Structure of Benzene

Introduction:

The structure of benzene, a six-carbon aromatic hydrocarbon, has been a subject of considerable importance in the history of organic chemistry. The traditional structural formulas used to describe benzene include alternating single and double bonds. However, experimental evidence did not support these classical structures. The actual structure of benzene is better described by a resonance hybrid, where the electrons are delocalized over all six carbon atoms.

Fig: Molecular Structure of Benzene

The structure of benzene has been derived as follows:

Analytical Evidence: –

A) Molecular formula: –

(1) Elemental analysis and molecular weight determination showed benzene had the molecular formula C₆H₆.

(2) This indicated benzene was a highly unsaturated compound (compare it with n-hexane, C6H14).

Synthetic Evidence:

Straight-chain structure not possible

(1) Benzene could be constructed as a straight-chain or ring compound having double(C=C) and triple bonds.

(2) But benzene did not behave like alkenes or alkynes.

(3) It did not decolorize bromine in carbon tetrachloride or cold aqueous potassium permanganate.

(4) It did not add water in the presence of acids