The molecular formula of benzene was suggested to be C6H6. This formula shows that benzene is a highly unsaturated compound. Various possible structures were suggested for benzene.
Straight-Chain Structures of benzene.
If we think that benzene is an open-chain compound then some of the following possibilities can be proposed.
STRAIGHT CHAIN STRUCTURE RULED OUT.
Anyhow, these structures were rejected due to the following reasons:
Benzene adds only six hydrogens and six halogen atoms to make it saturated, but these open-chain structures propose that eight hydrogen atoms or 8 halogen atoms can be added.
Benzene does not show the reaction with alkaline KMnO4 which is the best test of unsaturation.
Benzene does not react with bromine water which is one of the tests for unsaturated compounds. These facts tell us that there is unsaturation in the benzene but this
saturation is of a different kind from alkenes and alkynes.
Benzene gives substitution reactions just like saturated hydrocarbons. This shows that in spite of the presence of double bonds the benzene behaves like a saturated compound.
Benzene gives only one monosubstituted derivative, but the open-chain chemical structures propose at least three monosubstituted derivatives.
The molecular formula for benzene is C6H6. It does not correspond to the open-chain hydrocarbons.
In alkanes, the general formula is CnH2n+2 . Hence, six carbons should have 14 hydrogens and the formula should have been C6H14.
In alkynes, the general formula is CnH2n–2. Hence six carbon atoms should have ten hydrogen atoms and the formula should have been C6H10.
Possibility of open-chain ruled out:
Keeping all these things in view we can say that benzene does not belong to open chain hydrocarbon. The possibility for the straight-chain structure is ruled out.
KEKULE STRUCTURE OF BENZENE.
“Kekule Structure of Benzene” in 1865, after years of discovery of benzene, Kekule suggested that:
- The benzene molecule is made up of a hexagon of six carbon atoms.
- There are alternate single and double bonds and one hydrogen is attached to each carbon atom.
Evidence in favor of ring structure:The following evidence is there which support the Kekule Structure:Benzene adds three hydrogen molecules to form cyclohexane.
Benzene adds three molecules of chlorine in the presence of sunlight.
Benzene only gives one monosubstituted derivative. Actually, all the carbon atoms of benzene are alike.
Benzene gives three disubstituted derivatives that support the Kekule Structure.
RESONANCE STRUCTURE OF BENZENE.
The different structure which can be written under different pairing schemes of a compound is called resonance structures of benzene.
The two structures of benzene which have been mentioned above are called resonance structures of benzene.
They are also called contributing structures of benzene. Sometimes they are called canonical contributors, to the actual structure.
In addition to these two resonance structures, three other structures have been proposed by Dewar. They are called Dewar structures.
The five structures are joined by double-headed arrows:
Dewar structure is also called para-bond structures. The π-bonds between C1 – C4, C2 – C5, C3 – C6 are least favorable. For this reason, these structures III, IV, and V are least contributing, i.e., 7% each towards the resonance hybrid.
The contribution of Kekule structure (I) and (II) is 80 % and that of Dewar structures (III), (IV), (V) is 20 % to the actual or real structure of benzene.
Resonance Energy of Benzene:
The difference between this experimental and theoretical values of heats of hydrogenation is the amount of stability of benzene.
This is 150.5 kJ·mol−1 is the difference between 385.5 and 208. This amount of stability is gained by benzene, due to resonance. So this 150.5 kJ·mol−1 is called resonance energy of benzene.Following this diagram makes the idea clear.
Definition of resonance energy:
Resonance energy is the measure of how much more stable the actual structure of the compound is than its extreme resonance structure.
The heat of combustion and stability of benzene:
Similarly, it has been calculated that the observed heat of combustion of benzene is less than the calculated value. This also points to the fact that the benzene molecule has less energy than cyclohexatriene.
Substitution reaction and stability of benzene:
In most of the chemical reactions, benzene undergoes electrophilic substitution reactions. Benzene prefers to substitute the hydrogen of its ring rather than addition reaction at the double bond.
Properties of Benzene.
“Physical properties of benzene” and “chemical properties of benzene”
Physical properties of benzene.
- Benzene is a colorless liquid.
- Its boiling point is 80.1 °C.
- Its freezing point is +5.5 °C.
- It is insoluble in water and forms the upper layer.
- It is miscible with ether and alcohol in all proportions.
- It is a good solvent for fats, resins, sulfur, phosphorus, and iodine.
- its specific gravity is 0.884 cm-3 at 15 °C.
- It is extremely poisonous in nature. It destroys the red and white blood cells.
- It burns with a luminous sooty flame.
Chemical properties of benzene.
The principal types of reactions of benzene are the following:
- Electrophilic substitution reactions.
- Addition reaction.
- Oxidation reactions.