Alkanes highly branched

By combining the basic principles of lUPAC notation with the names of the various alkyl groups we can develop systematic names for highly branched alkanes We 11 start with the following alkane name it then increase its complexity by successively adding methyl groups at various positions... 

Even alkanes, when treated with superacids, can undergo oligocondensation. Eor example, highly branched polyalkanes, of molecular weight up to 700, were obtained by treating gaseous alkanes (C —with Hquid superacids at room temperature (81). 

The alcohol fraction is likewise a complex mixture of both aUphatic and cycHc compounds (Table 10). The principal components are cholesterol (34%), and lanosterol and dihydrolanosterol (38%). The aUphatic alcohols account for about 22% of the unsaponifiable products. Sixty-nine components of ahphatic alcohols had been reported up to 1974 (latest reported work as of ca 1997). The hydrocarbons (ca 0.5%) show stmctural similarity to the wool—wax acids or ahphatic alcohols and contain highly branched alkanes as well as cycloalkanes. 

Catalytic cracking When a mixture of alkanes from the gas oil fraction (C12 and higher) is heated at very high temperature (-500 °C) in the presence of a variety of catalysts, the molecules break apart and rearrange to smaller, more highly branched alkanes containing 5-10 carbon atoms. 

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. 

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 ... 

The ability of the Lindeman-Adams (LA) rule to predict, 3C chemical shifts of even highly branched alkanes satisfactorily was proven impressively by calculations of the shieldings of the dodecanes 292 to 294 (393). 

Greenwood PF, Arouri KR, Logan GA, Summons RE, Abundance and geochemical signihcance of Cj dialkylalkanes and highly branched alkanes in diverse Meso- and Neoproterozoic sediments, Org Geochem 35 331-346, 2004. 

Some of the cations may fragment or undergo ion-molecule reactions before neutralization. As shown in the older literature, the fragmentation has higher yield in the radiolysis of the smaller and/or highly branched alkanes such as neopentane or isooctane. Ion-molecule reactions, such as the H transfer, may also reduce the Si yield ... 

Alkane elimination is a basic photodecomposition mode of highly branched alkanes, e.g., most of the excited neopentane molecules split directly to methane and isobutene. 

Alkane isomerization equilibria are temperature-dependent, with the formation of branched isomers tending to occur at lower temperatures (Table 4.1). The use of superacids exhibiting high activity allows to achieve isomerization at lower temperature (as discussed below). As a result, high branching and consequently higher octane numbers are attained. Also, thermodynamic equilibria of neutral hydrocarbons and those of derived carbocations are substantially different. Under appropriate conditions (usual acid catalysts, longer contact time) the thermodynamic... 

Condensation of Q—C4 alkanes to produce highly branched oligomeric and polymeric hydrocarbons41 was also achieved by their condensation in FSO3H—SbF5. Block methylene units in the polymeric chain were observed even when methane was brought into reaction with alkenes under similar conditions.42... 

---