Hydrocarbon alkylation, mechanism

Only scant information is available about the influence of coke formation on the alkylation mechanism. It has been proposed that, similar to the conjunct polymers in liquid acids, heavy unsaturated molecules participate in hydride transfer reactions. However, no direct evidence was given for this proposition (69). In another study, the hydride transfer from unsaturated cyclic hydrocarbons was deduced from an initiation period in the activity of NaHY zeolites complete conversion of butene was achieved only after sufficient formation of such compounds (73). 

A new mechanism to interpret alkene formation in Fischer-Tropsch synthesis has been presented 499-501 There is a general agreement that hydrocarbon formation proceeds according to the modified carbene mechanism. Specifically, CO decomposes to form surface carbide and then undergoes hydrogenation to form surface methine (=CH), methylene (=CH2), methyl and, finally, methane. Linear hydrocarbons are formed in a stepwise polymerization of methylene species. When chain growth is terminated by p-hydride elimination [Eq. (3.61)], 1-alkenes may be formed,502 which is also called the alkyl mechanism ... 

However, there are certain anomalies with respect to the alkyl mechanism, namely, the relatively small amounts of C2 hydrocarbons in the Anderson-... 

The key to the mechanism of hydrocarbon alkylation was provided by the discovery by P. D. Bartlett, in 1940, that a carbocation can react rapidly with a hydrocarbon having a tertiary hydrogen to yield a new carbocation and a new hydrocarbon. Some of these hydrogen-transfer reactions are extraordinarily fast and may be complete in seconds or less. The hydrogen is transferred with both bonding electrons (H e). For example,... 

The question that has not yet been satisfactorily answered Is what reactions occur as the olefins enter the acid phase and how does the Isobutane eventually react In order to produce the final wide range of Isoparaffins. The mechanism (3,4) that has been widely accepted and that Involves a chain reaction In which Isobutane and the olefin react In consecutive reactions to form both TMP s and DMH s does not explain all the phenomena noted In the alkylation mechanism. Hofmann and Schrleshelm (5) for example, suggested that the acid-soluble hydrocarbons provided a source of hydride Ions that entered Into the reaction. More recently, Albright and LI (6) suggested that at least some olefins react In the acid phase with acid-soluble hydrocarbons. Goldsby (7,8) has shown that propylene often reacts with sulfuric acid to form sec-propyl sulfates. These sulfates then can be reacted with Isobutane to form alkylate. 

The alkylation mechanism for second-step reactions with isobutylene as the olefin have been significantly clarified by two runs, each conducted as follows. The acid and hydrocarbon phases produced by first-step reactions involving Isobutylene and sulfuric acid were separated. The acid phase which contained some acid-soluble hydrocarbons or reaction products such as perhaps t-butyl sulfate was then contacted with fresh Isobutane. This resulting mixture of reactants was designated as A below. The hydrocarbon... 

Slightly over half of the papers deal with the alkylation of isobutane widi light olefins to produce high quality gasoline blending hydrocarbons. New information is presented for isobutane alkylation relative to die chemistry and mechanism, process improvements, recovery of acid catalyst, and status of commercial units. Papers are also presented for die alkylation of aromatics, heterocyclics, coal, and other hydrocarbons. Alkylations using transition metal catalysts, strong acids, free radicals, and bases are also reported. 

Electrolysis of carbonyl compounds provides pinacols, alcohols or hydrocarbons, depending on the conditions, such as pH, the nature of the electrode, and its potential. Fundamental studies have been carried out on the mechanisms of hydrocarbon formation using acetone as a substrate. Although several electrodes, such as Cd, Pt, Pb or Zn, are recommended, carbonyl compounds, including aryl and alkyl derivatives, require strong aqueous acidic media for reduction to the hydrocarbons. The mechanism of the electrolytic reduction is probably similar to that of Clemmensen reduction, which starts from anion radical formation by one-electron transfer, as indicated in Scheme 3. The difference is that electrolytic reduction takes place in an electric double layer, rather than on the surface of the zinc metal. 

The formation of the Pt(II)-alkane complex as the first kinetically significant intermediate in the reaction has been proposed in the so called alkane-alkyl mechanism (see above) [14], This intermediate is analogous to alkonium ions of the type CHs whose formation is associated with the deformation of both the planar complex and of the tetrahedral hydrocarbon molecule. 

Halogenated Hydrocarbons - The destruction of cytochrome P-450 by CCl, first attributed to lipid peroxidation, has been shown to occur even under conditions where lipid peroxidation is not detectable.one possible explanation for this inactivation is that the trlchloromethyl radical or a related species obtained by reduction of the halocarbon reacts with the heme moiety or the apoprotein. The ill-defined radio-labeled porphyrins reported in Incubations of labeled CCI4 with hepatic microsomes would provide support for a heme alkylation mechanism were it not for the conflicting report that fluorescent N-alkylated porphyrins similar to those obtained with AIA are not isolated from CCl -incubated microsomes by procedures that result in isolation of the AIA adducts. ... 

At temperatures lower than those usually employed in catalytic cracking other polymolecular reactions become predominant namely, polymerization, and, in the presence of aromatic hydrocarbons, alkylation. These conversions can also be interpreted in terms of the conventionally assumed carbonium-ion mechanism. 

A hydrocarbon-pool mechanism. It is based on a carbonaceous species, (CH2)re. The species is alkylated by methanol or dimethyl ether until it eliminates an olefin and a new catalytic cycle starts. This mechanism can be represented by the scheme as shown in Figure 27. 

At temperatures above about 1200 K, hydrocarbon reaction mechanisms are simplified by the fact that alkyl radicals react primarily by means of p-decomposition. Thus the complex sequence (discussed below) of reactions initiated by addition of molecular oxygen to alkyl radicals... 

Kojima and co-workers [52] identified the major thermal decomposition products of plasma polymerised polypyrrole as nitriles with less than four carbons and alkyl pyrroles. Evolution of only monosubstituted alkyl pyrroles, such as 2-methylpyrrole and 2-ethylpyrrole, suggests that polypyrroles consist of monosubstituted pyrrole rings. This is also supported by the result that the IR spectrum of polypyrroles differs from that of the electrochemically polymerised pyrrole, which consists of disubstituted pyrrole rings. Evolution of linear nitriles shows evidence that a polypyrrole molecule has the main chain containing nitrogen atoms. The mechanism of polymerisation of pyrrole in the discharge is considered to be similar to that of aromatic hydrocarbons, which mechanism involves a process of production of acetylene. 

A hydrocarbon pool mechanism via alkylation/dealkylation of hydrocarbon scaffolds. Olefins interconversion via methylation, oligomerization, and cracking. 

Aromatic aldehydes are formed in the atmospheric oxidation of aromatic hydrocarbons. The mechanism of formation involves H abstraction by OH from an alkyl side group, followed by reaction with O2 and then NO to form the oxy radical. The oxy radical reacts with O2 to form the aldehyde and HO2. OH primarily adds to the ring, so the yield of the aldehyde is small, e.g., the yield of benzaldehyde from toluene is 7% (Bloss et al., 2005). Aromatic aldehydes are also emitted directly from vehicle tailpipes see chapter I. 

Friedel-Crafts (Lewis) acids have been shown to be much more effective in the initiation of cationic polymerization when in the presence of a cocatalyst such as water, alkyl haUdes, and protic acids. Virtually all feedstocks used in the synthesis of hydrocarbon resins contain at least traces of water, which serves as a cocatalyst. The accepted mechanism for the activation of boron trifluoride in the presence of water is shown in equation 1 (10). Other Lewis acids are activated by similar mechanisms. In a more general sense, water may be replaced by any appropriate electron-donating species (eg, ether, alcohol, alkyl haUde) to generate a cationic intermediate and a Lewis acid complex counterion. 

Aromatic ethers and furans undergo alkoxylation by addition upon electrolysis in an alcohol containing a suitable electrolyte.Other compounds such as aromatic hydrocarbons, alkenes, A -alkyl amides, and ethers lead to alkoxylated products by substitution. Two mechanisms for these electrochemical alkoxylations are currently discussed. The first one consists of direct oxidation of the substrate to give the radical cation which reacts with the alcohol, followed by reoxidation of the intermediate radical and either alcoholysis or elimination of a proton to the final product. In the second mechanism the primary step is the oxidation of the alcoholate to give an alkoxyl radical which then reacts with the substrate, the consequent steps then being the same as above. The formation of quinone acetals in particular seems to proceed via the second mechanism. ... 

In the 1950s it became recognised that one type of antioxidant also often behaved as an antiozonant. These were the branched alkyl, unsubstituted aryl-/7-phenylenediamines typified by A-isopropyl-A -/ -phenylenediamine (IPPD). The mechanism of their action is still not fully understood but it is to be noted that they are often improved by being used in conjunction with small amounts of hydrocarbon waxes. 

Many of the reactions of BF3 are of the Friedel-Crafts type though they are perhaps not strictly catalytic since BF3 is required in essentially equimolar quantities with the reactant. The mechanism is not always fully understood but it is generally agreed that in most cases ionic intermediates are produced by or promoted by the formation of a BX3 complex electrophilic attack of the substrate by the cation so produced completes the process. For example, in the Friedel-Crafts-type alkylation of aromatic hydrocarbons ... 

However, these reactions remain hypothetical, and the mechanism of alkylation of low-valent coordinatively insufficient ions during their interaction with hydrocarbons calls for a detailed study. When the activation by some additives is performed the formation of the active transition metal-carbon bond by oxidative addition is also possible, e.g. in the case of such additives as alkylhalogenides or diazocompounds according to the schemes ... 

In dibenzothiophene-S,S-dioxide the S atom is in a ring, and hence more constrained. The yield of SOz in the radiolysis is linear with the dose to about 13 Mrad after which it levels off as in p,p -ditolyI sulfone. However, the yield of S02 in this case is much lower (a factor of 25) than in the case of p,p -ditolyl sulfone (G = 0.002 compared to G = 0.05). This stability of the dibenzothiophene sulfone could be partially due to back reaction to reform the parent sulfone and partially due to more efficient energy delocalization. The expected biphenylene product was not detected due to limitations of the analytical method. Bowmer and O Donnell70 studied the volatile products in y-radiolysis of dialkyl, alkyl aryl and diaryl sulfones. Table 2 gives the radiolytic yields of S02 and of the hydrocarbon products of the alkyl or aryl radicals. The hydrocarbon products are those obtained either by H atom abstraction or by radical combination. The authors69 suggested the mechanism... 

Aquilante and Volpi indicate (2) that propanium ions formed by proton transfer from H3 + are not collisionally stabilized at propane pressures as great as 0.3 mm. and that they decompose by elimination of hydrogen or a smaller saturated hydrocarbon to form an alkyl carbonium ion. Others (16, 19) have proposed one or the other of these fates for unstabilized propanium ions. Our observations can be rationalized within this framework by the following mechanisms ... 

The mechanism of anionic polymerization of styrene and its derivatives is well known and documented, and does not require reviewing. Polymerization initiated in hydrocarbon solvents by lithium alkyls yields dimeric dormant polymers, (P, Li)2, in equilibrium with the active monomeric chains, P, Li, i.e. 

Variable valence transition metal ions, such as Co VCo and Mn /Mn are able to catalyze hydrocarbon autoxidations by increasing the rate of chain initiation. Thus, redox reactions of the metal ions with alkyl hydroperoxides produce chain initiating alkoxy and alkylperoxy radicals (Fig. 6). Interestingly, aromatic percarboxylic acids, which are key intermediates in the oxidation of methylaromatics, were shown by Jones (ref. 10) to oxidize Mn and Co, to the corresponding p-oxodimer of Mn or Co , via a heterolytic mechanism (Fig. 6). 

Predictive equations for the rates of decomposition of four families of free radical initiators are established in this research. The four initiator families, each treated separately, are irons-symmetric bisalkyl diazenes (reaction 1), trans-phenyl, alkyl diazenes (reaction 2), tert-butyl peresters (reaction 3) and hydrocarbons (reaction 4). The probable rate determining steps of these reactions are given below. For the decomposition of peresters, R is chosen so that the concerted mechanism of decomposition operates for all the members of the family (see below)... 

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