Dimethyl branched alkanes

Resolution of diastereomeric amides by preparative HPLC has been developed for a-branched alkanoic acids, and has been employed to prepare dimethyl branched alkanes implicated in tsetse fly sexual communication (6). In much of our own work we have made use of fractional crystallization whereby racemic acids are converted to amides of either (R)- or (S)-ot-methylbenzylamine. These amines are available from Hexcel Corp., Zeeland, MI, and the diastereomeric amides can be analyzed for purity chromato-graphically. 

Figure 5 Chromatograms on the ZIF-8-coated capillary (20-m long x 0.25-mm i.d.) for GC separation of (a) hexane and its branched isomers, (b) octane and its branched isomers, (c) 2,2-dimethyl-branched alkanes and hnear alkanes at a N2 flow rate of 1 mLmin under 170 °C, and (d) linear alkanes at a Nj flow rate of 1.5 mL min using a temperature program 140 °C for 1 min, then 20 °C min to 290 °C for the remainder. (Reproduced with permission from Ref 49. 2010 American Chemical Society, 2010.)...

From Mark s RIS model for ethylene-propylene copolymers (J. Chem. Phys. 1972, 57, 2541) it is determined that P(t) = 0.380, P g+) = 0.014, and Pig") = 0.606 in 2,4-dimethylhexane (2,4-DMH). Using this RIS model, furthermore, for all the branched alkanes considered whose isopropyl groups are separated by at least one methylene carbon from the next substituted carbon and the RIS model developed by Asakura et at. (Makromol. Chem, 1976, 177, 1493) for head-to-head polypropylene to treat 2,3-dimethyl pentane, AS s are calculated for a large number of branched alkanes. The agreement between the observed and the calculated nonequivalent 13C NMR chemical shifts is quite good, including the prediction that separation of the isopropyl group from the next substituted carbon by four or more methylene carbons removes the nonequivalence. 

The pore openings of zeolite A just allow normal alkanes to enter the pore system of the crystal. It might be of interest to have a zeolite available which adsorbs the normals and the mono methyl-branched paraffins and leaves the high octane dimethyl branched compounds unadsorbed. 

Shiea J., Brassell S. C., and Ward D. M. (1990) Mid-chain branched mono- and dimethyl alkanes in hot spring cyanobacterial mats a direct biogenic source for branched alkanes in ancient sediments Org. Geochem. 15, 223-231. 

One convenient simplification becomes clear when considering relatively simple or light components such as the branched C8H18. As shown in Table VI, only a few isomers describe the whole fraction of branched alkanes with eight carbon atoms three monomethyl-heptanes, ethyl-hexane and four dimethyl-hexanes with a tertiary C structure. In spite of the different origins of these feeds, there is clear regularity with regard to their composition. In fact,... 

Intramolecular oxidation proceeds with unusual facility in dimethyl-pentane compared with most other simple normal and branched alkanes because of structural and kinetic features that are particularly favorable. In the following reaction scheme (where HRH is dimethylpentane)... 

For branched alkanes, the fragmentation patterns could be very complex [3, 111]. The prompt yield of the -H radicals is always minor the highest yields are of the radicals formed by scission of skeletal C-C bonds next to the branches. For example, in radiolysis of isooctane only 15% of the radicals are of the -H type, the rest bemg tert-butyl and 2-propyl radicals [111]. In radiolysis of 2,3-dimethylbutane, 70% of radicals are 2-propyl and 30% are -H radicals (2,3-dimethyl-2-butyl). It is not known what species dissociate (singlets triplets excited holes excited radicals ) and what controls these fragmentation patterns. 

Methanol and Wood Conversion Product Classes. Methanol has been used in this screening work to ascertain catalyst activity. The methanol relative product distribution on an active, pure catalyst is shown in Figure lA. (Table II gives the identification of the ions observed). No methanol (m/z 31 and 32) breakthrough was observed, and the first formed product, dimethyl ether (m/z 45 and 46), has been consumed to form a mixture of C2 to Cg olefins and toluene, xylene, and trimethyl-benzene. Note the lack of benzene and alkanes. With lower space velocities and higher methanol partial pressure, the alkenes are known to disproportionate to branched alkanes and to form more aromatics (11). The absence of products above m/z 120 indicates the well-known shape selectivity of the catalyst. 

The liquid density, a measure of molecular packing and molar volume, has been modeled by Kier and HalF. For normal alkanes the density is inversely proportional to the count of carbon atoms. Hence in this model, an inverse x index is used. When dl is plotted against set of convergent lines is obtained for the various classes of branched alkane isomers, such as 3-methyl, 2-methyl, 2,3-dimethyl, etc. The focus of these convergent lines is a point on the density axis, 0.83. ... 

Well over 100 compounds have been determined in seawater (and sediments) using a considerable variety of techniques (Table 2). Methods include n-and branched alkanes (up to about C20, pristane/ phytane), alkenes and aromatic compounds (up to the disubstituted naphthalenes), halocarbons and chlorinated aromatic species, low relative molecular mass alcohols, organic sulfur compounds (notably dimethyl sulfide, a major product of some phytoplankton species, but ranging up to dimethyl trisulfide), and freons (11, 12, and 113 used in studies of oceanic mixing). 

Zeolite A is extensively used as selective adsorbent in several purification and separation processes [9]. The most prominent examples are the enrichment of oxygen from air [e.g. 9,10] and the separation of linear and branched alkanes [e.g. 9,1 Ij. Among others, zeolites Rho and ZK-5 have been studied in detail as catalysts for the selective amination of methanol with ammonia to dimethyl-amine [e. g. 12,13 ]. Also, the potential of small pore zeolites for the conversion of methanol to lower olefins has been explored [e.g. 14,15]. 

Different regio-selectivities are found in the sMMO-catalyzed hydroxylation of branched alkane. Sterically hindered tertiary carbon is not reactive. In the oxygenation of 2,3-dimethylpentane catalyzed by sMMO from M. capsulatus (Bath), 3,4-dimethyl-2-pentanol is the sole product as shown in eq. (6) The sMMO-catalyzed hydroxylation of isopentane occurs mainly at the primary carbons of the alkane (see Table 3). These different selectivities may depend on shape and size of the substrate binding site of sMMO. These reactivities are similar to the (o-hydroxylation of -alkane catalyzed by cytochrome P-450 [74]. 

Only large-pore zeolites exhibit sufficient activity and selectivity for the alkylation reaction. Chu and Chester (119) found ZSM-5, a typical medium-pore zeolite, to be inactive under typical alkylation conditions. This observation was explained by diffusion limitations in the pores. Corma et al. (126) tested HZSM-5 and HMCM-22 samples at 323 K, finding that the ZSM-5 exhibited a very low activity with a rapid and complete deactivation and produced mainly dimethyl-hexanes and dimethylhexenes. The authors claimed that alkylation takes place mainly at the external surface of the zeolite, whereas dimerization, which is less sterically demanding, proceeds within the pore system. Weitkamp and Jacobs (170) found ZSM-5 and ZSM-11 to be active at temperatures above 423 K. The product distribution was very different from that of a typical alkylate it contained much more cracked products trimethylpentanes were absent and considerable amounts of monomethyl isomers, n-alkanes, and cyclic hydrocarbons were present. This behavior was explained by steric restrictions that prevented the formation of highly branched carbenium ions. Reactions with the less branched or non-branched carbenium ions require higher activation energies, so that higher temperatures are necessary. 

Nooner and Oro, 1967). Nonnal (straight-chain) alkanes are most prominent, followed by methyl and dimethyl alkanes and structurally similar, slightly branched alkenes (Fig. 2). 

Heightened relative abundances of monomethyl, dimethyl, and other branched acyclic alkanes is another distinctive feature of Proterozoic... 

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