Structure of Benzene
Benzene is an aromatic hydrocarbon with the molecular formula C₆H₆, indicating a high degree of unsaturation which does not align with the characteristics of alkanes, alkenes, or alkynes. Isolated by Michael Faraday in 1825, benzene was originally thought to have alternating single and double bonds, which would suggest a high reactivity.
Kekulé proposed that benzene consists of a cyclic arrangement of six carbon atoms with alternating double bonds, yielding the potential for two isomeric dibromobenzenes. However, with experimental evidence showing that benzene forms only a single monosubstituted product and exhibits an extraordinary stability, Kekulé's structure was challenged.
The modern understanding attributes benzene's stability to resonance, where benzene can be represented as a hybrid of multiple structures, leading to equal bond lengths between all carbon atoms in the ring. This structure features sp² hybridized carbons and delocalized π electrons, fulfilling Hückel's Rule of aromaticity, characterized by planarity, complete electron delocalization, and the presence of 4n + 2 π electrons. Consequently, benzene is less reactive than expected for a compound with such unsaturation, favoring substitution reactions over addition reactions.