BIOL 3305 Lecture Notes - Lecture 19: Aromaticity, Cyclic Compound, Benzene
Document Summary
Historically, benzene and its first derivatives had pleasant aromas, and were called aromatic compounds. Kekul (1866) bravely proposed that benzene had a cyclic structure with three alternating c=c double and three c-c single bonds. Whilst this is reasonably close to accurate, it cannot be exactly correct since this would require that 1,2-dichlorobenzene existed as two isomeric forms, yet it was known that it did not. The kekul structure would have the single bonds of longer length than the double bonds, and thus an irregular hexagonal shape. But spectroscopy had shown that benzene had a planar ring, with all the carbon-carbon bond distances the same 1. 397 (c-c typically 1. 48 , c=c typically 1. 34 ). Since the atoms are the same distance apart, and the only difference is the location of the electrons in the two kekul structures, they are in fact resonance structures of one another. all cc bonds.
