Q.(a)

(i) Benzene forms from acetylene by cyclic trimerisation over a hot tube, and from phenol by reduction with zinc dust. (ii) DDT's chief use is as an insecticide (notably against malaria-carrying mosquitoes), though its environmental persistence has led to it being banned/restricted in many places.

Answering part (a), since (a)(i) and (a)(ii) are given as the primary alternative (the OR alternative (b), on the formation and effects of acid rain, is not required unless (a) is unanswerable):

(a)(i) Preparation of benzene:

(A) From acetylene: passing acetylene (ethyne, HC-tripleBond-CH) gas through a red-hot iron or copper tube at about 873 K causes three molecules of acetylene to undergo cyclic polymerisation (trimerisation), combining their six carbon atoms into the aromatic six-membered benzene ring:

3 HC-tripleBond-CH --(red-hot Fe/Cu tube, 873 K)--> C6H6

(B) From phenol: phenol (C6H5OH) is distilled with zinc dust. The zinc dust reduces/removes the hydroxyl group from the aromatic ring (replacing -OH with -H), converting phenol directly into benzene:

C6H5OH + Zn (dust) --(distillation)--> C6H6 + ZnO

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