Q.(a) Explain the structure of benzene.

This choice question asks either for the structure of benzene, or for a short note on the Friedel-Crafts reaction together with ways to protect the environment from pollution. …

This is a 5-mark either/or question; both alternatives are answered below.

PART (a) — Structure of benzene:

1. Benzene has the molecular formula C6H6 and is a cyclic molecule containing a ring of six carbon atoms.
2. The ring is planar and a regular hexagon; each carbon atom is sp2 hybridised, forming sigma bonds to two neighbouring carbons and one hydrogen, so all bond angles are 120°.
3. All six carbon-carbon bond lengths are equal (about 139 pm), a value intermediate between a normal C-C single bond (154 pm) and a C=C double bond (134 pm). This shows the bonds are neither pure single nor pure double.
4. Each carbon has one unhybridised p-orbital perpendicular to the ring; these six p-orbitals overlap sideways to form a delocalised π-electron cloud above and below the plane of the ring.
5. Because of this delocalisation, benzene is a resonance hybrid of the two Kekulé structures and is unusually stable (aromatic); it obeys Huckel's rule (4n+2 π electrons, with n = 1, giving 6 π electrons).

PART (b)(i) — Friedel-Crafts reaction:

The Friedel-Crafts reaction is an electrophilic aromatic substitution on benzene carried out in the presence of anhydrous aluminium chloride (a Lewis-acid catalyst). It is of two types:

• Alkylation: benzene + alkyl halide → alkylbenzene. C6H6 + CH3Cl --(anhyd. AlCl3)--> C6H5CH3 (toluene) + HCl.
• Acylation: benzene + acyl chloride → aromatic ketone. C6H6 + CH3COCl --(anhyd. AlCl3)--> C6H5COCH3 (acetophenone) + HCl.

PART (b)(ii) — Methods to protect the environment from pollution: …