Describe the reasons for the cracking of larger alkane molecules

Organic Chemistry – Alkenes (Cambridge IGCSE 0620)

Objective

To describe the reasons for cracking larger alkane molecules, to understand how cracking produces alkenes and hydrogen, and to link this process to other syllabus topics (fuels, petro‑chemical industry and polymers).

What is Cracking?

Cracking is an industrial process that breaks long‑chain (high‑molecular‑weight) alkanes into shorter‑chain alkanes and alkenes. Heat, pressure and/or a catalyst are applied to overcome the C–C bond energy.

Alkenes – Key Features (syllabus 11.5)

• Contain a carbon–carbon double bond (C=C); therefore they are unsaturated hydrocarbons.
• The double bond makes alkenes more reactive than alkanes and allows them to undergo addition reactions.

Why is Cracking Performed? – Syllabus‑aligned reasons

1. Obtain lighter, more valuable hydrocarbons – short‑chain alkenes (e.g. ethene, propene, butene) are essential feed‑stocks for the petro‑chemical industry.
2. Improve fuel quality – lighter molecules have lower boiling points, give a higher octane rating and burn more cleanly.
3. Meet market demand for specific fuel fractions – modern gasoline requires a higher proportion of C₄–C₈ hydrocarbons.
4. Reduce viscosity and improve handling – heavy fractions are viscous and difficult to pump; cracking produces liquids that flow easily.
5. Increase economic efficiency – heavy fractions are low‑value; converting them into lighter, higher‑value products maximises profit.
6. Remove impurities – cracking can break down contaminant molecules, giving a cleaner final product.

Link to Other Syllabus Areas

• Alkenes produced (ethene, propene, butene) are the feed‑stock for polymer and synthetic‑rubber topics:
  • Ethene → polyethylene
  • Propene → polypropylene
  • Butene → butadiene → synthetic rubber
• Lighter alkanes such as octane and hexane relate to the Fuels sub‑topic (octane rating, combustion).
• Understanding cracking explains how crude petroleum is refined into the range of products listed in the syllabus (gasoline, diesel, kerosene, etc.).

Typical Cracking Reactions

General thermal cracking:

$$\text{C}_{n}\text{H}_{2n+2}\;\xrightarrow{\text{heat}}\;\text{C}_{x}\text{H}_{2x+2} + \text{C}_{y}\text{H}_{2y}$$

where x + y = n and the second product is usually an alkene.

Example – Thermal cracking of dodecane (C₁₂H₂₆):

$$\text{C}_{12}\text{H}_{26}\;\xrightarrow{650^{\circ}\text{C}}\;\text{C}_{8}\text{H}_{18} + \text{C}_{4}\text{H}_{8}$$

Octane (alkane) and butene (alkene) are formed.

Types of Cracking (Cambridge IGCSE 0620, section 11.5)

Typical conditions for the three main cracking processes

Type	Operating Conditions	Typical Catalyst	Main Products
Thermal cracking	450–750 °C, low pressure, short residence time	None (or solid‑acid surface)	Mixture of alkanes and alkenes; higher temperature → more alkenes
Steam cracking	800–900 °C, very short residence time, steam as diluent	None (steam carries heat)	Predominantly alkenes (ethene, propene) **and hydrogen** as a by‑product
Catalytic cracking (Fluid‑Catalytic Cracking, FCC)	450–550 °C, moderate pressure, continuous catalyst circulation	Solid‑acid zeolites (e.g., zeolite Y)	High‑octane gasoline, light olefins, some aromatics

How the Catalyst Works (AO2 requirement)

• Acid sites on the zeolite donate a proton to the alkane, generating a carbocation intermediate.
• The carbocation undergoes β‑scission, breaking a C–C bond and producing a smaller alkane + alkene.
• Because the proton transfer step has a lower activation energy than direct bond cleavage, cracking can occur at lower temperatures and with greater selectivity.

Key Factors that Influence Cracking

1. Temperature – Provides the energy to break C–C bonds; higher temperatures shift the product distribution toward alkenes.
2. Pressure – Lower pressures favour formation of smaller molecules (Le Chatelier’s principle).
3. Catalyst acidity – Strong acid sites lower the activation energy, allowing cracking at lower temperatures and giving higher selectivity for desired products.
4. Residence time – Short residence times minimise secondary reactions such as coke formation.
5. Molecular size – Longer chains contain more C–C bonds that can be cleaved, making them more susceptible to cracking.

Tests for Saturated vs. Unsaturated Hydrocarbons (syllabus requirement)

• Bromine water test – Bromine water (Br₂ in water) is orange. When added to an alkene the orange colour disappears as Br₂ adds across the C=C double bond, forming a colourless dibromo product. No colour change occurs with a saturated alkane.

Addition Reactions of Alkenes (only one product is formed)

Alkenes undergo addition reactions in which the double bond is broken and a single product is obtained.

Typical addition reactions of alkenes

Reagent	Reaction	Product
Br₂ (aq.)	C=C + Br₂ →	Vicinal dibromoalkane (colourless)
H₂ / Ni catalyst	C=C + H₂ →	Alkane (saturation)
H₂O / H⁺ (acidic)	C=C + H₂O →	Alcohol (Markovnikov addition)

Industrial Benefits of Cracking

• Transforms low‑value heavy fractions into high‑value products such as ethene (precursor to polyethylene) and propene (precursor to polypropylene).
• Provides light olefins needed for synthetic rubbers (e.g., butadiene from butenes).
• Produces high‑octane gasoline that meets modern engine requirements.
• Generates solvents and intermediates used throughout chemical manufacturing.

Summary

Cracking of larger alkane molecules is carried out to obtain lighter, more valuable hydrocarbons, improve fuel performance, satisfy market demand, reduce handling problems, increase economic return and remove impurities. The process can be thermal, steam or catalytic; each has characteristic temperature, pressure and catalyst requirements. Cracking produces alkenes (unsaturated hydrocarbons) that undergo characteristic addition reactions (bromine water test, hydrogenation, hydration) and serve as feed‑stocks for polymers, synthetic rubber and many other chemicals. Understanding cracking therefore links the “Alkenes” sub‑topic to the broader IGCSE syllabus on fuels, petro‑chemical industry and polymer chemistry.