Alkene Synthesis by High-Temperature Industrial Methods

The least expensive way to make alkenes on a large scale is by the catalytic cracking A (0nG SvilthCSIS bv of petroleum heating a mixture of alkanes in the presence of a catalyst (usually ii x  

Because the products are always mixtures, catalytic cracking is not suitable for laboratory synthesis of alkenes. Better methods are available for synthesizing relatively pure alkenes from a variety of other functional groups. Several of these methods are discussed in Sections 7-9, 7-10, and later sections listed in the summary on page 314. 

Dehydrogenation is the removal of H2 from a molecule, just the reverse of hydrogenation. Dehydrogenation of an alkane gives an alkene. This reaction has an unfavorable enthalpy change but a favorable entropy change. 

The hydrogenation of alkenes (Section 7-7) is exothermic, with values of A//° around — 80 to —120 kJ/mol (—20 to —30 kcal/mol). Therefore, dehydrogenation is endothermic and has an unfavorable (positive) value of A//°. The entropy change for dehydrogenation is strongly favorable (AS° = +120 J/kelvin-mol), however, because one alkane molecule is converted into two molecules (the alkene and hydrogen), and two molecules are more disordered than one. 

In many ways, dehydrogenation is similar to catalytic cracking. In both cases, a catalyst is used to lower the activation energy, and both reactions use high temperatures to increase a favorable entropy term (—TAS) and overcome an unfavorable enthalpy term (A//). Unfortunately, dehydrogenation and catalytic cracking also share a tendency to produce mixtures of products, and neither reaction is well suited for laboratory synthesis of alkenes. 

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Alkene Synthesis by High-Temperature Industrial Methods 315 Problem-Solving Strategy Proposing Reaction Mechanisms 316 Summary Methods for Synthesis of Alkenes 320 EssentialTerms 322 Study Problems 323... 

Alkene Synthesis by High-Temperature Industrial Methods 315... 

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