2, 5-Dimethylhexane, from isobutane

Condon (130), in discussing the formation of 2,5-dimethylhexane from the reaction of isobutane with A1C13, postulates a protonated cyclopropane intermediate to explain the skeletal isomerization of the 2,2,4-trimethylpentyl ion, which he presumes was formed by attack of f-butyl ion on isobutylene. In this reaction the isobutylene was produced by removal of a proton from the initially formed f-butyl ion. The cyclic intermediate can be avoided if part of this isobutylene was first converted to butenyl ions which then added to the remaining isobutylene to provide the 2,5-dimethylhexyl skeleton directly. 

Octanes produced via the addition of (-butyl ester (from isobutane) to the butenes so formed will be the same regardless of what alkylating agent is used and the product will contain more trimethylpentanes than dimethylhexanes. If all of the alkylate were produced tlirough the olefins formed as in eq. (A), all three alkylating agents (1-butene, 2-butene and s-butyl ester) would yield identical products. However, part of the alkylate in the case of the butenes is formed by the direct reaction of the (-butyl ester with the olefin before the olefin has reacted with the acid catalyst this accounts for the small differences in composition. 

Gasoline contains more than 250 components of a mixture of C4-C12 hydrocarbons, which varies in concentration from batch to batch. Some of these components are isobutane, n-butane. isopentane, n-pentane, 2,3-dimethylbutane, 3-methylpentane, n-hexane, 2,4-dimethylpentane, benzene, 2-methylhexane, 3-meth-ylhexane, 2,2,4-trimethylpentane, 2,3,4-trimethylpentane, 2,5-dimethylhexane, 2,4-dimethylhexane, toluene, 2,3-dimethylhexane. ethylbenzene, methylethylbenzenes, m-, p-, and o-xylene, trimeth- ylbenzenes, naphthalene, methylnaphthalenes, and dimethylnaph-thalenes... 

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.

When alkylating isobutane, chain tennination forms primarily, but not entirely, 2,2,4-trimethylpentane the alkylate from chain termination very closely resembles isobutene alkylate. The similarity of alkylate compositions, particularly their C0 fractions, originating from various olefins and the distance from thermodynamic equilibrium composition indicates that alkylate molecules, once formed, are relatively stable under alkylation conditions and undergo little isomerization. Undesirable side products, e.g., dimethylhexanes and residue, are probably formed by buter e isomerization and polymerization (rather than by isomerization of alkylate or by isomerization of the C3 carbonium Ion which subsequently becomes alkylate). 

It follows from the above discussion that the chain mechanism requires that markedly different products be obtained in the alkylation of isobutane with 1- and 2-butene, respectively. The former should yield dimethyl-hexanes and the latter, trimethylpentanea, as the major products. Such has been showm (Schmerling, 14d) to be the case when aluminum chloride (particulary when modified to diminish side reactions) was employed as catalyst. The product of the alkylation with 1-butene in the presence of aluminum chloride monomethanolate contained about 60% by w eight dimethylhexanes and 10% trimethylpentanes, whereas the 2-butene product contained 65% trimethylpentanes and only 4% dimethylhexanes. The difference in composition was apparent also in the A.S.T.M. octane numbers of the 125° end-point gasolines. That from 1-butene had an octane number of only 76.1 that from 2-butene, 94.1. 

Batch alkylation of isobutane with 1-butene and with 2-butene at 30° resulted in deep-seated decomposition reactions, octanes comprising only 21-24% of the liquid products (Schmerling, 14d). Dimethylhexanes predominated in the 1-butene product (6.4% trimethylpentanes, 11.5% dimethylhexanes, and 3.5% methylheptanes). Trimethylpentanes were the principal octanes present in the 2-butene alkylate (14.5% trimethylpentanes, 5.8% dimethylhexanes, and 2.6% methylheptanes). The octane numbers of the 125° endpoint gasolines from the 1-butene and the 2-butene alkylations were 74.5 and 83.5, respectively. 

Reaction pressure was maintained with a dome-loaded back-pressure regulator (Circle Seal Controls). All heated zones were controlled and monitored with a Camile 2500 data acquisition system (Camile Products). Products were analyzed online by gas chromatography with an HP 5890 II GC, equipped with an FID, and a DB-Petro 100 m column (J W Scientific), operated at 35° C for 30 min, ramped at 1.5°/min to 100° C, 5°/min to 250° C for 15 min. An alkylate reference standard (Supelco) allowed identification of the trimethylpentanes (TMP) and dimethylhexanes (DMH). The combined mass of TMP and DMH is referred to hereafter as the alkylate product . As discussed elsewhere [19], propane, an impurity in the isobutane feed, was used as an internal standard for butene conversion calculations. Since isomerization from 1-butene to 2-butene isomers is rapid over acidic catalysts, reported conversion is for all butene isomers to C5 and higher products. Isobutylene formation was not observed under any conditions. 

Alkylation of Isobutane. When isobutene is alkylated with C3—C5 olefins, often at least 120-150 isoparaffins are detected, with carbon numbers varying from 5 to at least 16 (3). Using a strict definition of alkylation, only i-CyHie, i-CgHis, and j-C9H2o would be produced when isobutane is alkylated with propylene, C4 olefins, and C5 olefins, respectively. Considering t-Cg isoparaffins, trimethylpentanes (TMPs), dimethylhexanes (DMHs), and methyl heptanes are all produced. Yet, all C5—Cie isoparaffins produced are commonly considered to be alkylation products. 

However, experimentally, no 2,2-dimethylhexane is observed in the products whereas the amount of 2,2,3-TMP formed was very small (Tables 9 and 10). To explain the differences in the expected and observed product distribution, it has been assumed that a fast rearrangement of the isooctyl cations occurs (102). This was demonstrated not to be the full explanation since by working with isobutane and propylene marked with it was clearly shown (103) that under alkylation conditions the isomerization of a secondary carbenium ion to give a tertiary one by migration of a hydride ion or a methylene group is faster than the desorption, while the desorption becomes faster than the isomerization of the tertiary carbenium ions. Finally, from the results presented in the literature (104), it appears... 

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