Carbocations oxonium ion

Carbocation-oxonium ion equilibria are obvious complicating factors in studies of the kinetics of initiation of polymerisation and useful thermodynamic data for such equilibria involving Ph3C+ and a variety of linear and cyclic ethers have been reported by Slomkowski and Penczek (132). A dramatic increase in rates of initiation of polymerisation of THF induced by Ph3C+ salts is observed on addition of small amounts of epoxides such as propylene oxide (113a,b), which compete favourably with THF in the primary carbocation-oxonium ion equilibria and simplify the initiation reaction ... 

Nucleophilic addition of water to the carbocation yields another oxonium ion. . . ... 

Alkoxycarbenium ions are important reactive intermediates in modem organic synthesis.28 It should be noted that other names such as oxonium ions, oxocarbenium ions, and carboxonium ions have also been used for carbocations stabilized by an adjacent oxygen atom and that we often draw structures having a carbon-oxygen double bond for this type of cations.2 Alkoxycarbenium ions are often generated from the corresponding acetals by treatment with Lewis acids in the presence of carbon nucleophiles. This type of reaction serves as efficient methods for carbon-carbon bond formation. 

Zeolites are the main catalyst in the petrochemical industry. The importance of these aluminosilicates is due to their capacity to promote many important reactions. By analogy with superacid media (1), carbocations are believed to be key intermediates in these reactions. However, simple carbocationic species are seldom observed on the zeolite surface as persistent intermediates within the time-scale of spectroscopic techniques. Indeed, only some conjugated cyclic carbocations were observed as long living species, but covalent intermediates, namely alkyl-aluminumsilyl oxonium ions (2) (scheme 1), where the organic moiety is bonded to the zeolite structure, are usually thermodynamically more stable than the free carbocations (3,4). 

Numerous studies suggest that alkyl-aluminumsilyl oxonium ions should be the real intermediates in hydrocarbon reactions over zeolite, whereas carbocations should be just transition states (J). Equilibrium between the alkyl-aluminumsilyl oxonium ion and the carbocation, although suggested in some cases, has never been experimentally or theoretically proven, but recent calculations indicated that the tert-butyl carbenium ion is an intermediate on some specific zeolite structures 6,7). 

We have recently shown that metal-exchanged zeolites give rise to carbocationic reactions, through the interactions with alkylhalides (metal cation acts as Lewis acid sites, coordinating with the alkylhalide to form a metal-halide species and an alkyl-aluminumsilyl oxonium ion bonded to the zeolite structure, which acts as an adsorbed carbocation (scheme 2). We were able to show that they can catalyze Friedel-Crafts reactions (9) and isobutane/2-butene alkylation (70), with a superior performance than a protic zeolite catalyst. 

Nevertheless, the discussion whether the intermediates involved in the reactions of hydrocarbons over zeolite surface is the alkyl-aluminumsilyl oxonium ion or the carbocation could not be answered with these previous studies. 

There are no reported studies of this rearrangement on the zeolite surface and we argued that it could give some clues to the alkyl-aluminumsilyl oxonium ion/carbocation equilibrium. In this work we show experimental and theoretical results on the rearrangement of the cyclopropylcarbinyl chloride over NaY zeolite, aiming at demonstrating the equilibrium between the carbocation and the alkyl-aluminumsilyl oxonium ion. 

Table 1 Relative energy of the calculated alkyl-aluminumsilyl oxonium ions and carbocations, at B3LYP/6-31++G(d,p) MNDO.

The three alkyl-aluminumsilyl oxonium ions are more stable than the carbocations, with the allylcarbinyl aluminumsilyl oxonium ion lying 4.5 and 4.7 kcal.mor1 lower in energy than cyclobutyl. and cyclopropylcarbinyl aluminumsilyl oxonium ions, respectively. This result is in agreement with the thermodynamic stability of the respective chlorides. 

The protonated epoxides, i.e. the oxonium ions, could not be characterized as minima on the respective potential energy surfaces, as in every case the epoxide ring opened by a barrierless process upon O-protonation. Charge delocalization maps are shown in Figure 9, and some selected NPA-derived charges for the carbocations are displayed in Table 4. 

Diels-alder adducts at 0°C. This cation radical-vinylcyclobutane rearrangement is non-stereospecific, thus accounting for the formation of a cis-trans mixture of Diels-Alder adducts. Kinetic studies revealed (Scheme 8) that the ionization of these ethers involves an inner-sphere electron-transfer mechanism involving strong covalent (electrophilic) attachment to the substrate via oxygen (oxonium ion) or carbon (carbocation). 

The initiator used is important for copolymerizations between monomers containing different polymerizing functional groups. Basic differences in the propagating centers (oxonium ion, amide anion, carbocation, etc.) for different types of monomer preclude some copolymerizations. Even when two different monomer types undergo polymerization with similar propagating centers, there may not be complete compatibility in the two crossover reactions. For example, oxonium ions initiate cyclic amine polymerization, but ammonium ions do not initiate cyclic ether polymerization [Kubisa, 1996]. 

For this 2° alcohol, oxonium-ion formation is followed by loss of HjO to give a 2° carbocation that rearranges to a 3 carbocation. Both carbocations react by two pathways they form bonds to Br to give alkyl bromides or they lose H to yield alkenes. 

On treatment with HI, the ether is protonated. This oxonium ion cleaves readily to give t-butyl alcohol and the relatively stable /-butyl carbocation. Iodide ion adds to the carbocation, and the alcohol reacts with HI both give /-butyl iodide. 

The operation of the anomeric effect and the stabilization of carbocations are beautifully illustrated in a conformational study of 2-oxanol (2-oxacyclohexanol) (Smith, B. J., /. Am. Chem. Soc., 1997, 119, 2699-2706). 2-Oxanol prefers the OH axial form by 12 kJ/mol and, upon protonation of the OH group, spontaneously loses water to form the oxonium ion. Use principles of orbital interaction theory to explain ... 

Both stable oxonium ion (118,119) and carbocation salts (23,54,151) have been used in the study of 1,3-dioxolan polymerisations in methylene chloride solvent. Some confusion still surrounds the complex mechanism involved though the work of Penczek and his co-workers (54) has gone far towards clarifying the situation. 

Acidic conditions also can be used for the cleavage of oxacyclopropane rings. An oxonium ion is formed first, which subsequently is attacked by the nucleophile in an SN2 displacement or forms a carbocation in an SN1 reaction. Evidence for the SN2 mechanism, which produces inversion, comes not only from the stereochemistry but also from the fact that the rate is dependent on the concentration of the nucleophile. An example is ring opening with hydrogen... 

This reaction is an dehydration acid-catalyzed.12 The hexaaquocop-per cation behaves as a weak cationic acid in copper-salt solution.13 Protonation of the hydroxy group produces an oxonium ion that decomposes unimolecularly into carbocation 21 and water. Water is removed from the reaction equilibrium by means of a water-separating device. Carbocation 21 eliminates an -proton with formation of the energetically favorable conjugated diene 9. 

The capture of carbocations by alcohols involves a similar donation of a lone pair of electrons on oxygen to the vacant 2p atomic orbital of the sp2-hybridized, sextet carbocation. Note that charge must be conserved so the first formed product is a positively charged oxonium ion. 

Resonance stabilization can also make n -electron donation much more effective by avoiding the formation of a sextet carbocation. Lone-pair donation from tire oxygen of enol derivatives is very important to the good donor ability of these compounds. The resulting oxonium ion has all valence octets (although positively charged) and is thus stabilized over sextet canonical forms. 

The unimolecular step in the reaction of cyclohexanol with hydrogen bromide to give cyclohexyl bromide is the dissociation of the oxonium ion to a carbocation. 

Cumene hydroperoxide degradation. The degradation of the cumene hydroperoxide proceeds via a carbocation mechanism. In the first step, a pair of electrons on the oxygen of the hydroperoxide s hydroxyl group is attracted to a proton of the H30+ molecule, forming an oxonium ion. 

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