Crude Oil

Why cracking is needed

• The fractions from crude oil do not match consumer demand
• Demand is highest for short-chain fractions — petrol, kerosene, diesel — because these are the easiest fuels to burn in vehicles
• Supply from raw crude is heaviest in long-chain fractions — fuel oil, bitumen — which have fewer high-value uses
• Cracking balances this imbalance by breaking long-chain hydrocarbons down into shorter, more useful ones

What cracking is

• Cracking is the breaking of long-chain alkanes into smaller hydrocarbon molecules
• Every cracking reaction produces:
  • A shorter-chain alkane (still saturated, useful as fuel)
  • One or more alkenes (unsaturated, useful as feedstock for plastics, alcohols, detergents)
• The process is endothermic — energy has to be put in to break the C–C single bonds in the long chain

Conditions for catalytic cracking

• The long-chain alkane vapour is heated to about 600–700 °C
• The hot vapour passes over a hot powdered catalyst, usually aluminium oxide (Al₂O₃) or zeolite
• Some C–C bonds break at random points along the chain; the resulting fragments leave as a blend of shorter alkanes plus one or more alkene molecules

Example — cracking a C₁₂ alkane

• Dodecane (C₁₂H₂₆) is found in the fuel-oil fraction; cracking it gives shorter, more useful molecules:

C₁₂H₂₆(g) → C₈H₁₈(g) + 2 C₂H₄(g)

• One alkane (octane, useful in petrol) plus two alkenes (ethene, used to make poly(ethene) plastic and ethanol)
• The number of hydrogen atoms must balance: 26 H on each side; the number of carbon atoms must balance: 12 C on each side

Uses of the alkene products

• Ethene is polymerised industrially to make poly(ethene) — the plastic used in carrier bags, drink bottles and pipes
• Ethene also reacts with water to make ethanol for industrial solvent use
• Propene from cracking is polymerised to poly(propene), used in food containers and ropes

Alkanes versus alkenes — a useful contrast

• Alkanes are saturated — every carbon–carbon bond is a single bond
• Alkenes are unsaturated — they have at least one carbon–carbon double bond (C=C)
• The alkene's C=C double bond is the functional group that makes it useful for further reactions, including polymerisation (see topic 24 Alkenes)