Alcohols, general dehydrogenation

Fatty aldehydes are generally produced by dehydrogenation of alcohols in the presence of suitable catalysts. The alcohols are often cheap and available in good purity. Aldehyde synthesis via the oxo process is less suitable since the resultant products are often not pure enough for flavor and perfume purposes. Specific syntheses for the branched-chain and unsaturated aldehydes that are important in perfumery and flavoring techniques are described under the individual compounds. 

Table 1 shows that methyl dodecanoate is easily converted into amine in the presence of CuCr deposited on alumina or on titania. Nevertheless one can observe that the methylation reaction is rather difficult and favoured by alumina. Moreover, a significant increase of N-dimethyl dodecylamine is obtained when the reaction is carried out with a large excess of hydrogen. Due to the mechanism of the reaction this is unexpected indeed it is generally considered that the methylation of primary amine with methanol requires i) the dehydrogenation of alcohol into a carbonyl compound and ii) a further reaction of this compound with primary amine or secondary amine via imine or enamine intermediates. 

Fig. 18. Model for the dehydrogenation of ra-butyl alcohol. The model is of general significance for the hydrogenation of double bonds and the dehydrogenation of...

The anodic oxidation of secondary alcohols to the corresponding ketones is generally inferior to the catalytic dehydrogenation methods. Electrochemical syntheses are therefore of interest only in special cases. An example of this is the regioselective oxidation of an endo-hydroxyl group in 1,4,3,6-dianhydrohexitols 306) ... 

One exception to the general application of these ketone syntheses was failure of compounds having an alpha-substituted carbon atom such as isobutyl alcohol or 2-ethylhexanol to undergo the dehydrogenation (7) condensation reaction. This failure of alpha-substituted reactants to undergo the ketone synthesis was unexpected as the aldol condensation of alpha-substituted aldehydes with one labile hydrogen atom occurs readily. 

Tishchenko (79), using a modified form of Raney nickel, obtained a 95.7 % yield of camphor from the dehydrogenation of borneol. Rutovskii, (80) received a 93.5% yield of camphor with Raney alloy. Reeves and Adkins (81), studying the dehydrogenation of primary alcohols, removed the hydrogen with ethylene. It was found that, though Raney nickel could be used for a catalyst for the reaction, the yields were low and, in general, the Raney nickel was inferior to a catalyst composed of copper, zinc, nickel, and barium chromite. 

Isobutene is present in refinery streams. Especially C4 fractions from catalytic cracking are used. Such streams consist mainly of n-butenes, isobutene and butadiene, and generally the butadiene is first removed by extraction. For the purpose of MTBE manufacture the amount of C4 (and C3) olefins in catalytic cracking can be enhanced by adding a few percent of the shape-selective, medium-pore zeolite ZSM-5 to the FCC catalyst (see Fig. 2.23), which is based on zeolite Y (large pore). Two routes lead from n-butane to isobutene (see Fig. 2.24) the isomerization/dehydrogenation pathway (upper route) is industrially practised. Finally, isobutene is also industrially obtained by dehydration of f-butyl alcohol, formed in the Halcon process (isobutane/propene to f-butyl alcohol/ propene oxide). The latter process has been mentioned as an alternative for the SMPO process (see Section 2.7). 

Catalytic oxidations with dioxygen can also proceed via heterolytic pathways which do not involve free radicals as intermediates. They generally involve a two-electron oxidation of a (coordinated) substrate by a metal ion. The oxidized form of the metal is subsequently regenerated by reaction of the reduced form with dioxygen. Typical examples are the palladium(II)-catalyzed oxidation of al-kenes (Wacker process) and oxidative dehydrogenation of alcohols (Fig. 4.6). 

A considerable amount of work has been published dealing with the electro-oxidation of organics at Ni anodes in aqueous base [548-552], These reactions have generally been dehydrogenations, e.g. primary alcohols to aldehydes, secondary alcohols to ketones and primary amines to nitriles. The reactions occur on a relatively thick layer of oxide on the Ni anodes. Pletcher and co-workers [529, 548, 549] observed that most of the oxidizable compounds were found to oxidize at the same potential and this potential coincided with that at which the surface of the Ni became oxidized. A typical cyclic voltammogram recorded at Ni in dilute KOH in the presence and absence of n-propylamine is shown in Fig. 23. It can be seen that addition of n-propylamine results in an oxidation wave being observed which is... 

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