Carbenium ions properties

Stepanov AG, Luzgin MV, Romannikov VN, Zamaraev KI. Carbenium ion properties of octene-1 adsorbed on zeolite H-ZSM-5. Catal Lett 1994 24 271-84. 

Carbocation property Carbenium ions Carbonium ions... 

In order to think constructively about these problems we need some scale on which we can measure that intrinsic property of the ions which determines (in part) both stability and reactivity. In practice, we have to content ourselves, as far as carbenium ions are concerned, with the gas-phase standard enthalpy of formation, AHf, of a series of ions, and make estimates of the effects of solvation energies. In this way we can go quite a long way towards rationalising the reactivity of initiating ions with various monomers. 

Example The mass spectra of both acetone and butanone show typical acyiium ion peaks at m/z 43, whereas the signals in the spectra of isopropyl ethyl thioether (Fig. 6.9), of 1-bromo-octane, (Fig. 6.10), and of isomeric decanes (Fig. 6.18) may serve as examples for carbenium ion signals. The superimposition of both classes of ions causes signals representing an average pattern. The properties of larger carbenium ions are discussed in the section on alkanes (Chaps. 6.6.1 and 6.6.3). 

Several reviews appeared on the heavier congeners of the carbenium ions. Clearly, the silylium ion problem has received the most attention, and both theoretical as well as experimental aspects have been reviewed. The chemistry of cationic germanium, tin and lead is covered by a recent review by Zharov and Michl. We will concentrate in this review on the description of the progress made during the last 4 years and will try to give an account on the synthesis, the properties and the structure of organosubstituted three-coordinated, tiivalent group 14 element cations and closely related species in the condensed phase. 

A.l. Formation of Surface Alkoxy Species with Carbenium-Ion-Like Properties... 

So far, fewer than 10 types of carbenium ions have been reported to be persistent species formed upon adsorption of olefins or alcohols on acidic zeolites. Instead, surface alkoxy (alkoxide) species with carbenium-ion-like properties are suggested to act, most likely, as catalytic intermediates in reactions catalyzed by acidic zeolites. Various groups have observed that, upon adsorption of olefins or alcohols on acidic zeolites, alkoxy species are formed the observations are based on both in situ and ex situ A MAS NMR spectroscopy (49,50,71-80). 

Similar correlations between the acid-base properties of catalysts and activ-ity/selectivity were earlier observed in the rearrangement of simple oxiranes (refs. 5-8). In our case it seems reasonable to suppose that the observed changes are due to the different competing mechanisms discussed above. WO, with strong acidic sites in high concentration, is able to form the carbenium ion. Since the density and the strength of the basic sites on WO are low, formation of the double-bonded surface species depicted in Fig. 3 has only a low probability. The single-bonded open carbenium ion is then mainly transformed to ketone 3. In harmony with this, the isomers exhibit identical selectivity, a... 

As might be expected, NMR calculations that ignore electron correlation often give poor results, especially for molecules which typically require a correlated treatment in order to predict other properties accurately. For example, a good description of multiple bonds and lone pairs generally requires a correlated method. Thus, RHF NMR predictions for molecules such as CO and acetonitrile are poor (20). Furthermore, it has recently been shown that isotropic chemical shift calculations at the RHF level are unreliable for benzenium (21) and related carbenium ions which we often encounter in catalysis. 

This observation was explained in terms of the unusual electronic properties of pentaco-ordinated cations (Kollmar and Smith, 1970). The CHs species is best described as a hydrogen molecule with an abnormally long bond, 0-94 A, to which a deformed trivalent carbonium (carbenium) ion is coordinated. The energy of CHs was calculated to be 47 kcal mol-1 lower than that of H2 + CH3 (gas phase value) and two of the C—H bonds only of half the strength of a C—H bond in methane. This electronic structure, perhaps best being described as closely related to protonated hydrogen, is probably the reason for the reverse order of oxidation potentials observed. 

Many varieties of saturated ions are involved in the fruitful reactions of cracking, all participating in cycles of reactions which preclude deactivation. In order to explain catalyst decay, we must envision the formation of some other type of surface species. This must be a species which occasionally arises from the carbenium ions which normally participate in the mainline reactions. Where else could it come from Such a species - the species we believe to be responsible for decay - is unsaturated carbenium ions. Unsaturated ions may be expected to differ in their desorption and reactivity properties from the more common saturated ions perhaps these differences are sufficient to explain the accumulation of deactivating species. 

Within the scope of this review we shall only consider those compounds possessing one or more alkenyl functions susceptible to activation by electrc hilic attack. Included in this family is a vast array of monomers varying in basicity from ethylene, which is so resistant to protonation that the ethyl carbenium ion has hitherto eluded observations even under the most drastic conditions (see below), and which in fact is equally resistant to cationic polymerisation, to N-vinylcarbazole, whose susceptibility to this type of activation is so pronounced that it can be polymerised by almost any acidic initiator, however weak. We shall also deal with olefins which, because of steric hindrance, can only dimerise (e.g., 1,1-diphenylethylene) or cannot go beyond the stage of protonated or esterified monomeric species (e.g., 1,1-diphenylpropene). The interest of such model compounds is obvious they allow clean and detailed studies to be conducted on the kinetics and mechanism of the initiation steps and on the properties of the resulting products which simulate the active species in cationic polymerisation. The achievements and shortcomings of the latter studies will be discussed below. 

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