Alkane branched-chain

Compounds like butane and pentane, whose carbons are all connected in a row, are called straight-chain alkanes, or normal alkanes. Compounds like 2-methylpropane (isobutane), 2-methylbutane, and 2,2-dimethylpropane, whose carbon chains branch, are called branched-chain alkanes. The difference between the two is that you can draw a line connecting all the carbons of a straight-chain alkane without retracing your path or lifting your pencil from... 

Figure 6.2. Typical ignition delay of an alkane fuel as a function of the initial mixture s temperature. Three different kinetic models are shown (a) High temperature chemistry only that is, no peroxy radical chemistry, (b) Same as (a), but the Q OOH chain-branching channel of the peroxy radicals has been considered, (c) Same as (b), bnt the concerted elimination of RO2 to alkene + HO2 has been considered. (Figure courtesy of Timothy Barckholtz, ExxonMobil Research and Engineering.)...

After radical initiation of an alkane fuel, chain branching is immediately accessible via new high-temperature pathways with O2 ... 

Branching of the alkane chain lowers the boiling point (Table 4.2). 

The hydroisomerization of heavy linear alkanes is of a great interest in petroleum industry. Indeed, the transformation of long chain n-alkanes into branched alkanes allows to improve the low temperature performances of diesel or lubricating oils [1-3]. On bifunctional Pt-exchanged zeolite catalysts, n-CK, transformed into monobranched isomers, multibranched isomers and cracking products [4], The HBEA zeolite based catalyst was more selective for isomerization than those containing MCM-22 or HZSM-5 zeolites [4], This was explained on one hand by a rapid diffusion of the reaction intermediates inside the large HBEA channels, and on the other hand by the very small crystallites size of this zeolite (0.02 pm). 

Several reaction pathways for the cracking reaction are discussed in the literature. The commonly accepted mechanisms involve carbocations as intermediates. Reactions probably occur in catalytic cracking are visualized in Figure 4.14 [17,18], In a first step, carbocations are formed by interaction with acid sites in the zeolite. Carbenium ions may form by interaction of a paraffin molecule with a Lewis acid site abstracting a hydride ion from the alkane molecule (1), while carbo-nium ions form by direct protonation of paraffin molecules on Bronsted acid sites (2). A carbonium ion then either may eliminate a H2 molecule (3) or it cracks, releases a short-chain alkane and remains as a carbenium ion (4). The carbenium ion then gets either deprotonated and released as an olefin (5,9) or it isomerizes via a hydride (6) or methyl shift (7) to form more stable isomers. A hydride transfer from a second alkane molecule may then result in a branched alkane chain (8). The... 

The interdigital secretion of the red hartebeest, A. b. caama, consists of fewer compound classes. It contains a few alkanes and short-chain, branched alcohols, fatty acids, including a few of the higher fatty acids up to octadecanoic acid, an epoxide and the cyclic ethers, rans-(2 ,5.R)-furanoid linalool oxide 23, as-(2JR,5S)-furanoid linalool oxide 24 and ds-(2S,5i )-furanoid linalool oxide 25 (Fig. 5) in a ratio of 2.5 1 1.5 respectively [138]. From the point of view that many of the constituents of the interdigital secretion of this animal are probably of microbial origin, it is interesting that cis- and trans- furanoid linalool oxides have also been found in castoreum [77]. 

Summary of octane number aromatics, alkenes, and alkynes > cyclic alkanes and branched alkanes > straight-chain alkanes. 

Free-radical polyolefin reactions form polymers with many mistakes in addition to the ideal long-chain alkanes because of chain-branching and chain-termination steps, as discussed. This produces a fairly heterogeneous set of polymer molecules with a broad molecular-weight distribution, and these molecules do not crystallize when cooled but rather form amorphous polymers, which are called low-density polyethylene. 

Elevated concentrations of the n-Cl3 to -CIH alkanes and branched-chain and cyclic analogs were measured in a building having a history of air quality complaints the source was found to be volatilization from hydraulic fluids used in the building elevators (Weschler et al., 1990). 

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. 

The relative sensitivity of short-chain alkyl branches of different sizes to elimination on irradiation with formation of the corresponding alkane has been variously reported as being constant or varying (13,14). Figure 9 compares G values for formation of the alkane corresponding to the short-chain branch from samples of these three polymers with branch frequencies from 0.5 to 6 per 1,000 C atoms. There is a notably higher scission efficiency for ethyl branches. 

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... 

Alkene and alkane formation was suggested to take place through p cleavage and subsequent hydrogenation [Eq. (3.17)] chain branching involves the reaction of 1 with a half-hydrogenated intermediate [Eq. (3.18)] ... 

Several mechanisms were proposed to interpret bond shift isomerization, each associated with some unique feature of the reacting alkane or the metal. Palladium, for example, is unreactive in the isomerization of neopentane, whereas neopentane readily undergoes isomerization on platinum and iridium. Kinetic studies also revealed that the activation energy for chain branching and the reverse process is higher than that of methyl shift and isomerization of neopentane. 

Every branched or unbranched alkane chain is built as a zigzag chain. When a chain contains three or more C atoms a part of the molecule can turn freely around a single bond in relation to the rest of the molecule. In a spontaneous process the molecule converts from one three-dimensional structure to another. According to a chemists the molecule changes from one conformation into another . Figure 3.15 is a representation of an alkane chain and two conformations of the same molecule. 

Among these examples, a general trend is always observed third-phase formation is favored by larger alkane diluent molecules, the LOC is lower with a linear alkane chain diluent than with branched alkanes, and, generally, the third phase is prevented when aromatic diluents are used. In the language of surface wetting, short-chain solvents better wet the protruding chains. 

Two factors that influence the heats of combustion of alkanes are, in order of decreasing importance, (1) the number of carbon atoms and (2) the extent of chain branching. Pentane, isopentane, and neopentane are all C5H12 hexane is C6H14. Hexane has the largest heat of combustion. Branching leads to a lower heat of combustion neopentane is the most branched and has the lowest heat of combustion. 

The two main natural sources of alkanes are natural gas and petroleum. Alkanes are insoluble in and less dense than water. Their boiling points increase with molecular weight and, for isomers, decrease with chain branching. 

Know the relationship between boiling points of alkanes and (a) their molecular weights and (b) the extent of chain branching. 

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