Carbonium ion mechanisms

While a planar configuration characterizes the last monomeric unit of a polymeric chain growing by a radical or carbonium ion mechanism, a tetrahedral configuration should be attributed to the end of a growing polymeric carbanion. Hence an isotactic or a... 

Wiberg prefers mechanism A to the carbonium-ion mechanism with the proviso that the radical is oxidised before inversion occurs. The carbonium ion formed must rapidly acquire an oxygen atom to prevent inversion and the two processes may be synchronous. The minor role which free carbonium ions may play in the reaction has been discussed . 

Indeed, the acidity of the reaction mixture was found to increase upon continued photolysis, in accord with the carbonium ion mechanism. 

More than three decades ago, skeletal rearrangement processes using alkane or cycloalkane reactants were observed on platinum/charcoal catalysts (105) inasmuch as the charcoal support is inert, this can be taken as probably the first demonstration of the activity of metallic platinum as a catalyst for this type of reaction. At about the same time, similar types of catalytic conversions over chromium oxide catalysts were discovered (106, 107). Distinct from these reactions was the use of various types of acidic catalysts (including the well-known silica-alumina) for effecting skeletal reactions via carbonium ion mechanisms, and these led... 

A specific case of the carbonium ion mechanism [Eq. (5)] with reasonable plausibility is decarboxylation of metal arenoates by classic electrophilic aromatic substitution [Eq. (12)]. This mechanism would be favored by electron-donating substituents and has been invoked to explain the relative ease of decarboxylation of p-methoxybenzoic acid in molten mercuric trifluoroacetate (77) as well as the very facile decarboxylation on reaction of polymethoxybenzoic acids with mercuric acetate (18) (see below). 

It should be remembered, of course, that in the carbonium theory the word ion has a different connotation than it does in inorganic chemistry the degree to which the ester is dissociated may actually lie somewhere between the undissociated ester as in the ester mechanism and the free ions as usually written (although actually believed to be very short-lived) in the carbonium ion mechanism. ... 

Whitmore (16), when developing the idea of carbonium ions, included reactions over dehydrating catalysts. The application of carbonium ion mechanism to the dehydration of alcohols over alumina has found several supporters (17, 18). 

Pillai and Pines (84) found that neopentyl alcohol, mixed with 10% by weight of piperidine and passed over alumina prepared from aluminum isopropoxide, yielded 2-methyl-l-butene and 2-methyl-2-butene, in a maximum ratio of 3, and small amounts of 1,1-dimethylcyclo-propane. However, lert-pentyl alcohol yielded these two olefins in a maximum ratio of only 1.4, and none of the cyclopropane was produced (Table VI). Because of these facts a carbonium ion mechanism which is applicable to ferf-pentyl alcohol is not adequate to explain the rearrangement taking place during the dehydration of neopentyl alcohol,... 

From 3,3-dimethyl-2-butanol, the major product of rearrangement is 2,3-dimethyl-1-butene. The distribution of the primary dehydration products is far from equilibrium. The maximum ratio of 2,3-dimethyl-1-butene to 2,3-dimethyl-2-butene obtained from 2,3-dimethyl-2-butanol is about 10. This is higher than that to be expected if a proton is removed from the l,l,2-trimethyl-2-propyl carbonium ion in a statistical manner. The maximum ratio of the two olefins obtained from 2,3-dimethyl-2-butanol is also about 10. Hence it can be argued that the high yield of 2,3-dimethyl-1-butene from 3,3-dimethyl-2-butanol does not necessarily rule out a classical carbonium ion mechanism. It is very unlikely, however, that the same intermediate is involved from both alcohols. If such were the case the product of dehydration of 2,3-dimethyl-2-butanol would contain appreciable amounts of 3,3-dimethyl-l-butene. 

The dehydration of the two alcohols over alumina catalyst in the presence of piperidine was studied by Pillai and Pines [84). The experimental results which are given in Table X indicate that, although carbonium ion mechanism can interpret the products obtained from the tertiary alcohols, another mechanistic path has to prevail in order to account for the formation of the various dehydration products from 3,3-dimethyl-2-pentanol. The mechanism, as proposed above for the dehydration of 3,3-dimethyl-2-butanol, would also explain the hydrocarbons formed from the dehydration of 3,3-dimethyl-2-pentanol. 

The dehydration of tertiary alcohols over aluminas can be interpreted by a carbonium ion mechanism. 

Trimethylbutene (Triptene). The polymerization of trimethyl-butene is of interest because rearrangement of the olefin, unless of a very radical nature, can give only the starting material. It was found (Cook et al., 41) that polymerization in the presence of 75% sulfuric resulted in a 91% yield of polymer, 70% of which was 2,2,3,5,5,6,6-heptamethyl-3-heptene. The minor products of the reaction consisted of 3.1% of unreacted triptene, 0.9% of 8- to 10-carbon atom olefins, 3.0% of 10-carbon atom olefins, 9.0% of 11- to 14-carbon atom olefins and 12.0% of residue. The formation of the heptamethylheptene is to be expected on the basis of the carbonium ion mechanism ... 

As was shown above, only the latter is obtained when sec-butyl alcohol is treated with 75% sulfuric acid at 80° under pressure. It is significant to note that this shortcoming of the methyl separation mechanism as compared to the carbonium ion mechanism was pointed out in a later paper (Drake and Veitch, Jr., 30) by one of the original proponents of the methyl separation mechanism. 

The fact that tetramethylethylene which contains no hydrogen on either of the double-bonded carbon atoms undergoes polymerization to yield dimer might be considered as a means of choosing between the carbonium ion mechanism and the hydrogen separation mechanism. However, regardless of which mechanism is used, it is necessary to assume that the olefin first undergoes isomerization to terf-butylethylene. 

This mechanism does not seem to take into account the formation of products having a different carbon skeleton than that of the primary product i.e., it does not explain the formation of those products which in accordance with the carbonium ion mechanism are formed by the migration of a methyl group. 

At the time of maximum rate of hydrocarbon formation the composition of reaction product shows the pattern of a carbonium ion mechanism (much i-butane, i-pentane, Cg and Cy monomethyl paraffins, some aromatics and olefins C3, (Table 1). The zeolite hosts a large amount of non volatile hydrocarbons. 

Cracking reactions can take place on either the acid site or the platinum site. Acid cracking is characterized by the formation of C3 and C4 paraffins due to the carbonium ion mechanism. Metal cracking (hydrogenolysis), as shown by Sinfelt (19), is random and forms more C, and C2 gases relative to acid cracking. 

Possibly the most convincing evidence for positive ion-molecule reactions in polymers is the high rate of decay of vinyl unsaturation during the radiolysis of polyethylene, as recently discussed by Dole, Fallgatter, and Katsuura (13). The ideas of these authors with respect to the carbonium ion mechanism for vinyl decay by means of a dimerization reaction were largely suggested by the mechanisms proposed by Col-linson, Dainton, and Walker (5) for vinyl decay (polymerization) in the radiolysis of n-hexa-l-decene, Reactions 3 and 4 of Table I. 

Since the sensitivity towards water in many organic reactions lies in the order carbanion > carbonium ion > free radical, it appears likely that as water is progressively removed from a-methylstyrene—and, perhaps, other vinyl monomers—the free radical propagation is augmented or supplanted by a carbonium ion mechanism, which, in turn, is further enhanced at low water content, by a carbanion mechanism. Under the latter conditions, one would expect a termination mechanism which is bimolecular with regard to the total concentration of propagating species and hence a square-root dependence of the polymerization rate on the dose rate. This is the order dependence observed in a-methylstyrene at the highest polymerization rates and lowest water content. 

Richards and Hill have recently obtained quantitative evidence of the stabilization of a -metallocenyl carbonium ions (38, 95). They have shown that sol-volyses of methylmetallocenylcarbinyl acetates proceed via a carbonium ion mechanism, and that these acetates solvolyze with rates greater than even tri-phenylmethyl (trityl) acetate. Further, the relative rates of solvolysis and therefore the order of carbonium ion stabilities increas.e, proceeding from the iron to the osmium acetate. A portion of these data is summarized in Table II. 

The catalytic cracking of four major classes of hydrocarbons is surveyed in terms of gas composition to provide a basic pattern of mode of decomposition. This pattern is correlated with the acid-catalyzed low temperature reverse reactions of olefin polymerization and aromatic alkylation. The Whitmore carbonium ion mechanism is introduced and supported by thermochemical data, and is then applied to provide a common basis for the primary and secondary reactions encountered in catalytic cracking and for acid-catalyzed polymerization and alkylation reactions. Experimental work on the acidity of the cracking catalyst and the nature of carbonium ions is cited. The formation of liquid products in catalytic cracking is reviewed briefly and the properties of the gasoline are correlated with the over-all reaction mechanics. 

The acid-catalyzed reactions of olefin polymerization and aromatic alkylation by olefins have been very well explained by the carbonium ion mechanism developed by Whitmore (21). This mechanism provides the basis of the ensuing discussion, which is devoted to the application of such concepts (7,17) to catalytic cracking systems and to the provision of much added support in terms of recently developed structural energy relationships among hydrocarbons and new experimental evidence. 

The presence of at least three different glycosidic linkages, unequivocally demonstrated by the changes in rotation during hydrolysis, requires the assumption of a carbonium-ion mechanism in this polymerization, and a detailed, conformational analysis has been proposed. Some steric control was also noted with change in the solvent.147... 

The detailed mechanism, or mechanisms, of the solvolysis of acid chlorides is still a matter of dispute. There are at least four possible mechanisms, (a)-(d) below, all of which have been proposed either to act separately or in various combinations, and there is a unified mechanism, that of Minato93 which will be discussed later. The bimolecular mechanisms (a) and (b) differ in that (a) includes a tetrahedral intermediate whereas (b) does not. The former is commonly accepted as the most likely for the bimolecular mechanism and the arguments against (b) have been stated in the introduction. There is, however, good evidence for (A), at least in the case of the hydrolysis of chloracetyl chloride94. The acylium ion mechanism (c) and the hydrated carbonium ion mechanism (d) are both unimolecular mechanisms. Whereas the acylium ion XXVII has never been directly observed in hydrolysis or alcoholysis reactions, it is favoured as an intermediate by many workers, although it is kinetically indistinguishable from XXVIII. 

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