Alkanes quaternary carbon

For hydrocarbon studies, analyses can be made without prior assumptions, since the carbons not carrying protons can be excited directly, this of course not being the case for hydrogen (e.g., quaternary carbons in alkanes, substituted carbons in aromatic rings). 

Table I gives the compositions of alkylates produced with various acidic catalysts. The product distribution is similar for a variety of acidic catalysts, both solid and liquid, and over a wide range of process conditions. Typically, alkylate is a mixture of methyl-branched alkanes with a high content of isooctanes. Almost all the compounds have tertiary carbon atoms only very few have quaternary carbon atoms or are non-branched. Alkylate contains not only the primary products, trimethylpentanes, but also dimethylhexanes, sometimes methylheptanes, and a considerable amount of isopentane, isohexanes, isoheptanes and hydrocarbons with nine or more carbon atoms. The complexity of the product illustrates that no simple and straightforward single-step mechanism is operative rather, the reaction involves a set of parallel and consecutive reaction steps, with the importance of the individual steps differing markedly from one catalyst to another. To arrive at this complex product distribution from two simple molecules such as isobutane and butene, reaction steps such as isomerization, oligomerization, (3-scission, and hydride transfer have to be involved.

The GP rule (eq. [21]) suffers from the poor statistical analysis for parameters describing methine and quaternary carbon atoms in highly branched alkanes. This was improved by Lindeman and Adams (120), who introduced another equation based on data for 39 alkanes, C5 through C9 ... 

In the case of alkanes with tertiary or quaternary carbon atoms the characteristic decomposition mode is usually the fragment alkane elimination (Sec. 2.6) and its yield can be used to estimate G(S i) values. Based on the fragment alkane elimination yield Pitchuozhkin et al. [162] calculated a singlet G value of 3.3 1 for 2,2,4-trimethylpentane. 

When a nonacidic catalyst such as nickel on kieselguhr is used, demethylation of alkanes occurs almost exclusively.66 It is significant that the demethylation is very selective—a methyl group attached to secondary carbon is more readily cleaved off than is one attached to a tertiary, which in turn is more readily eliminated than one at a quaternary carbon. Demethylation of 2,2,3-trimethylpentane yielded product consisting of 90% 2,2,3-trimethylbutane, and only 7% 2,3- and 3% 2,2-dimethylpentane. 

Exercise 22-23 Show explicitly how an alkyl side chain of alkylbenzenesulfonates could be formed with a quaternary carbon, if the C12 alkane used at the start of the synthesis contained any branched-chain C12 isomers. 

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. 

Saturated compounds. The position of absorptions of methyl, methylene, meth-ine and quaternary carbon atoms in the alkanes is shown in Fig. 3.49. Within each group the exact position of absorption is determined by the number and nature of substituents on the p and y carbons. Replacement of a proton by CH3 results in a downfield shift of c. 8 p.p.m. at C-l, and c. 10 p.p.m. at C-2, and an upfield shift at C-3 of c. 2 p.p.m. Polar substituents result in a downfield shift in the position of absorption Table A3.12 in Appendix 3 shows the effect on 13C chemical shifts of replacing a methyl group by various polar substituents. 

Very similar relationships were observed for other series of alkanes in which the weakest bond is that between two tertiary carbons (23, C — C, series) Eq. (5 and 6)16) or a tertiary and a quaternary carbon 25, C— Cq series) Eq. (7 and 8) ... 

Kenig F., Simons D.-J. H., Critch D., Cowen J. P., Ventura G. T., Brown T. C., and Rehbein T. (2002) Alkanes with a quaternary carbon centre a 2,200 Myr record of sulfide oxidizing bacteria. Geochim. Cosmochim. Acta 66, A393 (abstr.). 

Valuable information can be derived from the comparison of reaction rates of various hydrocarbons, measured by the decrease of their concentration due to oxidation [18], It is essential to note the important role of steiic factors which manifest themselves in a very similar way to the case of H-D exchange. Thus methyl and methylene groups adjacent to the quaternary carbon atom practically do not participate in the oxidation. 2,2-Dimethylpropane and 2,2,3,3-tetramethylbutane were shown to be oxidized by platinum(lV) much more slowly than alkanes with the same number of carbon atoms but with no quaternary carbon. According to [18], the number of carbon atoms ( o) accessible to attack by the platinum compounds can be approximately determined by the formula... 

Provided a branched alkane has only primary, secondary and tertiary carbon atoms, all hydrogen atoms can be exchanged by the a/S mechanism the carbon skeletons (devoid of hydrogen atoms) of some such molecules are shown in the first row of Table 6.5. The presence of a quaternary carbon atom as in neopentane (2,2-dimethylpropane) prevents the formation of an aa-diadsorbed species, so that multiple exchange, if it oecurs, must of necessity proceed through either aa- or ay-diasorbed structures, or where possible through anaS structure. Carbon skeletons of molecules of this type are shown in the lower part of Table 6.5, but... 

The introduction of single or double branches (i.e. of tertiary or quaternary carbon atoms) into alkane molecule immediately further differentiates the C—C bonds thus for example 2-methylpentane has four. The presence of branches also allows a greater variety of modes of attachment to the surface by dissociation of C—H bonds it is generally assumed that reactive species must be cr-diadsorbed. 

The CNMR data for selected substituted bicyclic and polycyclic alkanes are listed in Table 9. Noteworthy is the upheld shift for the quaternary carbon atom of the tert-buiy group in 24, which can be attributed to the special bonding character in the tetrahedrane... 

The fragmentation of an alkane can be understood by assuming that a C—C bond is broken, sometimes accompanied by a hydrogen shift in order to minimize the energy requirements this is, to a first approximation, a rather satisfactory model. In the case of a branched alkane the bond to a tertiary or quaternary carbon atom is more easily broken, a fact that allows the determination of the branching point(s) with a good level of confidence. Except for small alkanes, the cleavage of C—H bonds seems to be a rare event. 

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