Inertness of alkanes

Saturated hydrocarbons are the main constituents of petroleum and natural gas. Mainly used as fuels for energy production they also provide a favorable, inexpensive feedstock for chemical industry [74]. Unfortunately, the inertness of alkanes renders their chemical conversion challenging with respect to selectivity. Clearly, the development of new and improved methods for the selective transformation of alkanes belongs to the central goals of catalysis. Iron-catalyzed processes might be a smart tool for such transformations (for reviews see [75-77]). 

Chemical inertness of alkanes is reflected in one of their old names, paraffins , from the Latin parum affinis (without affinity). However, saturated hydrocarbons can be involved very easily in a total oxidation with air (simply speaking, burning) to produce thermodynamically very stable products water and carbon dioxide. It should be emphasized that at room temperature alkanes are absolutely inert toward air, if a catalyst is absent. At the same time, some active reagents, e.g., atoms, free radicals, and carbenes, can react with saturated hydrocarbons at room and lower temperatures. These compounds are easily transformed into various products under elevated (above 1000 °C) temperatures, in the absence of other reagents. 

Some important reactions of alkanes have been developed, e.g., autoxi-dation by molecular oxygen at elevated temperatures, which proceeds via a radical chain mechanism. The main feature of this and many other reactions is a lack of selectivity. Reactions with radicals give rise to the formation of many products all possible isomers may be obtained. As far as burning is concerned, this process can be very selective, producing solely carbon dioxide, but apart from being an important source of energy, is useless from the viewpoint of the synthesis of valuable organic products. Chemical inertness of alkanes is due to... 

The chemical inertness of alkanes can be overcome if the transformations are carried out at high temperatures. However, the low selectivity of such processes motivates chemists into searching principally for new routes of alkane conversion which could transform them into very valuable products (hydroperoxides, alcohols, aldehydes, ketones, carboxylic acids, olefins, aromatic compounds etc.) under mild conditions and selectively. This is also connected with the necessity for the development of intensive technologies and for solving... 

The lowercase n in Reaction 2.14 represents a large, unspecified number. From this reaction, we see that polyethylene is essentially a very long chain alkane. As a result, it has the chemical inertness of alkanes, a characteristic that makes polyethylene suitable for food storage containers, garbage bags, eating utensils, laboratory apparatus, and hospital equipment... 

The different PFCLs used as interoperative tools can be categorised as a group of compounds with more or less uniform behaviour. On the contrary, the partially fluorinated species have to be regarded as individuals each. This is due to the hybrid character of these compounds, which combine the behaviours of alkanes and perfluoroalkanes. Especially, the toxicological behaviour cannot be optimised by simple modifications of the degree of fluorination. If the two molecule parts are combined in the right balance, the cytotoxic behaviour of the alkyl chain can be overcompensated by the inert perfluorinated part of the compound. But each compound should be tested individually to ensure its suitability as a candidate for ophthalmic use. 

Problem 4.18 (a) Why are alkanes inert (b) Why do the C—C rather than the C—H bonds break when alkanes are pyrolyzed (c) Although combustion of alkanes is a strongly exothermic process, it does not occur at moderate temperatures. Explain. 4... 

Chlorine or bromine reacts with alkanes in the presence of light (hv) or high temperatures to give alkyl halides. Usually, this method gives mixtures of halogenated compounds containing mono-, di-, tri- and tetra-halides. However, this reaction is an important reaction of alkanes as it is the only way to convert inert alkanes to reactive alkyl halides. The simplest example is the reaction of methane with CI2 to yield a mixture of chlorinated methane derivatives. 

Arguments similar to those stated above can be used to explain the relative chemical inertness of fluoropolymers. Consider the reactivity of alkanes vs. perfluoroalkanes as shown in Table 4.2 (abstracted from Sheppard and Sharts Statistically, FA based materials will have many more types of bonds, in addition to C—F, than fluoropolymers. These bonds will be subject to the same chemical fate during assault by aggressive reagents as bonds in their hydrocarbon counterparts. Similar reasoning can be used to explain the relative thermal stability of FAs compared to fluoropolymers. Thus, incorporation of perfluoroalkyl groups will not make the modified material less stable than the native one. 

These polysiloxanes were also found to be synthesized by the condensation reaction between organosilanes and organoalkoxysilanes with release of alkane as an inert by-product. For this polycondensation, B(C6F5)3 was used as an effective catalyst [88]. This process involves cleavage of C-0... 

Alkanes, which are the principal components of natural gas and crude oil, are still the preferred energy source of our society. In regard to the prime importance of alkanes as feedstock for the chemical industry, it appears a waste of resources simply to burn these precious raw materials. Unfortunately, attempts to transform alkanes into more valuable products are hampered by their low reactivity, as best illustrated by the use of alkanes as inert solvents. For example, the cracking process requires temperatures of about 1000 °C in order to convert long-chain alkanes into short-chain alkanes. Controlled conversion of hydrocarbons is difficult to achieve and limited to partial oxidations, such as the conversion of butane into acetic acid. It is obvious that processes that would enable efficient functionalization to occur at low temperature would have enormous potential application. Achievements towards this goal will almost certainly rely on the use of catalysts, which will have to activate the stable C-H bond (375-440 kf mol-1) in order to induce its scission. 

Inert solvents alkanes and fluoroalkanes constitute the main members of this class, which is quite large, but does not play a great role in... 

TS-1 catalyzes the hydroxylation of alkanes with dilute solutions of hydrogen peroxide in water, in a biphasic system of alkane and aqueous H2O2, or in aqueous-organic solution. The rate of reaction decreases in the solvent order butanol > butanol/water > methanol = acetonitrile = water [24, 25]. The temperature is generally lower than 55 °C in methanol, close to 100°C in water and of intermediate values in other solvents. Hydroxylation occurs at secondary and tertiary C—H bonds, while primary ones are completely inert (Equations 18.3 and 18.4). 

FIGURE 25.27 Schematic representation of the Oj profile generated in a tubular reactor for the partial oxidation of alkanes (a) by a classical inert uniform porous membrane, and (b) by the suggested chemical valve membrane. (From Julbe, A., Farrusseng, D., and Guizard, C /. Membr. ScL, 181, 3, 2001.)... 

Farrusseng D, Julbe A, and Guizard C. Evaluation of porous ceramic membranes as O2 distributors for the partial oxidation of alkanes in inert membrane reactors. Sep Purif Technol 2001 25 137-149. 

Recently, Sen has reported two catalytic systems, one heterogeneous and the other homogeneous, which simultaneously activate dioxygen and alkane C-H bonds, resulting in direct oxidations of alkanes. In the first system, metallic palladium was found to catalyze the oxidation of methane and ethane by dioxygen in aqueous medium at 70-110 °C in the presence of carbon monoxide [40]. In aqueous medium, formic acid was the observed oxidation product from methane while acetic acid, together with some formic acid, was formed from ethane [40 a]. No alkane oxidation was observed in the absence of added carbon monoxide. The essential role of carbon monoxide in achieving difficult alkane oxidation was shown by a competition experiment between ethane and ethanol, both in the presence and absence of carbon monoxide. In the absence of added carbon monoxide, only ethanol was oxidized. When carbon monoxide was added, almost half of the products were derived from ethane. Thus, the more inert ethane was oxidized only in the presence of added carbon monoxide. 

In principle, the adsorption of alkane molecules on C(OOOl) would appear unlikely because of the inert character of the substrate. In this case, however, besides van der Waals forces, other contributions come into play and can make energy adsorption reach values of up to lOOkJ/mol, which are comparable to those of chemisorption processes. This enables determining the structure of aliphatic hydrocarbons adsorbed on C(OOOl) by AFM or STM because the adsorbate withstands tip—sample interaction forces. 

Paraffin — A waxy substance obtained from the distillation of crude oils and often contained in the crude oils. Paraffin is a complex mixture of alkanes with higher numbers of carbon that is resistant to water and water vapour and is chemically inert. The term is sometimes used to refer to alkanes as a class of compounds. (See also Alkanes, Waxes.)... 

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