Alkanes oxidation level

We must include simple alkanes, which have no bonds to heteroatoms, as an alkane oxidation level . 

Sophorolipid is a glycolipid, ie it is composed of carbohydrate and lipid. It therefore contains moieties of widely different oxidation levels and its synthesis from single demand carbon sources has a high ATP demand. However, the demand for ATP is reduced if a mixture of glucose and C-18 alkane is used. If glucose and fatty add is used the ATP demand is reduced further and relatively high spedfic production rates can be achieved. 

Considering the various alkanes shown below, one sees that alkanes can have several different oxidation levels for carbon. Oxidation levels can range from —4 for methane and —3 for the carbon atom of methyl groups all the way to 0 for the quaternary carbon of neopentane. In spite of the several oxidation levels possible in alkanes, the functional group approach tells us that all are saturated alkanes and thus have the same functional equivalency and similar reactivity patterns. 

This conclusion is made from the fact that all the carbons in alkanes have four single bonds originating from them and those a bonds go to either carbon or hydrogen. Thus one cannot automatically assign a molecule to a functional class based solely on a certain oxidation level of the carbons that it contains. 

Furthermore it is evident that because the lowest possible oxidation level of a single carbon atom in an alkene is —2 while the lowest possible oxidation level of a carbon atom in an alkane is —4, alkenes are thus oxidized relative to alkanes. 

The same process can be carried out to determine the oxidation levels of carbon atoms in several common functional types. It is clear that by using these procedures we can assign oxidation levels to carbon atoms in a wide variety of compounds. It is also clear that knowing the oxidation level is insufficient to assign the functional group present. For example, the alkane neopentane, the alkene isobutylene, the alkyne propyne, the alcohol isopropanol, and formaldehyde all have a carbon with an oxidation level of 0 yet all belong to completely different functional classes and have different physical and chemical characteristics. 

The Grignard reaction is often one of the first reactions encountered for the preparation of organometallic compounds. As such it provides a method for the conversion of an alkyl bromide to an alkane. From the example shown below it is seen that the overall oxidation level change from the organic reactants to the products is from 0 to —2, so a reduction has occurred. Magnesium is the reductant and is itself oxidized from 0 to +2 oxidation state. The actual reduction takes place in the first step of the process in which the C-Br bond is converted to a C-Mg-Br bond. The reaction with water is merely a hydrolysis that does not change the oxidation state of carbon. 

C. Alkanes are at the lowest oxidation level, and CO2 is at the highest level. 

A list of compounds of increasing oxidutlou level Is shown in Figure 10.6. Alkanes are at the lowest oxidation level because they have the maximum possible number of C-H bonds, and 00 is a the highest level because it lias the maximum possible number of C O bonds. Any reaction that converts a compound from a lower level to a higher level is an oxidation, any reaction that converts a compound from a higher level to a lower lewl is s reduction, and any reaction that doesn t change the level is neither an oxidation nor a reduction. 

Consider the C-H bond in alkanes. Carbon is a more electronegative element than hydrogen. Consequently, the electron pair that forms this bond is shifted towards the carbon atom. In the extreme, an ionic representation of this bond can be given as pictured in 122 (Scheme 2.45). Within these conventions the carbon atom in an alkane can be approximated as a carbanion (oxidation level 0 by definition). Using this definition it becomes possible to apply oxidation-reduction terminology to the processes as if they occurred to ion pair 122. Thus, oxidation of 122 with the loss of one electron leads to the radical 123. With the loss of two electrons, the oxidation leads to carbocation 124. Similarly, the conversion of an alkane to an alcohol and the alcohol into an aldehyde and the aldehyde eventually to a carboxylic acid can unambiguously be classified as an oxidation sequence with the loss of two, four, and six electrons. The oxidation levels 1, 2, and 3 are ascribed respectively to these functional derivatives. The conversion of an alkane to an alkene or alkyne can be interpreted in an analogous fashion. 

Dehydrogenation of alkanes such as ethane (1.38) relates them to alkenessuch as ethene (ethylene, 1.39). The same functional group may be obtained by dehydration of ethanol (1.40). Further dehydrogenation of ethene would generate an alkyne, ethyne (acetylene, 1.41). In terms of oxidation level, the alkene is related to the alcohol and the alkyne is related to the ketone. 

It follows that one may conclude that basic properties as well as atomic arrangements at molecular level play an important role in alkane oxidative dehydrogenation reactions. Such a conclusion could also be reached from the study of VMgO catalysts [42] for oxidative dehydrogenation of several alkanes as ethane, propane and butane under similar conditions (see e g. fig. 5 in ref 42). 

Of course, such accounting for mass-transfer is an oversimplification of real processes taking place during alkane oxidation over real catalysts. Additional studies are required to estimate the possibility to integrate a detailed microchemical (and micro-kinetic) description with methods capable of advanced accounting of mass-transfer on the inter- and intra-particle level and in the bulk of reactor (see, for instance, Couwenberg, 1994 Hoebink et al, 1994). 

The main conclusion that could be derived from the above analysis is the ascertaining of the extraordinary complexity of the system we attempt to model. Even staying at the level of micro-chemical modeling we must accept as a fact the fundamental difference between the phenomenon and its model and, as a result, the impossibility to develop the description that reflects the reality in all its manifestations. A possible solution of the modeling problem, as applied to light alkane oxidative processing, can be found on the way of stepwise and successive execution of the basic principles stated below. 

The oxidation of light alkanes by air or O2 at supercritical temperatures and pressures was explored by Standard Oil in the mid-1920s [153]. Experiments were performed at the laboratory and then semicommercial plant level. The primary products were alcohols. For example, the oxidation of pentane was performed at supercritical conditions (240 °C, around 200 bar and a few mole per cent O2) and produced primarily C2-C3 alcohols and acids. However, the oxidation of heptane was performed at subcritical temperatures (225 °C) and produced primarily Cg-Cy alcohols. The change in selectivity was attributed to either the difference in phase or more likely the difference in temperature. Other commercial processes for the formation of alcohol denaturants or formaldehyde were reported in the same decade [154,155], but it is unclear whether those reactions were operated at supercritical pressures. Modem processes involving alkane oxidation are heterogeneously catalyzed and operated at sub-critical pressures [156]. 

Sen and coworkers have examined systems based on Pd(OAc)2 in triflic acid (TfOH) at 80 °C (equation 5). Na2Cr207/Pd(0Ac)2/Tf0H aromatizes cyclohexanes for example, decalin gives naphthalene (9%) and a-tetralone (4%). Normally the functionalization product of an alkane is more reactive than the alkane itself and so only the low conversion prevents the initial product from being oxidized further. Shilov and Sen s use of triflic acid in this context means that the initial functionalization product at the alcohol oxidation level is protected as the triflate ester, which is relatively insensitive to oxidation. 1,4-Dimethylbenzene is oxidized to triflates with a 50 1 preference for oxidation at the ring rather than of the side chain, unlike the selectivity expected in a radical process. A kinetic isotope effect of 5 was measured. In certain cases the reaction can be... 

An aldehyde (RCHO) or ketone (RCOR) can therefore be reduced to an alkane via a hydrazone. This transformation can also be achieved using the Oxidation levels can be used to Mozingo reduction (see Section 8.3.6) or the Clemmensen reduction (shown assess if a reaction involves... 

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