Carbenium ion-type mechanism

Sommer and coworkers have made important observations with respect to the activation of alkanes over sulfated zirconia, a new type of solid superacid. Whereas isotope exchange of small alkanes occurs with the involvement of the corresponding pentacoordinate ions, a classical carbenium ion-type mechanism was found to be operative for larger homologs (propane, isobutane). The exception is the isomerization of n-butane over sulfated zirconia promoted by Pt and alumina, where the initiation step for isomerization was suggested to be the protolysis of the C-H bond. ... 

The alkylation of benzene with ethylene is an electrophilic substitution on the aromatic ring. Alkylation reactions are commonly considered as proceeding via carbenium-ion-type mechanisms. On a Bronsted acid site ethylene is protonated to form the active species. The latter can follow two major routes ... 

Evidence for the SNl-type mechanism is provided by fluorodehydroxylation of diastereomeric benzylic alcohols both diastercomers give the same mixture of products which implies that bond breaking precedes bond making in precursors that readily form carbenium ions. Evidence for an SN2 displacement is based on the results of fluorodehydroxylation of epimeric a-amino-/ -hydroxypropanoic acids which proceeds with predominant inversion of configuration.40... 

This potential-acidity diagram (Pourbaix s type) has been determined for a large series of alkanes.79 All of these results indicate two types of oxidation mechanism of the C—H bond (i) oxidation of alkanes into carbenium ion at high acidity levels and (ii) oxidation of alkanes into radicals at low acidity levels. 

Although the reactions have been generally described in terms of a carbenium ion mechanism, this does not altogether explain the catalytic behavior of the alkali metal ion-exchanged zeolites or the selectivity behavior. An ionic mechanism of the type previously described for cyclopropene dimerization would seem to be more appropriate for the alkali metal ion-exchanged zeolites, where the activity does seem to correlate qualitatively with the electrostatic field (e/r) exerted by the cation. 

Propagation is the most important elementary reaction in which a macro-molecular chain is formed. Control in new carbocationic polymerizations, in which well-defined polymers are prepared, might be explained by new mechanisms of propagation and new types of active centers involved. However, as discussed briefly in Section IV.B.4, we believe that only two types of species with different degrees of ionization are involved sp3-hybridized dormant species and sp2-hybridized carbenium ions [Eq. (43)] ... 

A more complex picture was painted in a further study by Rapoport, which indicated that both the mechanism and reactivity sequence are dependent upon the alkene structure and reaction conditions 1,2-disubstituted alkenes (1) reacting via an oxaselenocyclobutane intermediate with a reactivity sequence CH > CH2 > CH3 geminally disubsdtuted alkenes (2) with a reactivity sequence CH > CH2 > CH3 and trisubstituted alkenes (3) with a reactivity sequence CH2 > CH3 > CH, ( )-allylic alcohols being the preferred products as established by Blichi types (2) and (3) reacting via carbenium ion intermediates (4) without four-membered ring closure or by unspecified cyclic transition states. Rapoport s evidence also showed the final step to occur by 5n1 or 5n1 processes and not by 5n2. Monosubstituted alkenes, particularly arylpropenes, commonly react with rearrangement. ... 

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. 

Exposure of I to another efficient dienophile, quinone II, yielded the abnormal product III. This compound shows signs of C- and O-alkylation processes that call for the operation of a mechanism other than the normal [2+4] cycloaddition. 1,4-Benzoquinone is not only susceptible to Diels-Alder operations but also to Michael-type 1,4 additions. In this case the required nucleophile would be the vinyl ether portion of I while the alkyl dithiane group would remain unaltered throughout the sequence. Its role would be limited to provide substantial stabilization to the carbenium ion in V. The second C-alkylation step that would lead to four- and six-membered ring structures VI and IV, respectively, is overcome by proton transfer and D-alkylation to yield III (see Scheme 13.2). 

Trioxane is the cyclic trimer of formaldehyde and it can be polymerized to yield polyoxymethylene having the same structure as polyformaldehyde. Polymerization has been carried out with or without catalyst in the liquid, solid, and sublimed states. All polymerizations appear to proceed by a cationic mechanism and the usual type of cationic initiators are effective [122,138—141]. The structure of the cationic chain ends is not clear and two types of props ating centres have been proposed [142], namely, tertiary oxonium ions and carbenium ions. Their propagation reactions are... 

Unfortunately no carbenium ion-forming equilibrium is known where it is sure that the positive charge is localised solely on the central carbon and which would be convenient to use as a reference reaction for SnI-type processes. Further difficulties in the interpretation of the Grunwald-Winstein m values arise because the solvent itself could almost certainly be involved in the mechanism of the standard reaction. Internal return could also interfere with the value of the rate constant if decomposition of the carbenium ion became rate limiting as a result... 

This type of primary KIE is observed when the a-carbon is labeled in an SN2 reaction, a-carbon KIEs have been used extensively to determine the mechanism of SN reactions.20 Small a-carbon KIEs near 1 % are indicative of a carbenium ion SN (an SN1) reaction while larger KIEs of up to 8% for a 12C/13C KIE,21 16% for a 12C/14C KIE22 and 22% for an nC/14C KIE23 are indicative of an SN2 mechanism. 

---