Carbenium ions intermediates

Catalytic cracking proceeds mainly via carbenium ion intermediates. The three dominant reactions are cracking, isomerization, and hydrogen... 

The reaction mechanism involves carbonium and carbenium ion intermediates. The first and difficult step is the generation of carbonium ions from alkanes ... 

The data are summarized in Table II. They have been normalized to kx x s i for each zeolite catalyst. In general it is seen that the7transfer of an ethyl group (E,E E,X) occurs faster than that of a methyl group (X,E X,X). This is in agreement with the indicated mechanism for transalkylation (Figure 4) which involves a benzylic carbenium ion intermediate. In the case of methyl transfer, this is a primary cation,... 

Subsequent studies on cyclopentadiene, in which the antarafacial concerted 1,4-addition is impossible because of interference between the antimony system and the methylene of cyclopentadiene, have however shown70 that both butadiene and cyclopentadiene react with SbCIs through a stepwise mechanism involving a carbenium ion intermediate. In agreement with a non-concerted mechanism are also the data related to the 1,4-addition... 

The Ritter reaction [6] proceeds by the electrooxidation of alkyl iodides (56) in an MeCN-(Pt) system to form Ai-alkyl acetamides (58) (Scheme 21). Attack of carbenium ion intermediate - from dissociation of the initially formed alkyl cation radical - to acetonitrile would give the iminium cation (57). However, a different mechanism is proposed, whereby the alkyl iodide reacts with the electrogenerated iodo cation [I]" " [73]. 

An ab initio study of the l-azabicyclo[1.1.0]butyl cation (97) and its isomers shows that (98) and (99) are much less stable than (97), and that the transition states between (97), (98), and (99) are too high in energy to allow (99) to form. The 3-halobicyclo[l.l.l]pent-l-yl cation (101) has been shown to be an intermediate in the addition of halogens to (100). The only product observed was (102) no rearranged products were detected. The Diels-Alder-type reaction of (103) to give (104) is said to involve several carbenium ion intermediates. 

The clearest and most direct experimental evidence from zeolite studies for the existence of a free carbenium ion intermediate obtained by any means is summarized in Fig. 14 (113). We followed the dimerization of styrene to form cis and trans isomers in a series of low-temperature MAS NMR experiments. Identification of the dimeric products was further... 

An extremely wide variety of catalysts, Lewis acids, Brinnsted acids, metal oxides, molecular sieves, dispersed sodium and potassium, and light, are effective (Table 5). Generally, acidic catalysts are required for skeletal isomerization and reaction is accompanied by polymerization, cracking, and hydrogen transfer, typical of carbenium ion intermediates. Double-bond shift is accomplished with high selectivity by the basic and metallic catalysts. 

If the reduction step is the rate-determining one, path 1 (see Scheme 14), which appears to be the plausible one, fails to explain the different selectivity for the exo- and endo-oxolane acetals, because these would then proceed through the same oxo-carbenium ion intermediate. 

There have been a number of reports of improved selectivity with sulfonic acid resin catalysts compared with conventional liquid acid catalysts[6—9]. Various explanations have also been proposed. If mechanisms usually postulated for condensation reactions with liquid Br0nsted acid [10] and solid acid catalysts [11] are adopted, the sequence of steps shown in Fig. 2 could be considered for the condensation of MFC. Both mechanisms incorporate the essential features of known carbenium ion chemistry, i.t., electrophilic attack on the aromatic ring by polar carbenium ion intermediates. Note that MDU is formed by this attack on the benzene ring of MPC, while the N—benzyl compound by the attack on nitrogen atom. 

The Biginelli synthesis (Scheme 3) is an important route to dihydropyrimidilies, e.g. (25),46a with many variants of the original reactants now established. The mechanism has now been re-investigated using 1H- and 13C-NMR.46b The first step does not appear to involve aldol condensation or a carbenium-ion intermediate rather, condensation of benzaldehyde and urea gives an A-acyliminium ion intermediate (26), which then goes on to react with ethyl acetoacetate. 

This reaction occurs with a variety of fully substituted 1,2-diols, and can be understood to involve the formation of a carbenium ion intermediate that subsequently undergoes a rearrangement. The first generated intermediate, an a-hydroxycarbenium ion, rearranges through a 1,2-alkyl shift to produce the carbonyl compound. If two... 

Negative p values of -1.51 and -0.96 versus o+ were found for the syn/anti ratio in the reaction between l-aryl-l,2-epoxycyclohexanes and MeOH in 0.2 n H2S04/ MeOH and in the gas phase catalysed by D3+ respectively.45 This led the authors to suggest that these reactions occur via a mechanism where the methanol attacks an ion-dipole pair rather than a full carbenium ion intermediate, except when the para-substituent is MeO or N02. The p-MeO reaction was thought to be an SnI process whereas the p-N02-substituted compound reacts via an SN2 mechanism. 

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