Structure of Benzene

The molecular formula of benzene is C6H6 which indicates that benzene is an unsaturated hydrocarbon.

Benzene does not respond to the tests of unsaturation which are shown by alkenes and alkynes. both the alkenes and the alkynes decolourize bromine water and alkaline solution of potassium permanganate (Bayer’s Reagent). However, benzene undergoes substitution reactions.

Kekule Structure

A ring structure for benzene was proposed by Kekule in 1865. According to him, six carbon atoms are joined to each other by alternate single and double bonds to form a hexagon ring.

As Kekule’s structure contains three single bonds and three double bonds, one may expect that in benzene there should be two different bond lengths - 154 pm for C-C single bond and 134 pm for C=C double bond. But, the experimental studies show that benzene is regular hexagon with an angle of 120° and all the carbon-carbon bond lengths are equal to 139 pm.

Kekule’s structure does not explain the stability of benzene and its some unusual reactions. Resonance can explain the unusual behaviour of benzene.

Resonance

The phenomenon by virtue of which a single molecule can be represented in two or more structures is called resonance. The actual structure is the resonance hybrid of all the canonical or resonating structure.

Heat of hydrogenation data provides proof for resonance stabalization in benzene. The heat of hydrogenation is the amount of heat liberated when hydrogen is added to one mole of an unsaturated compound in the presence of a catalyst.

If the three double bonds in benzene do not interact, then it should behave like cyclohexatriene and the amount of heat liberated on adding there molecules of hydrogen should be 358.5 KJ mol–1. But, the actual heat of hydrogenation of benzene is 208.2 KJ mol–1.

This difference of (358.5 - 208.2) 150.3 KJ mol–1 in the heat of hydrogenation is the measure of stability of benzene. Benzene acquires stability due to resonance and hence, this energy is called resonance energy of benzene.