Carbonium special

Acidic, high area silica-almnina had received substantial attention in ICC 1, 52-58). Perhaps the most dramatic change in the subsequent catalytic literature was the debut of zeolites. Why acid catalyzed reactions are so much faster on zeolites than on silica-alumina has been extensively discussed but probably not conclusively. One should be able to know the exact structures of catalytic sites in zeolites, but initial hopes that this would do wonders for mechanistic imderstanding have not been fully realized. Super acids and carbonium ions came into heterogeneous catalysis from homogeneous chemistry and in special cases reaction via carbonium ions seems to occur. 

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. 

Since the condition for formation of high polymers is that the propagation reaction must be faster than all other reactions of the growing species, and since carbonium ions are highly reactive, it is evident that very special conditions are required for the formation of high polymers by cationic polymerization. The general conditions which must be satisfied are ... 

The degree of dissociation is a = pic = 1 - qlc. We now consider the three special cases shown in Table 3. In order to ascertain which, if any, of the two approximations may be valid, we need to examine the magnitudes of K, Ky and c which are relevant to the systems under discussion. To determine a value of K which will be relevant to polymerizing systems, we need the dissociation constant of a carbonium salt with a large anion, in a solvent of e about 10, at 25 °C. The only relevant information is Longworth and Mason s value of K for triphenylmethyl perchlorate in ethylene dichloride [53], and values of K for two quaternary ammonium perchlorates [140] (see Table 4). 

Concerning the (generally complicated) polymerisation of higher alkenes, it is shown that the transfer of CHf, analogous to H transfer, may play a significant part, except for isobutene. The energetic reasons for the distinctive polymerisation behaviour of isobutene are analysed, with special reference to the energetics of the transfer of protons or carbonium ions to monomer. The hypothetical termination reaction for the isobutene-BF3 polymerisation. 

It now remains to place the concept of an ester as an active species into a wider chemical context, with special reference to polymerisation catalysts. Sinn and Patat [39] have emphasised the distinction between monofunctional and bifunctional catalytic systems and this distinction is obviously and necessarily related to the idea, explained above, that there is a difference in kind between polarised molecules and the ions which can be formed from them. Whereas the carbonium and other cations as reactive species are monofunctional, the esters evidently belong to the class of bifunctional catalysts their mode of action - the addition of their constituent parts across a double bond - is, in modern terminology, an insertion reaction. In this context, we must note the important... 

Any discussion of the mechanism of xanthine oxidase should attempt to incorporate the special features of xanthine oxidase (and xanthine dehydrogenase and aldehyde oxidase) which are not present, for example, in sulfite oxidase. There are two such features at least (a) the involvement of two protons rather than the one found for sulfite oxidase, and (b) the presence of the cyanolyzable sulfur atom. The mechanistic features discussed so far involve the abstraction of two electrons and a proton. This means that a carbonium ion is generated, which could undergo attack by a nucleophile. Thus, the presence of a nucleophile at the active site could lead to the formation of a covalent intermediate that will break down to give the products.1032 The nucleophile could either be the cyanolyzable sulfur atom or a group associated with the second proton. A possible scheme is shown in Figure 41. 

Examples for frequently encountered intermediates in organic reactions are carbocations (carbenium ions, carbonium ions), carbanions, C-centered radicals, carbenes, O-centered radicals (hydroxyl, alkoxyl, peroxyl, superoxide anion radical etc.), nitrenes, N-centered radicals (aminium, iminium), arynes, to name but a few. Generally, with the exception of so-called persistent radicals which are stabilized by special steric or resonance effects, most radicals belong to the class of reactive intermediates. 

All of the reactions of such vinylic derivatives that we have studied proceed in this manner (46). This reaction is of special utility in the generation of tertiary cluster-substituted carbonium ions ... 

Boron alkyls are expected to be inactive for coordinated anionic polymerization of olefins because the boron-carbon bond is not sufficiently ionic. The diazomethane polymerization with boron alkyl catalyst reported by Bawn, Ledwith and Matthies (275) is a special case of the growth reaction. A coordination mechanism seems most probable, but it has not been ascertained whether the polymer chain migrates as a car-banion or as a radical. If the complex between diazomethane and boron decomposes into a boron-carbene complex, then the polymer chain could migrate as a carbanion with the driving force provided by the electrophilic carbonium ion ... 

Comparisons between diazonium ion reactions and solvolyses involving tertiary alkyl structures are of special interest because both reactions are likely to yield unsolvated carbonium ions. Cannell and Taft (1956) have measTu-ed the ratio of thiocyanate to isothiocyanate obtained when the carbonium ions formed in the solvolysis of t-butyl chloride and the deamination of t-butylamine and isobutylamine in ethanol are trapped with large amormts of added sodium thiocyanate. For the t-butyl... 

It is of special interest that the orbital symmetry rules can be applied to sigmatropic migrations within ionic species. E.g. the suprafacial 1,2 shift within a carbonium ion is symmetry-allowed ... 

Three-c(H)rdinate carbenium ion and five-coordinate carbonium ion intermediates satisfactorily account for many of the acid-catalyzed reactions of hydrocarbons at high temperatures. Yannoni et al. have characterized the structure and dynamics of several carbenium ions trapped in (noncatalytic) solids at low temperatures [32,94,95), but lifetimes of such ions on active surfaces at higher temperatures would preclude NMR observation in all but special cases. Maciel observed triphenyl carbenium ion on alumina 196). The alkyl-substituted cyclopentenyl ions discussed earlier are also special ions they are commonly observed products in conjunct polymerization reactions of olefins in acidic solutions. The five member ring cannot easily rearrange to an aromatic structure, and ions like I and II are apparently too hindered to be captured by the framework to form alkoxy species. 

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