Hydrocarbons, hydrocarbon carbonium ions

Similar results have been obtained with substituted triphenylcarbinols in sulfuric acid.174 Some unsaturated hydrocarbons form carbonium ions by simple addition of a proton, as do some carboxylic acids.175-177 But other carboxylic acids behave like the triarylcarbinols, the most prominent example being mesitoic acid. 

In these sections of our chapter, we emphasize research advances in the area of surface acidity of specific solids that have occurred during the period from 1970 to the fall of 1976. As stated earlier, the class of solids with which we are chiefly concerned are metal oxides that catalyze skeletal rearrangements of hydrocarbons via carbonium ion intermediates. However, we have included reviews of silica gel and alumina, which are relatively inactive, because the properties of these solids form a useful frame of reference. The initial sections (Sections III.A-III.D) deal predominantly with amorphous catalysts the final sections (Sections III.E and III.F), with crystalline catalysts. 

Fig. 19. a) Bridgehead derivatives used in the calculation b) An example of correlation between -log of the experimental tosylate acetolysis rate constants at 70° and the calculated hydrocarbon-carbonium ion strain energy... 

The superacid media can induce hydride abstraction, H-D exchange and other reactions even with saturated hydrocarbons.25 Carbonium ions are formed, some of which, notably the trimethylcarbonium ion, are quite stable Me3CH - Me3C+ <- CH3CH2CH2CH3... 

Organosilicon hydride Hydrocarbons from carbonium ions... 

Reaction (15) is equivalent to the condensed-phase alkylation of aromatic hydrocarbons by carbonium ions. In general, when MW + H) is an abundant ion, MW -I- is also abundant that is, the stability of the... 

Since the structure of these ions is similar to the structure of the protonated aromatic hydrocarbons MH+ (carbonium ions) we shall designate these negative ions as carbanions. The name negative ions will be reserved for hydrocarbon ions that differ from the parent compounds only by the presence of more electrons. The carbanions are obtained by means of the following reaction... 

That is, hydrocarbon carbonium ions will nearly always undergo exothermic hydride transfer reactions with alkanes having a larger number of carbon atoms, but this is apparently not the case In fluorocarbon systems. An examination of the thermochemistry of these reactions explains these di fering trends.Table X contrasts the nergy gained in going from R to RH for, and sec-... 

Trivalent carbenium ions are the key intermediates in electrophilic reactions of Tt-donor unsaturated hydrocarbons. At the same time, pen-tacoordinated carbonium ions are the key to electrophilic reactions of cr-donor saturated hydrocarbons through the ability of C-H or C-C single bonds to participate in carbonium ion formation. 

The reverse reaction of the protolytic ionization of hydrocarbons to carbocations, that is, the reaction of trivalent carbocations with molecular hydrogen giving their parent hydrocarbons, involves the same five-coordinate carbonium ions. 

The cationic polymerization of cardanol under acidic conditions has been referred to earlier [170,171], NMR studies [16] indicated a carbonium ion initiated mechanism for oligomerization. PCP was found to be highly reactive with aldehydes, amines, and isocyates. Highly insoluble and infusible thermoset products could be obtained. Hexamine-cured PCP showed much superior thermal stability (Fig. 12) at temperatures above 500°C to that of the unmodified cardanol-formaldehyde resins. However, it was definitely inferior to phenolic resins at all temperatures. The difference in thermal stability between phenolic and PCP resins could be understood from the presence of the libile hydrocarbon segment in PCP. 

Reaction of a carbonium ion formed from step 1 or 2 with another hydrocarbon by abstraction of a hydride ion... 

The new carbocation may experience another beta scission, rearrange to a more stable carbonium ion, or react with a hydrocarbon molecule in the mixture and produce a paraffin. 

Several types of proton transfer reactions can be studied conveniently by a neutral product analysis. Until now, the most extensive investigations have been concerned with (1) proton transfer from H3+ and CH5 + to various hydrocarbon molecules, and (2) the transfer of a proton from carbonium ions to larger olefins or other organic compounds. 

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 ... 

This reversal has been demonstrated by both product and kinetic studies. In the absence of solvent nucleophilic assistance and of substituents favouring P-bromo-carbonium ion intermediates, the ionization of CTCs to bromonium (poly)bromide has been shown to occur not only for congested olefins, but more generally for "normal olefins both in aprotic chlorinated hydrocarbons and in protic solvents like acetic acid and methanol. 

The more basic of the polynuclear benzenoid hydrocarbons dissolve in concentrated sulfuric acid, probably with the formation of simple carbonium ions.263 Benzene itself is only slightly soluble, but exchanges hydrogen with DaS04.866 Benzene and toluene will dissolve in a mixture of hydrogen fluoride and boron trifluoride.867 268 Pyrene in hydrogen... 

Two structures are possible for the interaction of aromatic hydrocarbons with acids.270 In the a-structures a covalent bond is established between the acidic reagent and a particular carbon atom of the benzene ring. The a-structures are essentially classical carbonium ions. In the -structures a non-classical bond is established, not to any particular atom, but to the -electron cloud in general. It is quite likely that both types of structure are represented by actual examples. Thus m-xylene interacts more strongly with hydrogen chloride than does o-xylene, but the difference between the two hydrocarbons is much more pronounced when their interactions with a boron trifluoride-hydrogen fluoride mixture are compared. This is readily understandable... 

The site of reaction on an unsaturated organometallic molecule is not restricted to the most probable position of the metallic atom or cation or to a position corresponding to any one resonance structure of the anion. This has been discussed in a previous section with reference to the special case of reaction with a proton. Although the multiple reactivity is particularly noticeable in the case of derivatives of carbonyl compounds, it is not entirely lacking even in the case of the derivatives of unsaturated hydrocarbons. Triphenylmethyl sodium reacts with triphenylsilyl chloride to give not only the substance related to hexaphenylethane but also a substance related to Chichi-babin s hydrocarbon.401 It will be recalled that both the triphenyl-carbonium ion and triphenylmethyl radical did the same sort of thing. 

The direct protonation of isobutane, via a pentacoordinated carbonium ion, is not likely under typical alkylation conditions. This reaction would give either a tertiary butyl cation (trimethylcarbenium ion) and hydrogen, or a secondary propyl cation (dimethylcarbenium ion) and methane (37-39). With zeolites, this reaction starts to be significant only at temperatures higher than 473 K. At lower temperatures, the reaction has to be initiated by an alkene (40). In general, all hydrocarbon transformations at low temperatures start with the adsorption of the much more reactive alkenes, and alkanes enter the reaction cycles exclusively through hydride transfer (see Section II.D). 

Carbocations are central to hydrocarbon chemistry (/). Much of this chemistry is based on acid catalysis, which leads to generation of positive ions of carbon. The resulting intermediates are classified as carbenium and carbonium ions, as proposed by Olah (2-4). Carbonium ions are the penta- or higher coordinate carbocations that maintain 8 valence electrons via 2-electron/3-center bonding, quite different from carbenium ions that possess only 6 valence electrons. Figure 1 shows a systematic classification of carbocations. 

Aspects of bonding and structure/dynamics in selected carbonium ions were presented and discussed. These representative studies demonstrate the power of structural theory in the development of concepts that could lead to new and efficient processes, especially in the area of hydrocarbon chemistry and catalysis. There is no doubt that as newer theoretical and experimental techniques and models are introduced, they will be applied to the study of carbonium ions. A deeper understanding of structure/dynamics of hypervalent non-classical carbonium ions will not only deepen our knowledge of structural theory in chemistry, but could also help in the development of new processes and materials useful in our daily life. 

All the amendments to Bredt s rule that have been presented in the past decade have been more or less violated. One reason for these failures is that the past rules ignored the strain in parts of the molecule other than at the bridgehead double bond. Schleyer defines the strain at the bridgehead double bond, or olefin strain (OS), as the difference in strain between the olefin and the parent hydrocarbon, analogous to for carbonium ions (293). He... 

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