Kekule was to say later that he must have dozed off at this point. In his dream the black balls of carbon turned into black imps with forked tails that began racing around the room and would soon be upsetting the apparatus of the laboratory. He was ready to run the rascals out. Then, almost suddenly, the confusion died away as each imp grabbed the tail of the one ahead of him, the six forming a whirling circle. One hand of each imp held a tail, the other a white handkerchief--and they waved to him as the group whirled by. He said that he came awake with a start, realizing that the imps were acting out the formula for benzene. As his hand grabbed the sketching pencil, the imps were back to black balls again and the handkerchiefs had changed to hydrogen atoms. How simple the arrangement turned out to be. "The carbon atoms of benzene form a ring."

In 1858, August Kekule proposed a rapid oscillation (I & II) between the 3 C=C and the 3 alternating C-C of hexagonal benzene (C₆H₆).

However, Kekule's representation of benzene had serious flaws.
 

Flaws with Kekule Model
1.  Heat of Hydrogenationis the heat liberated when hydrogen adds to C=C.  Benzene is compared to cyclohexene because both form cyclohexane upon complete hydrogenation.  Since benzene supposedly contains 3 C=C, it should release three times the heat of hydrogenation reported for cyclohexene with only 1 C=C.  Heats of hydrogenation for cyclohexene and benzene are reported as:

Since cyclohexane releases 120 kJ/mol, benzene is expected to release (3x120) or 360 kJ/mol.

Why does benzene possess 150 kJ/mol less energy than predicted?
 

2.  Addition versus Substitution:Compounds with C=C (alkenes) typically undergo addition reactions such as bromination.  Shown below are reactions of ethene/bromine and benzene/bromine:

Why does benzene with 3 C=C resist addition and prefer substitution?
 

3.  X-Ray studiesreveal the following bond lengths: 0.153nm for typical C-C, 0.134nm for typical C=C, and 0.139nm for the 6 carbon-carbon bonds in benzene.

Why does benzene possess 6 bonds intermediate between C=C and C-C?
 

Resonance
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 150 kJ/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 150 kJ/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.
 

Aromatic Compounds
Because of its odor benzene was referred to as "aromatic" but today aromatic compounds have come to represent benzene-like structures. Characteristics of aromatic compounds include:
Must be Cyclic
Must have (4n + 2) pi Electrons (n = 1,2,3,4,...)
Resist Addition but Prefer Substitution
Must Possess Resonance Energy

Examples of aromatic compounds:


Why is Trinitrotoluene (TNT) Explosive?  Because nitrogen in the nitro group (NO2) has a charge of +1, the nitro group has a very strong tendency to withdraw (pull) electrons from the aromatic ring.  Therefore, attaching three nitro groups to a benzene ring leads to an extremely unstable or explosive compound.
 

Hazardous Chemical
Its hexagonal shape allows benzene to fit between base pairs in DNA to cause genetic mutations. Benzene's use as a solvent has been replaced by the non carcinogen toluene--the minute change of replacing a hydrogen with CH3 alters the shape and does not allow toluene to get between DNA base pairs.  Short-term health risks attributed to benzene exposure include temporary nervous system disorders, immune system depression, and anemia.  Long-term risks include chromosome damage and forms of cancer that include leukemia.