Isomerization hydrocarbon branching

Branched-chain alkanes do not exhibit the same smooth gradation of physical properties as do the continuous-chain alkanes. Usually there is too great a variation in molecular structure for regularities to be apparent. Nevertheless, in any one set of isomeric hydrocarbons, volatility increases with increased branching. This can be seen from the data in Table 4-2, which lists the physical properties of the five hexane isomers. The most striking feature of the data is the 19° difference between the boiling points of hexane and 2,2-dimethylbutane. 

Naturally the differences in energy content of isomeric hydrocarbons, cis-trans isomerism etc., are proof that additivity only holds approximately. These differences are, however, of the order of magnitude of 1 to 2 kcal per side-chain for example wobutane—butane 1.6 kcal, 2.2.4 trimethylpentane— zz-octane 3.1 kcal, in which the branched hydrocarbon has always the lower energy content. 

Fig. 2. Gas chromatogram of hydrocarbons in the range Qj to C,6 (Studier et al., 1972). Only 6 of the 10 isomeric hydrocarbons with 16 C atoms are present in appreciable abundance 5 of them (underlined) are common to all three samples. The sample of the Precambrian Nonesuch shale (courtesy W. G. Meinschein) is a pure aliphatic fraction from which aromatic hydrocarbons had been removed by silica gel chromatography it will be discussed in Sec. 8.2. Abbreviations Me, methyl B.P., branched paraffin...

Of course, Kekule continued, structure theory specifies only the bonding sequences of the atoms, not their spatial positions inside the molecule. Of two isomeric hydrocarbons, one possessing a highly branched structure and the other a straight-chain structure, the former is always found to be more volatile this must obviously have to do with the differing centers of gravity and moments of inertia of the two molecules. This is one example of many ways that we can indirectly learn about the average spatial positions of the atoms and the molecules. 

Metallocarbenes have been implicated in the iridium-catalysed isomerization of branched hydrocarbons, such as that of 2-methylpentane (1) to 3-methylpentane (3). Studies with C-labelled (1) support a mechanism which proceeds via (2) as intermediate. Polymer-bound triphenylphosphine-lithium diorganocuprates may offer advantages in the Wurtz-type coupling of alkyl halides, in that work-up is easier and the product is not contaminated with residual tertiary phosphine. However, yields in general are comparable with those from the corresponding homogeneous reagents. ... 

Three series of epoxides, accounting together for ca 4% of dry biomass were isolated by TLC from the external lipids of the Austin collection strain 56). EIMS, and C-NMR data showed that these were epoxyalkenes (64), epoxybotryals (65) and epoxyalkylphenols (66) derived from the parent compounds (1), (58) and (60) respectively, by epoxidation of the C(co9)-C(o)10) double bond. All these compounds contain a cis epoxide ring as deduced from the chemical shifts of the a-methylene carbons. In the case of the epoxybotryals (65) the location of the epoxide relatively to the two hydrocarbon branches was not ascertained and two isomeric series might occur. Neither the diepoxyalkanes, nor the 1,2-epoxyalkenes derived from n-alkadienes (1) were detected in the lipids of the Austin strain. 

Although isomerization processes have developed slowly because of the cost of handling corrosive catalytic agents and the cost of separating the isomers of hydrocarbons that contain five or more carbon atoms, the increasing need for high-octane-number fuels will require a more extensive use of isomerization processes. Branched-chain compounds have much... 

Isomerization is a small-volume but important refinery process. Like alkylation, it is acid catalyzed and intended to produce highly-branched hydrocarbon mixtures. The low octane C5/C6 fraction obtained from natural gasoline or from a light naphtha fraction may be isomerized to a high octane product. 

A branched hydrocarbon C4H10 reacts with chlorine in the presence of light to give two branched structural isomers with the formula C4H9C1. Write the structural formulas of (a) the hydrocarbon (b) the isomeric products. 

Products in several isomeric forms can occur in systems with fewer atoms than considered above the association reaction between C3H+ and H2 to produce both cyclic and noncyclic C3H3 is a case in point, although the branching ratio in this instance seems to be noncontroversial.30 The problem of whether product hydrocarbon ions are cyclic or noncyclic extends to other classes of ion-molecule reactions such as condensation and carbon insertion reactions, where studies of product reactivity have only been undertaken in a few instances. In general, cyclic ion products are less reactive than their noncyclic counterparts. For systems with a... 

The isomerization of the butanes and of neopentane has been studied over various types of evaporated platinum films by Anderson and Baker (68) and Anderson and Avery (108,24). Table II gives some typical results. It is clear that the proportion of parent hydrocarbon reacting to isomeric rather than to hydrogenolytic product is considerably smaller for a hydrocarbon with an unbranched as opposed to a branched chain containing an isostructural unit indeed, neopentane was studied as the archetypal molecule of the latter class. 

A similar mechanism of chain oxidation of olefinic hydrocarbons was observed experimentally by Bolland and Gee [53] in 1946 after a detailed study of the kinetics of the oxidation of nonsaturated compounds. Miller and Mayo [54] studied the oxidation of styrene and found that this reaction is in essence the chain copolymerization of styrene and dioxygen with production of polymeric peroxide. Rust [55] observed dihydroperoxide formation in his study of the oxidation of branched aliphatic hydrocarbons and treated this fact as the result of intramolecular isomerization of peroxyl radicals. 

Isomerate A continuous hydrocarbon isomerization process for converting pentanes and hexanes to highly branched isomers. Developed by the Pure Oil Company, a division of the Union Oil Company of California. The catalyst, unlike those used in most such processes, does not contain a noble metal. 

IsoSiv [Isomer separation by molecular sieves] A process for separating linear hydrocarbons from naphtha and kerosene petroleum fractions. It operates in the vapor phase and uses a modified 5A zeolite molecular sieve, which selectively adsorbs linear hydrocarbons, excluding branched ones. Developed by Union Carbide Corporation and widely licensed, now by UOP. The first plant was operated in Texas in 1961. By 1990, more than 30 units had been licensed worldwide. See also Total Isomerization. 

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 conversion of a chemical with a given molecular formula to another compound with the same molecular formula but a different molecular structure, such as from a straight-chain to a branched-chain hydrocarbon or an alicyclic to an aromatic hydrocarbon. Examples include the isomerization of ethylene oxide to acetaldehyde (both C2H40) and butane to isobutane (both C4H10). 

Based on the various hybridization states of carbon, (Figure 1.2) at least four major carboskeletal architectures are known [6, 15]. They are recognized as (I) linear, (II) bridged (2D/3D), (III) branched and (IV) dendritic. In adherence with skeletal isomerism principles demonstrated by Berzelius (1832) these major architectural classes determine very important differentiated physicochemical properties that define major areas within traditional organic chemistry (e.g. linear versus branched hydrocarbons). It is interesting to note that analogous... 

It has been suggested [18] that the greater tendency for long-chain hydrocarbons to knock as compared to smaller and branched chain molecules may be a result of this internal, isomerization branching mechanism. 

Isomerization the conversion of a normal (straight-chain) paraffin hydrocarbon into an iso (branched-chain) paraffin hydrocarbon having the same atomic composition. 

Pines and Csicsery (90, 90a) proposed three and/or four-membered cyclic intermediates in the isomerization of various branched alkanes over non-acidic chromia-alumina. A similar, 1,3-methyl shift has recently been reported with an oxygenated reactant (tetramethyloxetane) over supported Pt, Pd, and Rh (90b). Future experiments are necessary to elucidate whether hydrocarbons, too, can form C4 cyclic intermediates over metal catalysts. Some products assumedly formed via ethyl shift could be interpreted by C4 cyclic isomerization. 

Isomerization, A process used to convert straight-chain to branch-chain hydrocarbons as in a butane isomerization plant. 

Olefin isomerization reactions range from some of the most facile using acid catalysts to moderately difficult and, as components of more complex reaction schemes such as catalytic cracking, may be among the most common reactions in hydrocarbon processing. As stand-alone reactions, they are primarily used to shift the equilibrium between terminal and internal olefins or the degree of branching of the olefin. While olefin isomerization was considered for the production of MTBE, today stand-alone olefin isomerization processes are only considered for a few special situations within a petrochemical complex. 

The molecular formulas just shown for 10 alkane hydrocarbon molecules represent the proportions of carbon to hydrogen in each molecule. These formulas do not reveal much about their structures, but rather indicate the proportions of each element in their molecules. Each molecule may have several different structures while still having the same formula. Molecules with different structures but the same formulas are called isomers. For example, n-butane is formed in a straight chain, but in an isomer of butane, the CH branches off in the middle of the straight chain. Another example is ethane, whose isomeric structure can be depicted as H,C H,C-CH,. The name for the normal structure sometimes uses n in front of the name. 

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