Alkanes reactions, summary

Figure 8 Summary of the reactions of osmium atoms with alkanes and alkylbenzenes. Osmium atoms were co-condensed with the indicated substrates at...

The choice of appropriate reaction conditions is crucial for optimized performance in alkylation. The most important parameters are the reaction temperature, the feed alkane/alkene ratio, the alkene space velocity, the alkene feed composition, and the reactor design. Changing these parameters will induce similar effects for any alkylation catalyst, but the sensitivity to changes varies from catalyst to catalyst. Table II is a summary of the most important parameters employed in industrial operations for different acids. The values given for zeolites represent best estimates of data available from laboratory and pilot-scale experiments. 

This paper is a summary of our current understanding of this system. In particular, we will be discussing the observations in terms of selectivity with respect to the availability of reactive lattice oxygen. The organization of the paper is as follows. First, the general features of the reaction scheme for alkane oxidation on vanadate catalysts will be presented. This is followed by a discussion of results on the effect of ease of removal of oxygen from the lattice on the selectivity, and then a discussion on the importance of the atomic arrangement of the active sites. 

The initial coordination of reactants has indeed been proposed to explain the selective oxidation of alkenes in the presence of saturated hydrocarbons. It was argued that, owing to the hydrophobic nature of titanium silicates, the concentration of both hydrocarbons inside the catalyst pores is relatively high and hence the alkenes must coordinate to TiIv. Consequently, the titanium peroxo complex will be formed almost exclusively on Tilv centers that already have an alkene in their coordination sphere, and will therefore oxidize this alkene rather than an alkane which may be present in the catalyst (Huybrechts et al., 1992). Objections to this proposal are based on the fact that the intrinsically higher reactivity of alkenes with respect to saturated hydrocarbons is sufficient to account for the selectivity observed (Clerici et al., 1992). But coordination around the titanium center of an alcohol molecule, particularly methanol, is nevertheless proposed to explain the formation of acidic species, as was previously discussed. In summary, coordination around Tiiv could play a more important role than it does in solution chemistry as a consequence of the hydrophobicity of the environment where the reactions take place. 

In summary, catalytic C-H transformations in small unfunctionalized alkanes is a technically very important family of reactions and processes leading to small olefins or to aromatic compounds. The prototypical catalysts are chromia on alumina or vanadium oxides on basic oxide supports and platinum on alumina. Reaction conditions are harsh with a typical minimum temperature of 673 K at atmospheric pressure and often the presence of excess steam. A consistent view of the reaction pathway in the literature is the assumption that the first C-H abstraction should be the most difficult reaction step. It is noted that other than intuitive plausibility there is little direct evidence in heterogeneous reactions that this assumption is correct. From the fact that many of these reactions are highly selective toward aromatic compounds or olefins it must be concluded that later events in the sequence of elementary steps are possibly more likely candidates for the rate-determining step that controls the overall selectivity. A detailed description of the individual reactions of C2-C4 alkanes can be found in a comprehensive review [59]. 

Summary Rules for Naming Alkanes 94 3-4 Physical Properties of Alkanes 95 3-5 Uses and Sources of Alkanes 97 3-6 Reactions of Alkanes 99 3-7 Structure and Conformations of Alkanes 100 3-8 Conformations of Butane 104 3-9 Conformations of Higher Alkanes 106 3-10 Cycloalkanes 107 3-11 Cis-trans Isomerism in Cycloalkanes 109 3-12 Stabilities of Cycloalkanes Ring Strain 109 3-13 Cyclohexane Conformations 113... 

Hydrocarbon polymers are particularly susceptible to attack by atomic oxygen in LEO. The reactions of atomic oxygen with hydrocarbon molecules in the gas phase serve as models for the relatively unstudied reactions of atomic oxygen with a hydrocarbon surface. A wealth of knowledge of gas-phase reactions is available, largely because these reactions are important in combustion and in atmospheric chemistry. Studies of both reaction kinetics and dynamics have revealed many of the mechanisms by which atomic oxygen reacts with gaseous alkanes and alkenes. A summary of probable reaction pathways is presented in Fig. 4. 

On the other side, the addition of a second metal like Re, Ir or Sn, which increases the stability of Pt/Al203 catalyst, has been studied extensively in the literature. In summary, the effect of Sn is ascribed to i) a reduction of the number of Lewis sites [17] ii) an increase of selective hydrogenation of coke precursors (dienes) [18] Hi) a suppression of hydrogenolysis reactions [19]. Moreover, Sn can promote alkane oligomerization or even Diels-Alder type reactions [20]. 

During World War II, the great demand for aircraft fuel necessitated the production of large quantities of isobutane, a basic raw material in the production of high octane aviation gasoline. (See chapter on Alkylation of Alkanes. ) Various processes have been developed for the isomerization of n-butane to isobutane all of them employed aluminum chloride-hydrogen chloride as catalyst. The difference between the various processes consisted either in the method of introduction of aluminum chloride to the reaction zone, the catalyst support, or the state of the catalyst. The following summary describes some of the main features of the various processes which were developed ... 

In summary, it seems obvious that a generally applicable, simultaneous solution of the spin and stoichiometry problems is rather difficult to achieve. Indeed, as far as single 0-atom transfer is concerned, the two problems are inherently linked. Thus, a metal catalyst enable of both activating 02 and mono-oxygenating a substrate has to release one 0-atom equivalent. For example, in the oxygenation of an alkane (R-H) by dioxygen, the residual O-atom may be bound as a metal-oxo species or as a peroxide (Reaction 7). 

Aldehyde dlacetates. 1,1-Dlacetates of aldehydes can be prepared by using soluble protlc acids. With a catalytic amount of NATION, the same dlacetates can be obtained by vigorously stirring equivalent amounts of the aldehyde and freshly distilled acetic anhydride (25). The general reaction In equation 10 is exemplified by the summary In Table VIII. The yields are good with aromatic aldehydes and alkanals. 

Summary The synthesis of a,(0-bis[tris(trimethylsilyl)silyl] alkanes was achieved by the reaction of ct,(0-ditosylalkanes with tris(trimethylsilyl)silyl potassium. Reaction of these with one equivalent of potassium /er/-butoxide resulted in clean formation of monopotassium silyl anions. Addition of another equivalent of the transmetallating agent led to the formation of the dipotassium compounds in cases where the alkyl spacer contained at least three methylene units. Partial hydrolysis of the dipotassium compounds induced an intramolecular reaction yielding a cyclic silyl potassium compound. 

Cis-Trans Isomerism in Cycloalkanes A WORD ABOUT... Isomers—Possible and Impossible Summary of Isomerism Reactions of Alkanes A WORD ABOUT... Alternative Energy The Benefits of Hydrogen A WORD ABOUT... Methane, Marsh Gas, and Miller s Experiment... 

In summary, the carbonylative oxidation of arenes, alkanes, and alkenes can be carried out in the presence of a catalytic amount of a palladium catalyst and an oxidant. The various carbonyl compounds can be prepared directly from simple hydrocarbons by these reactions. 

The concept of fluorous biphase hydroformylation of heavy olefins was introduced by Horvath at Exxon in 1994 [42, 43]. Fluorocarbon-based solvents, especially perfluorinated alkanes and ethers, are of modest cost, chemically inert, and nonpolar and show low intermolecular forces. Most of them are immiscible with water and can be therefore used as the nonaqueous phase. Moreover, their miscibility with organic solvents such as toluene, THF, or alcohols at room temperature is quite low. Only at elevated temperature miscibility occurs. These features allow hydroformylation at smooth reaction conditions at 60-120 °C in a homogeneous system [44]. Upon cooling, phase separation takes place. The catalyst is recovered finally by simple decantation. One of the last summaries in this area was given by Mathison and Cole-Hamilton in 2006 [45]. 

Table 24.1. Summary of the best results in the most interesting partial oxidation reactions of C2-C3 alkanes.

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