To help explain molecules like benzene, Linus Pauling proposed resonance theory in 1931 (Pauling also gave us hybrid orbitals, electronegativity, and valence bond theory). As a result of Pauling's resonance, benzene is viewed as a hybrid of III& IVand represented byV. 
The six carbons are arranged in a hexagon with one hydrogen atom attached to each carbon. The 12 atoms of benzene are planar with carbon in the sp2 hybrid state and 120° bond angles. Because the six bond lengths are equivalent, Kekule's rapid equilibrium must be ruled out. To describe benzene we need to use contributing structures III& IVor one structure with a circle in the middle (V). Structures III &IV do not exist! Whatever benzene might be, it displays characteristics of its contributing structures. For an analogy, consider crossing a bloodhound with a greyhound: Although the offspring is unique, it displays characteristics (smell & speed) of its parents--it does not flip flop between a greyhound one instant and a bloodhound the next.
Why does benzene possess 36 kcal/mol less energy than predicted?
This missing energy, called resonance energy, is attributed to overlap of p-orbitals. Structures VI and VII show pi bonding for the contributing structures while VIII illustrates delocalization of pi electrons over the 6 carbon atoms. Benzene can be represented as IX using molecular models with p-orbitals. The circle in the middle of V is an abbreviated way to represent the delocalization of the 6 pi electrons. Resonance energy is the difference in energies between III (or IV) and V (V has lower energy). Due to resonance, benzene is 36 kcal/mol more stable than calculations would predict.
Why does benzene resist addition and prefer substitution?
If benzene were to undergo addition,resonance would be disrupted and this would render the structure less stable. Therefore benzene prefers to substitute (Br for H) and maintain resonance.
Why does benzene possess 6 bonds intermediate between C=C and C-C?
According to resonance, the bonds are not C-C or C=C but a hybrid of the two. X-ray studies confirms this with the intermediate bond lengths.
Moderation Team
![[Post New]](/templates/default/images/icon_minipost_new.gif){kind=icon}
![[Up]](/templates/default/images/icon_up.gif "[Up]"){kind=icon}
41
