Q.Explain the following reactions of alkanes :

Alkanes undergo three characteristic high-temperature/catalytic reactions besides substitution: isomerisation (rearrangement to branched isomers), aromatization (cyclization + dehydrogenation to benzene-type rings), and pyrolysis (thermal cracking into smaller fragments).

1. Isomerisation Straight-chain (n-) alkanes are converted into their branched-chain isomers when heated in the presence of anhydrous aluminium chloride (AlCl3) and hydrogen chloride (HCl) gas as catalyst. This process is called isomerisation. Example: n-butane, on heating with anhydrous AlCl3/HCl at about 573 K under pressure, isomerises to isobutane (2-methylpropane). CH3–CH2–CH2–CH3 --(AlCl3/HCl, heat)--> (CH3)3CH
2. Aromatization n-Alkanes having six or more carbon atoms, when heated to about 673–773 K under high pressure in the presence of oxide catalysts such as Cr2O3, V2O5 or Mo2O3 supported over alumina, undergo cyclization followed by dehydrogenation (aromatization or reforming) to give aromatic hydrocarbons. Example: n-hexane is converted to benzene, and n-heptane is converted to toluene. CH3(CH2)4CH3 --(Cr2O3/Al2O3, 773 K, high pressure)--> benzene + 4H2
3. Pyrolysis …