Dehydrogenations of alcohols to aldehydes

Some information about structure effects on the rate of dehydrogenation of alcohols to aldehydes and ketones on metals is found in the older literature 129-132) from which it follows that secondary alcohols react more easily than the primary alcohols 129) and that the reactivity decreases with the length of the carbon chain 131). Some series can be correlated by the Taft equation using a constants (Ref. 131, series 103, Cu-Cr203 catalyst, 350°C, four points, slope 18 Ref 132, series 104, Cu catalyst, four points, slope 22). Linear relationships have been used in a systematic way by... 

Oxidation or Dehydrogenation of Alcohols to Aldehydes and Ketones C,0-Dihydro-elimination... 

Platonov and co-workers have made a significant contribution to the study of alcohol dehydrogenation by means of rhenium catalysts (310). Rhenium disulfide was found to be efficient in promoting dehydrogenation of alcohols to aldehydes or ketones (acetone), and in the dehydrogenation of cyclohexanol to phenol and a small amount of cyclohexanone (308),... 

Oxidations by oxygen and catalysts are used for the conversion of alkanes into alcohols, ketones, or acids [54]-, for the epoxidation of alkenes [43, for the formation of alkenyl hydroperoxides [22] for the conversion of terminal alkenes into methyl ketones [60, 65] for the coupling of terminal acetylenes [2, 59, 66] for the oxidation of aromatic compounds to quinones [3] or carboxylic acids [65] for the dehydrogenation of alcohols to aldehydes [4, 55, 56] or ketones [56, 57, 62, 70] for the conversion of alcohols [56, 69], aldehydes [5, 6, 63], and ketones [52, 67] into carboxylic acids and for the oxidation of primary amines to nitriles [64], of thiols to disulfides [9] or sulfonic acids [53], of sulfoxides to sulfones [70], and of alkyl dichloroboranes to alkyl hydroperoxides [57]. 

Acetone, cyclohexanone, benzophenone, cinnamaldehyde, and other carbonyl compounds are hydrogen acceptors in the Oppenauer oxidation of alcohols to carbonyl compounds. The reaction is catalyzed by Raney nickel [961], aluminum alkoxides [962], tris(isopropoxide), or tris(tert-bu-toxide) as bases soluble in organic solvents [963, 964]. These dehydrogenations of alcohols to aldehydes and ketones require refluxing or distillations and have given way to dimethyl sulfoxide oxidations, which take place at room temperature. 

Dimethyl sulfide and chlorine or, better still, dimethyl sulfide and N-chlorosuccinimide, form a system capable of the selective dehydrogenation of alcohols to aldehydes or ketones. The intermediates, such as (013)28 C1 Cr, react with bases according to the scheme in equation 25 [720],... 

Introduction.—The oxidative dehydrogenation of alcohols to aldehydes and ketones over various catalysts, including copper and particularly silver, is a well-established industrial process. The conversion of methanol to formaldehyde over silver catalysts is the most common process, with reaction at 750—900 K under conditions of excess methanol and at high oxygen conversion selectivities are in the region 80—95%. Isopropanol and isobutanol are also oxidized commercially in a similar manner. By-products from these reactions include carbon dioxide, carbon monoxide, hydrogen, carboxylic acids, alkenes, and alkanes. 

Industrially, the dehydrogenation of alcohols to aldehydes and ketones in the presence of metal catalysts is conducted in some instances in apparatus similar in principle and construction to a water tube boiler with inclined tubes. In this case, however, the tubes contain the catalyst and vapors of aldehyde and hydrogen are removed in place of steam.- 1 The catalyst may be so distributed in the tubes and the rate of alcohol feed so regulated that the proper time of contact of alcohol vapors is maintained at the correct operating temperature.21... 

Complexes of these metals catalyze the dehydrogenation of alcohols to aldehydes or ketones, producing gaseous or transferring hydrogen... 

Dehydrogenation of alcohols to aldehyde or ketone allows subsequent bond construction steps which would not be possible for the parent alcohols. Hence, a variety of iridium, rhodium or ruthenium phosphine, pincer and related complexes, that are efficient catalysts for the dehydrogenation of alcohols, can potentially be appHed for the related hydrogen-transfer reactions, thus leading to new added-value compounds. The hydrogen atoms transfer to a sacrificial hydrogen acceptor, such as a carbonyl compound or an olefin which is reduced to the corresponding alcohol or alkane. 

Rhenium oxides have been studied as catalyst materials in oxidation reactions of sulfur dioxide to sulfur trioxide, sulfite to sulfate, and nitrite to nitrate. There has been no commercial development in this area. These compounds have also been used as catalysts for reductions, but appear not to have exceptional properties. Rhenium sulfide catalysts have been used for hydrogenations of organic compounds, including benzene and styrene, and for dehydrogenation of alcohols to give aldehydes (qv) and ketones (qv). The significant property of these catalyst systems is that they are not poisoned by sulfur compounds. 

The above-described reverse reaction (viz. the Fe-catalyzed dehydrogenation of alcohols to ketones/aldehydes) has been reported by Williams in 2009 (Table 9) [58]. In this reaction, the bicyclic complex 16 shows a sluggish activity, whereas the dehydrogenation of l-(4-methoxyphenyl)ethanol catalyzed by the phenylated complex 17 affords the corresponding ketone in 79% yield when 1 equiv. (relative to 17) of D2O as an additive was used. For this oxidation reaction, l-(4-methoxyphenyl) ethanol is more suitable than 1-phenylethanol and the reaction rate and the yield of product are higher. 

Dehydrogenation. The removal of one or more hydrogen atoms from a molecule by chemical means, as in the conversion of alcohols to aldehydes. For example, methanol (CH3OH) can be oxidized to formaldehyde (HCHO) plus H2. ... 

Table 1 summarizes the experimental results obtained in our laboratory on the kinetics of the normal dehydrogenation of hydrocarbons (hexahydro-aromatics to aromatics, the open chain compounds butylene to butadiene, and ethylbenzene to styrene), of amines to ketimines, and of alcohols to aldehydes or to ketones, respectively, in the presence of metallic or oxide catalysts. Equation (1) was found to apply in all cases. Ko and h are given by... 

Elemental copper can be used as an unsupported catalyst for the oxidative dehydrogenation of alcohols to their respective aldehydes. There are two main reaction paths partial oxidation to formaldehyde and total oxidation to carbon dioxide, which is thermodynamically favored. The... 

Copper chromite, CuCr204, and mixtures of cupric oxide with chromium sesquioxide and special additives (the Adkins catalyst), dehydrogenate primary alcohols to aldehydes [354, 355] and secondary alcohols to ketones [354, 355, 356]. 

Benzeneseleninic anhydride, C5HjSe(0)0(0)SeC5Hs, which is prepared in situ from diphenyldiselenide and tert-hniyX hydroperoxide, is used for the oxidation of alcohols to aldehydes or ketones [525]. This reagent is a suitable dehydrogenating agent for the introduction of double bonds a to carbonyl groups [526] and the regeneration of ketones from their oximes, semicarbazones, and phenylhydrazones [527]. 

Dimethyl sulfoxide (DMSO), which is successfully used to dehydrogenate primary alcohols to aldehydes, converts secondary alcohols into ketones in very high yields and under very gentle conditions. The mechanism is discussed in a previous section. Dehydrogenation and Oxidation of Primary Alcohols to Aldehydes (equation 217). The first oxidations were carried out in the presence of dicyclohexylcarbodiimide and an acid catalyst such as pyridinium trifluoroacetate [1016], which protonates the diimide and facilitates the attack by dimethyl sulfoxide (equation 259). 

An interesting variation of the chromic acid oxidation of alcohols to aldehydes was discovered by Oppenauer and Oberrauch.411 When dissolved in tert-butyl alcohol and an organic solvent, alcohols are dehydrogenated to aldehydes by tert-butyl chromate in 90% yield. Experimental details are given in Houben-Weyl s reference work.ln... 

When palladium black is brought into contact with alcohol at room temperature in the absence of oxygen, a part of the alcohol is converted into aldehyde and palladium hydride is formed. If oxygen is now admitted the hydride is converted into water. This fact indicates that in the catalytic oxidation of alcohol to aldehyde by oxygen the process consists in dehydrogenation, followed by the oxidation of the hydrogen withdrawn. 

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