Industrial Preparation and Use of Alkenes

Core Concepts

Cracking: thermal breakdown of alkanes

Cracking breaks carbon–carbon bonds of large alkanes into smaller molecules, including alkenes. In steam cracking (the dominant industrial method), a hydrocarbon feedstock (ethane, propane, naphtha, or gas oil) mixed with steam passes through tubes heated to roughly 750–900 °C for a fraction of a second. The high temperature breaks C–C bonds homolytically; the radicals fragment and disproportionate, and the process is quenched quickly to preserve the reactive alkene products. A representative change for ethane feedstock is:

CH₃CH₃ ^{750--900 °C}⟶ CH₂=CH₂ + H₂

For a larger alkane, cracking produces a mixture — an alkene plus a smaller alkane:

C₈H₁₈ ^Δ⟶ C₄H₁₀ + CH₂=CH-CH₂CH₃ (one possible split)

The product slate (ethylene, propylene, butenes, butadiene, plus hydrogen and methane) is separated by fractional distillation. Because the reactions are radical chain processes at high temperature, the product distribution depends on feedstock and conditions; industry tunes temperature, residence time, and steam ratio to favor ethylene or propylene.

Dehydrogenation of alkanes

Dehydrogenation removes H2 from an alkane to make an alkene. It is the reverse of hydrogenation (Topic 6 of this chapter) and is endothermic, requiring heat and a catalyst:

CH₃CH₂CH₃ ⇌ CH₃CH=CH₂ + H₂  (ΔH > 0)

Industrially, propane is dehydrogenated over a chromium oxide or platinum-based catalyst at roughly 550–650 °C to make propylene; isobutane is dehydrogenated to isobutene. Because the reaction is reversible and endothermic, high temperature shifts the equilibrium toward products (Le Chatelier's principle), and the hydrogen is removed to drive the reaction further. The same chemistry runs in reverse in refineries, where hydrogenation saturates alkenes when stability is wanted.

Dehydration of alcohols

Alcohols can be dehydrated to alkenes with an acid catalyst, the microscopic reverse of acid-catalyzed hydration (Chapter 8). Industrially, ethanol is dehydrated over alumina (Al2O3) at about 350 °C:

CH₃CH₂OH ^{Al₂O₃, 350 °C}⟶ CH₂=CH₂ + H₂O

This route is used when ethylene is needed from renewable ethanol (bio-ethylene) or when local economics favor alcohol feedstock. At the laboratory scale, dehydration uses concentrated sulfuric or phosphoric acid at elevated temperature, and the regiochemistry follows Zaitsev's rule (the more substituted alkene is favored) — a preview of Chapter 8.

What the alkenes are used for

Ethylene is the largest-volume product: most goes to polyethylene, the rest to ethylene oxide (ethylene glycol, polyester), ethanol, vinyl chloride (PVC), and styrene (with benzene, Chapter 16). Propylene goes largely to polypropylene, plus propylene oxide, acrylonitrile (acrylic fibers), and cumene (phenol and acetone). Butenes and butadiene feed synthetic rubber and other polymers. In every case the π bond is the reactive handle for polymerization or functionalization — which is why alkenes, not alkanes, are the strategic intermediates of the petrochemical industry.

Process considerations: hazards and design principles

Industrial alkene chemistry operates at high temperatures, sometimes high pressures, with flammable gases and liquids. The general safety principles governing such processes — ventilation, fire and explosion hazard management, pressure-relief design, material compatibility — are engineering standards, not laboratory procedures. The study takeaway is thermodynamic and kinetic: cracking needs high temperature because C–C bonds are strong (bond dissociation energies roughly 350–400 kJ/mol) and the desired unsaturated products are higher in energy; catalysts lower activation energies so viable rates are reached at achievable temperatures.

Common Confusions