RE): Benzene > Phenanthrene ~ Naphthalene > Anthracene. In fact other fused polycyclic aromatic hydrocarbons react faster than benzene.
Note- Phenanthrene
Halogens like Cl2 or Br2 also add to phenanthrene. In phenanthrene, C9-C10 has 4/5 double bond character hence it is shorter than C1–C2.

Addition of Br2 to C9–C10 results in a biphenyl compound hence leave two benzene rings intact hence loss of resonance energy is minimum. The addition to terminal rings gives a naphthalene product with less
The reactivity order is as follows : benzene <naphthalene< anthracene.
- The difference in between the energy of one of the most stable mesomeric structure and a said structure of the same compound is called mesomeric stabilisation energy (MSE).
- The stability of cyclic compounds is judged by its MSE. The more the MSE, the more stable a compound is.
- The mesomeric(resonance) energy of benzene is the highest as it has one benzene ring followed by naphthalene which has two benzene rings and anthracene which has three benzene rings.
- Thus benzene is most stable followed by naphthalene and anthracene.
- The way a compound reacts is inversely proportional to stability.
- Hence, the order of reactivity will be benzene <naphthalene< anthracene.