After completing this section, you should be able to
- discuss the industrial importance of ethylene (ethene) and propylene (propene).
- describe, briefly, the industrial process known as thermal cracking.
Among the most important and most abundant organic chemicals produced worldwide are the two simple alkenes, ethylene and propylene. They are used as the starting materials to synthesize numerous valuable compounds.
Produced from ethylene (ethene)
|ethanol||solvent; constituent of cleaning preparations; in synthesis of esters|
|acetaldehyde||slug killer, in the form of methaldehyde (CH3CHO)4|
|acetic acid||manufacture of vinyl acetate polymers, ethyl acetate solvent and cellulose acetate polymers|
|ethylene oxide||“cellosolves” (industrial solvents)|
|ethylene glycol||anti-freeze; production of DacronOR|
|ethylene dichloride||solvent; production of vinyl chloride|
|vinyl chloride||manufacture of poly (vinyl chloride)—PVC|
|vinyl acetate||manufacture of poly (vinyl acetate) used in paint emulsions, plywood adhesives and textiles|
|polyethylene||“plastic” bags; toys; packaging|
Produced from propylene (propene)
|isopropyl alcohol||rubbing alcohol; cosmetics; synthesis of acetone|
|propylene oxide||manufacture of polyurethanes; polyesters|
|cumene||industrial preparation of phenol and acetone|
|polypropylene||molded articles (e.g., kitchenware); fibres for indoor-outdoor carpeting|
Catalytic cracking to form ethylene
Cracking is the name given to breaking up large hydrocarbon molecules into smaller and more useful bits. This is achieved by using high pressures and temperatures without a catalyst, or lower temperatures and pressures in the presence of a catalyst. The source of the large hydrocarbon molecules is often the naphtha fraction or the gas oil fraction from the fractional distillation of crude oil (petroleum). These fractions are obtained from the distillation process as liquids, but are re-vaporized before cracking.
The hydrocarbons are mixed with a very fine catalyst powder. These days the catalysts are zeolites (complex aluminosilicates) - these are more efficient than the older mixtures of aluminium oxide and silicon dioxide. The whole mixture is then blown rather like a liquid through a reaction chamber at a temperature of about 500°C. Because the mixture behaves like a liquid, this is known as fluid catalytic cracking (or fluidized catalytic cracking). Although the mixture of gas and fine solid behaves as a liquid, this is nevertheless an example of heterogeneous catalysis - the catalyst is in a different phase from the reactants. The catalyst is recovered afterwards, and the cracked mixture is separated by cooling and further fractional distillation.
There is not any single unique reaction happening in the cracker. The hydrocarbon molecules are broken up in a fairly random way to produce mixtures of smaller hydrocarbons, some of which have carbon-carbon double bonds. One possible reaction involving the hydrocarbon C15H32 might be:
Or, showing more clearly what happens to the various atoms and bonds:
This is only one way in which this particular molecule might break up. The ethene and propene are important materials for making plastics or producing other organic chemicals. You will remember that during the polymeriation of ethene, thousands of ethene molecules join together to make poly(ethene) - commonly called polythene. The reaction is done at high pressures in the presence of a trace of oxygen as an initiator.