8.16 - Cracking long-chain hydrocarbons

From a large alkane to smaller products

The starting molecules here are alkanes: saturated hydrocarbons containing only carbon and hydrogen, with no carbon–carbon double or triple bonds. Heat supplies energy for bonds to break; atoms rearrange and new bonds form in smaller molecules. This is a chemical change. Fractional distillation instead separates existing molecules by their boiling points.

A typical simplified pattern is a larger alkane forming a smaller alkane and an alkene. Alkenes contain a carbon–carbon double bond and are unsaturated: they have fewer hydrogen atoms than the corresponding alkane with the same number of carbon atoms.

[DIAGRAM: asset_name: cracking_reaction_scheme: Cracking a larger alkane into smaller alkane and alkene - diagram 1; asset_slug: 194_8_16_cracking_involves_the_breaking_down_of_larger_saturated_hydrocarbon_molecu_diagram1; file: 194_8_16_cracking_involves_the_breaking_down_of_larger_saturated_hydrocarbon_molecu_diagram1_imagegen_v1.png; recommended_method: imagegen; description: Reaction schematic: dodecane C12H26 forms octane C8H18 and butene C4H8. Carbon chains contain 12, 8 and 4 atoms respectively; the alkene has a visible double bond. Hydrogen atoms are omitted.]

The picture shows only carbon atoms and carbon–carbon bonds. Hydrogen atoms are omitted: each carbon must have four bonds in total. In the four-carbon product, the middle double bond counts as two bonds at each carbon, giving CH₃–CH=CH–CH₃ and C₄H₈. The octane product has only single bonds and is saturated. Real cracking produces a mixture, not necessarily just these two products.

Smaller hydrocarbons can be useful fuels; alkenes can be starting materials for other chemicals.

Conserving atoms in cracking equations

Cracking equations do not require oxygen. Oxygen reacting to produce carbon dioxide and water would describe combustion.