Reforming

Reforming converts straight-chain alkanes into branched-chain alkanes and cyclic hydrocarbons (cycloalkanes and arenes such as benzene). The chain is rearranged, not broken into smaller pieces as in cracking.The reason is combustion. Straight-chain alkanes in petrol tend to ignite too early in an engine, before the spark, giving uneven burning called knocking. Branched and cyclic hydrocarbons burn more smoothly and efficiently, so they are blended into petrol to improve its quality.Reforming is carried out by heating the alkanes with a platinum catalyst (often on an aluminium oxide support). Hydrogen is a useful by-product.Types of reaction:Isomerisation (straight to branched chain, same formula):
CH₃CH₂CH₂CH₂CH₃ → CH₃CH(CH₃)CH₂CH₃
pentane → 2-methylbutaneCyclisation (chain to ring, losing hydrogen):
C₆H₁₄ → C₆H₁₂ + H₂
hexane → cyclohexane + hydrogenAromatisation (ring to arene):
C₆H₁₂ → C₆H₆ + 3H₂
cyclohexane → benzene + hydrogenWorked example: heptane, C₇H₁₆, is reformed into methylcyclohexane, C₇H₁₄. The carbon atoms balance (7 = 7) and 16 - 14 = 2 hydrogen atoms are lost, so the other product is H₂:
C₇H₁₆ → C₇H₁₄ + H₂