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Organic Chemistry

7.1 Industrial Preparation and Use of Alkenes

Organic Chemistry7.1 Industrial Preparation and Use of Alkenes

7.1 • Industrial Preparation and Use of Alkenes

Ethylene and propylene, the simplest alkenes, are the two most important organic chemicals produced industrially. Approximately 220 million tons of ethylene and 138 million tons of propylene are produced worldwide each year for use in the synthesis of polyethylene, polypropylene, ethylene glycol, acetic acid, acetaldehyde, and a host of other substances (Figure 7.2).

Ethylene derivatives include ethanol, ethylene glycol, ethylene dichloride, acetaldehyde, acetic acid, ethylene oxide, vinyl acetate, polyethylene, and vinyl chloride. Propylene derivatives include isopropyl alcohol, propylene oxide, polypropylene, and cumene.
Figure 7.2 Compounds derived industrially from ethylene and propylene.

Ethylene, propylene, and butene are synthesized from (C2–C8) alkanes by a process called steam cracking at temperatures up to 900 °C.

An alkane consisting of 2 to 8 carbons undergoes steam cracking at 850-900 degrees Celsius to form hydrogen, ethene, propene, and butene.

The cracking process is complex, although it undoubtedly involves radical reactions. The high-temperature reaction conditions cause spontaneous breaking of C−C and C−H bonds, with the resultant formation of smaller fragments. We might imagine, for instance, that a molecule of butane splits into two ethyl radicals, each of which then loses a hydrogen atom to generate two molecules of ethylene.

Butane at 900 degrees Celsius forms two ethyl radicals, which convert to two ethene molecules and hydrogen.

Steam cracking is an example of a reaction whose energetics are dominated by entropy (ΔS°) rather than by enthalpy (ΔH°) in the free-energy equation ΔG° = ΔH° − TΔS° discussed in Section 6.7. Although the bond dissociation energy D for a carbon–carbon single bond is relatively high (about 370 kJ/mol) and cracking is endothermic, the large positive entropy change resulting from the fragmentation of one large molecule into several smaller pieces, together with the high temperature, makes the TΔS° term larger than the ΔH° term, thereby favoring the cracking reaction.

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