Question

Knowing the value of the hydrogenation enthalpy of 1,2,3,4,4a,5,8,8a-octahydronaphthalene (-120 kJ/mol) and the value of enthalpy of hydrogenation of naphthalene (-350 kJ/mol) calculate the magnitude of resonance energy for naphthalene in kJ/mol.

Answer 1

230 kJ/mol

Answer 2

1. The resonance energy is defined as the difference in hydrogenation enthalpy between a hypothetical localized (non-aromatic) system and the actual aromatic compound.

2. For naphthalene, the full hydrogenation releases –350 kJ/mol.

3. The corresponding hypothetical non-aromatic (localized) compound, 1,2,3,4,4a,5,8,8a‑octahydronaphthalene, hydrogenates with an enthalpy of –120 kJ/mol.

4. Therefore, the resonance energy is given by the difference:
     Resonance energy = |(–350) – (–120)| = |–350 + 120| = 230 kJ/mol.

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Minimal Explanation:
Difference in hydrogenation enthalpies: 350 kJ/mol – 120 kJ/mol = 230 kJ/mol → Resonance energy is 230 kJ/mol.