Valence Bond Theory And Resonance

A C-C single bond typically has a bond length of 1.54 Å and a C=C double bond has a bond length of 1.34 Å. In benzene, all six C-C bond lengths are experimentally found to be 1.40 Å, which falls exactly between these two values. This uniform, intermediate bond length cannot be explained by a single Kekule structure, which would predict alternating single and double bonds. Instead, the concept of resonance explains that benzene is a hybrid of two Kekule structures (and other contributors), with the pi electrons delocalised equally over all six carbon atoms. This delocalisation makes every C-C bond equivalent and partially double-bond in character, explaining the observed bond length of 1.40 Å. The inductive effect involves sigma bond electron shifts and does not account for pi electron distribution or bond length uniformity. Hyperconjugation is relevant in alkyl-substituted systems and involves sigma-pi electron delocalisation, not the inherent pi system of benzene itself. sp^3 hybridisation would give tetrahedral carbon with no pi system at all, which contradicts benzene's planar structure. This concept is central to NCERT Class 11 and JEE questions on aromaticity and resonance. Plausibility: the resonance energy of benzene (approximately 152 kJ/mol) directly supports extensive pi electron delocalisation and the resulting bond equalisation.