Q.(a) Explain the structure of Benzene. OR

Benzene is a planar, hexagonal ring of six sp2 carbons with a delocalised pi-electron cloud above and below the ring plane, making all six C-C bonds identical and giving the molecule its characteristic aromatic stability. (This answers part (a) of the OR question.)

Molecular formula: C6H6.

Kekule structure: August Kekule first proposed benzene as a hexagonal ring of six carbon atoms with alternating single and double bonds (cyclohexatriene), each carbon also bonded to one hydrogen atom. This structure explained benzene's formula and monosubstitution behaviour but had two well-known problems: (a) it predicted two different C-C bond lengths (shorter double bonds, longer single bonds), while experimentally all six C-C bonds in benzene are found to be equal (about 139 pm, intermediate between a typical C-C single bond of about 154 pm and a C=C double bond of about 134 pm); (b) it predicted benzene should show the high reactivity typical of alkenes (e.g. readily undergoing addition reactions), but benzene is unusually unreactive towards addition and instead favours substitution reactions.

Modern (resonance/MO) picture: Each of the six carbon atoms in benzene is sp2 hybridised, using its three sp2 orbitals to form sigma bonds -- two to neighbouring ring carbons and one to a hydrogen atom -- giving a completely planar, regular hexagonal ring with all C-C-C and H-C-C bond angles equal to 120 degrees. Each carbon also has one unhybridised p-orbital, oriented perpendicular to the plane of the ring, each containing one electron. These six p-orbitals overlap sideways (laterally) all around the ring to form a continuous, delocalised pi-electron cloud spread above and below the plane of the ring, rather than three separate, localised double bonds. …