Dehydration/hydrogenation reactions

Alkane production from sugars by aqueous phase dehydration/hydrogenation reactions has the advantage that most of the alkane fraction is spontaneously separated from the aqueous phase. Unfortunately, the major compound produced by this process is hexane, which has a low value as a gasoline additive due to its relatively high volatility. This limitation has been partially overcome by promoting a base-catalyzed aldol condensation step which links carbohydrate-derived units via formation C-C bonds to form heavier alkanes ranging from C to C15 [151]. 

The reaction of 17-keto steroids with hydrogen cyanide (or acetone cyanohydrin) to form a mixture of the 17-cyano-17-hydroxy compounds, followed by dehydration and reaction with methyl Grignard reagent, is one of the earliest methods for the conversion of androstanes to pregnanes. 

A palladium phosphine complex [e.g., BCPE = l,2-bis(l,5-cyclooctylenephos-phino)ethane] was also reported to produce propanediols and n-propanol from glycerol at 443 K under 6 MPa CO/H2 atmosphere in acidic conditions, n-Propanol is the dominant product, while a slight preference for the formation of propane-1,3-diol is seen in the diol fraction. Reactions were performed at different temperatures in the range 413-448 K. Since acrolein was monitored at high temperature, a reaction network was proposed following a sequential dehydration/hydrogenation pathway [20]. 

The diacetone alcohol reaction is a catalytic liquid-phase aldol condensation. The subsequent reactions involve dehydration, hydrogenation, and esterification. 

Figure 5.1 illustrates the key reactions implied in the manufacturing of cyclohexanone by phenol hydrogenation. The reactions are of consecutive type, in which the desired product is an intermediate. Small amounts of cyclohexene might appear at higher temperature by cyclohexanol dehydration. Additional reactions can lead to heavies by polymerization or benzene and cyclohexane by disproportionation. 

Hydrate and dehydrate Hydrogenation progesterone. Hydration is the addition of water in a chemical reaction and dehydration is the taking away of water from a substance. The addition of hydrogen gas to a double-bonded unsaturated carbon = carbon bond, making the compound saturated and containing only single C—C bonds. 

The Guerbet reaction is a complex combination of oxidation of a primary alcohol to an aldehyde, aldol condensation followed by dehydration, hydrogenation of the a,p-unsaturated aldehyde to a saturated alcohol, and oxidation of the aldehyde to an acid. The reaction is catalyzed by sodium and copper bronze and is carried out by heating the mixture in an autoclave at 270-300 °C and 50-60 atm for 6-10 h (equation 352). 

Similar mechanisms can be provided for other reactions, such as parahydrogen conversion, hydrogen exchange reactions, dehydrations, etc. Reactions of the exchange type, where the energy levels in the two species are close, if not identical, can proceed in the van der Waals layer at low temperatures by virtue of the quantum mechanical exchange interaction. However, other classes of reactions are unlikely to proceed exclusively in other than the chemisorbed state. 

Scheme 11.5 Dehydration - hydrogenation, Two-Step, one-pot reaction sequence.

There is a remarkable parallelism between the exchange and the hydrogenation reactions over alumina, although these reactions occur at temperatures several hundreds of degrees apart. For both reactions, the initial activity increases as the drying temperature is increased addition of 0.15 wt. % water to the dehydrated oxide results in almost complete loss of catalytic activity the full poisoning effect of water is exhibited only when the water is added at temperatures of about 300° or higher and the active sites... 

Deliberate dehydrogenation of natural hydroazulenes and other sesquiterpene derivatives, sometimes after preceding dehydration, hydrogenation, and reduction, has been the first synthetic path to azulenes (55FCF(3)334, p. 355 59MI2, p. 301). Similar reactions yielded... 

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