Stepwise dehydrogenation

The desaturation of l5-cis phytoene into lycopene occurs in four stepwise dehydrogenations, yielding phytofluene, ( -carotene, neurosporene and lycopene... 

The mechanism of thermal decomposition (pyrolysis) of methane has been extensively studied [90,91]. Because C-H bonds in the methane molecule are significantly stronger than C-H and C-C bonds of the products, the secondary and tertiary reactions contribute at the very early stages of the reaction, which obscure the initial processes. According to Holmen et al. [92], the overall methane thermal decomposition reaction at high temperatures can be described as a stepwise dehydrogenation as follows ... 

Aromatization, however, may also be envisaged as taking place via stepwise dehydrogenation of an unbranched hydrocarbon molecule followed by ring closure of the polyunsaturated intermediates. In fact, the formation of dienes was proved during the aromatization of C6 and C7 alkanes to the corresponding aromatics over monofunctional metal oxides and metal black, and bifunctional catalysts.307 308 Radiotracer studies even allowed the detection in very low concentration of hexatriene during the aromatization of n-hexane over Pt black.309 It was also proposed that aromatics are formed from the cis isomers, whereas trans isomers may be coke precursors.213 Direct experimental evidence has recently been... 

In a further study of routes to steroids with an aromatic ring C, 18-norandrost-13-enes (273) were found to give 7a-hydroxy-18-norandrosta-8,l 1,13-trienes (274) on reaction with selenium dioxide, while the 18-norandrosta-7,13-dienes reacted with mercury(n) acetate to give the 7a-acetate (276), as well as its 7j8-epimer. Mechanisms, including stepwise dehydrogenations, are discussed.226... 

The reaction mechanisms for thermal coupling of methane has been studied by various researchers [175], The overall reaction in thermal coupling of methane can be described as a stepwise dehydrogenation at high temperature. 

It should be assumed that in all the cases of stepwise dehydrogenation listed above, a doublet mechanism [edgewise orientation (56)] takes place, as in the already familiar case of dehydrogenation of ethane on palladium at elevated temperatures (220). 

Assuming a stepwise dehydrogenation from the octane to the allQrlbenzene (via olefin, diene, triene, cyclodiene) such behaviour would be expected with decreasing dehydrogenation activity as postulated in [5]. 

Dehydrocyclisation of alkanes to aromatic compounds is one of the basic reactions of naphtha reforming, which is one of the most important industrial catalytic processes. IThe nature of the ring closure step is one of the main questions of understanding the chemistry of dehydrocyclisation. Sharan reviewed earlier work on tracer studies of this reaction.[ 1 An excellent discussion on the development of ideas and the role of [ C] isotope in elucidating reaction pathways has been published by Davis.Two basic ideas have competed in Scheme 2. One assumed stepwise dehydrogenation of open-chain alkanes then cyclisation as one of... 

The reaction hexenes —> hexadienes was demonstrated without using radiotracers both on oxide and metal catalysts, Nil 1 and Ptj l Mixtures containing [ " CJ-hexene contributed to the clarification of the further reaction pathway. These studies showed that neither the hexene cyclohexane nor the hexene —> cyclohexene ring closure pathway took place.Table 2 indicates that radioactivity appeared in both the hexatriene and 1,3-cyclohexadiene fractions when their inactive form was admixed to radioactive hexene. The aromatisation of both inactive components was much more rapid than that of hexene, therefore their specific radioactivities showed very low absolute values, however, these were still higher than that of benzene produced mainly from these non-radioactive precursors. The true precursor of ring closure should have been cis-cis-1,3,5-hexatriene. Its ring closure takes place without any catalyst from 513 The stepwise dehydrogenation of open-chain hydrocarbons produces cis- and trans-isomers of alkenes and alkadienes. Any c s-c s-triene... 

The kinetic isotope method was also applied to study whether the stepwise or direct route is valid for the dehydrogenation of the cyclohexane ring.[bi4,67] mentioned before, plotting specific radioactivities as a function of the contact time confirmed the predominance of direct pathway on Re and the stepwise dehydrogenation on chromia. 

Giannetto has discussed dehydrocyclodimerization, i.e., the conversion of propane and butane to BTX aromatics over bifunctional catalysts (e.g., Ga-ZSM-5). The reaction occurs stepwise dehydrogenation, oligomerization, cyclization, and dehydrogenation follow each other. Some of these reaction steps are catalysed by the dehydrogenation function of the gallium, some by the acid sites of the ZSM-5, and some by both. Aromatization is a primary reaction on Ga-ZSM-5 but only a secondary one on H-ZSM-5. 

Though Eqs. 2.18 to 2.22 look simple from a global perspective, in reality these reactions proceed in a much more complex fashion such as adsorption of the gas-phase species on the surface followed by reaction among various surface adsorbed species and desorption from the surface into the gas-phase. Chemisorption of CH4 (or any hydrocarbon) on metals involve the cleavage of C-H bond [58]. CH4 undergoes stepwise dehydrogenation on Ni leading to surface intermediates as proposed by Alstrup et al. [61] ... 

Biosynthesis. Tail-to-tail condensation of two molecules of geranylgeranylpyrophosphate (see Terpenes) gives phytoene, which undergoes stepwise dehydrogenation to produce the all-trans configuration of the true carotenoids (Fig. 1). Hie ionone rings of cyclic carotenoids arise by addition of a proton at C3, and formation of a bond between C2 and C7. This is followed... 

A new mild and stepwise dehydrogenation of tetrahydroisoquinolines involves the use of Fremy s salt (potassium nitrosodisulfonate) at room temperature. Best results were obtained when no substituent or only small alkyl substituents were present at... 

Figure 4.4. CH3OH stepwise dehydrogenation pathway and corresponding heats of formation on Pt, Ir, Os, Pd, Rh, and Ru [65]. (Reproduced with permission from J Am Chem Soc 1999 121 10928-41. Copyright 1999 American Chemical Society.)...

The stepwise dehydrogenation of CH3OH on Pt and oxidation to CO2 through the COad route can be summarized based on the theoretical model of Kua and Goodard (see also Figures 4.3-4.5) as follows ... 

Plants can introduce double bonds into fatty acids in both directions towards the terminal CH3-group or towards the carboxyl end. Oleic acid (oleoyl-CoA ester or P-oleoyl-phosphatidylcholine) is thus dehydrogenated to linoleic and then to linolenic acid. In addition synthesis of the latter can be achieved by another pathway involving stepwise dehydrogenation of lauric acid with chain elongation reactions involving C2 units (Fig. 3.5). 

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