Branched alkanes from alkenes

Contaminants in recycled plastic packaging waste (HDPE, PP) were identified by MAE followed by GC-MS analysis [290]. Fragrance and flavour constituents from first usage were detected. Recycled material also contained aliphatic hydrocarbons, branched alkanes and alkenes, which are also found in virgin resins at similar concentration levels. Moreover, aromatic hydrocarbons, probably derived from additives, were found. Postconsumer PET was also analysed by Soxhlet extraction and GC-MS most of the extracted compounds (30) were thermally degraded products of additives and polymers, whereas only a few derived from the original contents... 

Kerogens isolated from the Fig Tree cherts produced very complex mixtures of pyrolysis products, dominated by a series of methyl branched alkenes with each member of the series having 3 carbon atoms more than the previous member. At each carbon number a highly complex mixture of branched alkanes and alkenes plus various substituted aromatic compounds was found. The highly branched structures may have actually incorporated isoprenoids originally present in the Precambrian microorganisms (Philp Van DeMent, 1983)6>. 

The ability of insects to withstand desiccation was recognized in the 1930s to be due to the epicuticular layer of the cuticle. Wigglesworth (1933) described a complex fatty or waxy substance in the upper layers of the cuticle which he called cuticulin . The presence of hydrocarbons in this wax of insects was suggested by Chibnall et al. (1934) and Blount et al. (1937), and over the next few decades the importance of hydrocarbons in the cuticular wax of insects was established (Baker et al., 1963 and references therein). The first relatively complete chemical analyses of the hydrocarbons from any insect, the American cockroach, Periplaneta americana (Baker et al., 1963), occurred after the development of gas-liquid chromatography (GLC). The three major components of the hydrocarbons of this insect, //-pen taco sane, 3-methylpentacosane and (Z,Z)-6,9-heptacosadiene, represent the three major classes of hydrocarbons on insects, n-alkanes, methyl-branched alkanes and alkenes. Baker and co-workers (1963) were able to identify n-pentacosane by its elution time on GLC to a standard and its inclusion in a 5-angstrom molecular sieve. 3-Methylpentacosane... 

The outer covering of insects consists of a layer of water repellent lipids, frequently made up of alkanes, methyl-branched alkanes and alkenes. This lipid layer is important to prevent dehydration and to repel rain and in social insects (bees, wasps, ants and termites), the mixture is characteristic of the group, and the available evidence suggests the mixture helps individuals to distinguish between nestmates and individuals from... 

The distribution of volatile products of low molar mass from the irradiation of poly (olefin) s is strongly dependent on the nature of substituents (short-chain branches) on the backbone chain. Hydrogen is the main volatile product with smaller quantities of alkanes and alkenes. 

We have studied the alkane and alkene yields from the radiolysis of copolymers of ethylene with small amounts of propylene, butene and hexene. These are examples of linear low density polyethenes (LLDPE) and models for LDPE. Alkanes from Ct to C6 are readily observed after irradiation of all the polymers in vacuum. The distribution of alkanes shows a maximum corresponding to elimination of the short-chain branch. This is illustrated in Figure 8 for the irradiation of poly (ethylene-co-1-butene) containing 0.5 branches per 1,000 carbon atoms at 20 C. 

Alkenes are only produced in significant amounts above ca. 80 C. Ethylene is produced with the highest yield, which may be comparable to that for alkanes from short-chain branches after irradiation above 150 C (15). Typical results for the increasing yields of alkanes and alkenes with irradiation temperature are shown in Figure 10. Closer examination of the butane and butene produced has shown that they include considerable proportions of isobutane and isobutene. Typical G values for the formation of the butenes at... 

Several alkene isomers vary structurally in the position of a methyl branch on their parent 1-alkene chain. Generally, moving the methyl group from C3 to positions further from the double bond results in an exothermic enthalpy of isomerization. That is, the isoalkyl-1-alkenes are the most stable isomers and the 3-methyl-1-alkenes are (presumably) the least stable. Because of the problematic 5-methyl-1-hexene data and the lack of data for 3-methyl-1-heptene, nothing more quantitative can be said other than each methyl re-positioning down the chain results in about 1-2 kJmol-1 stabilization. A similar change in branching position from 3-methyl-n-alkanes to 2-methyl-n-alkanes releases about 3 kJmol-1. 

Most oils contain low levels of saturated and unsaturated hydrocarbons. In olive oil, the unsaturated hydrocarbon squalene can constitute up to 40% of the unsaponifiable fraction (Boskou, 1996). Other hydrocarbons commonly present in olive oil are straight chain alkanes and alkenes with 13 to 35 carbon atoms, along with very low amounts of branched chain hydrocarbons. Variations are found between different olive varieties but the main hydrocarbons are those with 23, 25, 27 and 29 carbon atoms (Guinda et al., 1996). Olive oil can clearly be differentiated from other vegetable oils on the basis of hydrocarbon components, and levels of 2.6% crude rapeseed oil or crude sunflower oil can be detected by hydrocarbon analysis (Webster et al., 1999). Terpenes have been identified in the volatile fraction of crude sunflower oil (Bocci and Frega, 1996). 

Aromatic Hydrocarbons and Light Alkanes. Carbonaceous chondrites contain a wide range of aromatic hydrocarbons, from benzene through alkylbenzenes and -naphthalenes to polynuclear hydrocarbons of up to six fused benzene rings (see the reviews cited above). At higher carbon numbers, aromatics tend to be less abundant than normal alkanes, but below about C j, the reverse is true (Fig. 3, top). In fact, virtually no normal alkanes at all are found between and Cg, their place having been taken largely by benzene, toluene, xylene, and various alkenes or branched alkanes, notably butene (Studier et al., 1965 b, 1968, 1972 Hayes and Biemann, 1968 Levy et al., 1973). ... 

A composition of a typical gasoline from a refinery is given in Table 7.1. The main constituents are branched alkanes, aromatic hydrocarbons with one ring and alkenes. Toluene and isopentane (methyl butane) often occur in the largest concentrations. 

Alkylation is a synthetic process in which lower alkenes (from catalytic cracking) are reacted in an acid medium (sulphuric or hydrofluoric acid) with small branched alkanes to produce Cg to Cg branched alkanes. These are probably the most desirable constituents of a gasoline, with good knock resistance and with fewer undesirable properties, such as tendencies to... 

One general approach is to consider the balance between radical production and loss by the different components of a mixture. Alkanes, because of their low-temperature chemistry have active chain-branching reactions, while alkenes and aromatics have efficient termination reactions through the production of stabilized radicals, such as allyl and benzyl radicals. While the rates of the branching and termination processes arise from contributions by each of the constituents in the mixture in a way that depends linearly on their composition, the overall rate of the autoignition reactions depends on branching and termination in a non-linear fashion. 

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