Carbocations as intermediates

The reaction mechanism is based on protonation of the hydroxyl moiety, rearrangement of the phenyl group and simultaneous cleavage of water, creating a carbocation as intermediate [135]. This cation is hydroxylated by water. Thereby, an unstable hemiacetal is formed that splits into two molecules, phenol and water. 

Several reaction pathways for the cracking reaction are discussed in the literature. The commonly accepted mechanisms involve carbocations as intermediates. Reactions probably occur in catalytic cracking are visualized in Figure 4.14 [17,18], In a first step, carbocations are formed by interaction with acid sites in the zeolite. Carbenium ions may form by interaction of a paraffin molecule with a Lewis acid site abstracting a hydride ion from the alkane molecule (1), while carbo-nium ions form by direct protonation of paraffin molecules on Bronsted acid sites (2). A carbonium ion then either may eliminate a H2 molecule (3) or it cracks, releases a short-chain alkane and remains as a carbenium ion (4). The carbenium ion then gets either deprotonated and released as an olefin (5,9) or it isomerizes via a hydride (6) or methyl shift (7) to form more stable isomers. A hydride transfer from a second alkane molecule may then result in a branched alkane chain (8). The... 

Cationic polymerization of alkenes involves the formation of a reactive carbo-cationic species capable of inducing chain growth (propagation). The idea of the involvement of carbocations as intermediates in cationic polymerization was developed by Whitmore.5 Mechanistically, acid-catalyzed polymerization of alkenes can be considered in the context of electrophilic addition to the carbon-carbon double bond. Sufficient nucleophilicity and polarity of the alkene is necessary in its interaction with the initiating cationic species. The reactivity of alkenes in acid-catalyzed polymerization corresponds to the relative stability of the intermediate carbocations (tertiary > secondary > primary). Ethylene and propylene, consequently, are difficult to polymerize under acidic conditions. 

In an attempt to study the l-methylcyclopentyl-[70] to cyclohexyl-[71] cation interconversion, Olah et al. (1967) tried a number of cyclohexyl- and methylcyclopentyl-precursors under different superacidic conditions at —60°C. However, the only observed product was ion [70]. For the facile rearrangement of [71] to [70] Olah et al. favoured protonated cyclopropanes over primary carbocations as intermediates. 

According to this scheme, the first step of the reaction is the formation of a hydrogen-bonded precursor FH B, followed by the protonation of the monomer, leading to the formation of FBH". Examples of this type are shown in Section IV.B, where the oligomerization of unsaturated molecules in protonie zeolites is discussed. It is important that this first step is common to other reactions catalyzed by Bronsted acid sites. For example, in Section IV.A, the formation of methyl-substituted benzene carbocations as intermediate species involved in the MTO process in Hp zeolite is diseussed. 

The coupling of an allyl or acyl moiety onto carbon atoms is achieved by anodic oxidation of a-heteroatom substituted organostannanes or Oj -acetals in the presence of allylsilanes or silyl enol ethers. The reaction probably involves carbocations as intermediates that undergo electrophilic addition to the double bond [245c]. 

Thls trick was used to show the existence of simple carbocations as intermediates in the SnI mechanism in Chapter 17. 

It is interesting to see completely different regiochemistries in nucleophilic and electrophilic reactions to 1,1-difluoroethene as a model. Scheme 1.55 shows a typical electronic effect of the fluorine atom as a substituent. Electrophiles mostly attack the (3-carbon of 1,1-difluoroethene generating a-difluorinated carbocations as intermediates in contrast to regioselective nucleophilic additions on the a-carbon of 1,1-difluoroethene generating (3-fluorocarbanions. 

Here, reactivity indexes cannot be computed in the way defined previously (as a reciprocal value of the activation barrier for certain reactions), but the relative reactivity of carbocations or reactions that have those carbocations as intermediates can be determined. The carbocations are a very reactive species... 

Quite early. Ingold and coworkers (Baker et al., 1928) proposed carbocations as intermediates in these reactions, as the product pattern and the stereochemistry display similarities to those of nucleophilic aliphatic substitutions of molecules with anionic leaving groups like halogenide ions, arenesulfonate ions, etc. (7-2). 

In strong acid, the electrophile is a proton and it is actually possible to observe this cationic intermediate. The trick is to pick a non-nucleophilic and non-basic counterion X-, such as SbF. In this octahedral anion, the central antimony atom is surrounded by the fluorine atoms with the negative charge spread over all seven atoms. The protonation is carried out using FSO3H and SbF at -120 °C. A similar trick was described in Chapter 15 as a means to show the existence of simple carbocations as intermediates in the mechanism. 

Pearson, A.J. (1984) Iron-stabilised carbocations as intermediates for organic synthesis. Science, 223, 895. 

When studying reactions that are believed to involve carbocations as intermediates, it is common to test this proposal by assessing the stereochemical relationship between the organic reactant and its product. For example, if a carbocation is an intermediate in the reaction of tertiary alcohols with hydrogen halides, both stereoisomers of 4-tert-butyl-l-methylcyclohexanol are converted to the same carbocation ... 

In Summary Ethers can be prepared by treatment of alcohols with acid through Sn2 and SnI pathways, with alkyloxonium ions or carbocations as intermediates, and by alcoholysis of secondary or tertiary haloalkanes or alkyl sulfonates. 

Why do we not simply write the isomeric free carbocations as intermediates in the acid-catalyzed ring openings The reason is that the cyclic oxonium ion has an octet sfructure, whereas the carbocation isomer has a carbon with an electron sextet. Indeed, experimentally, inversion is observed when reaction takes place at a stereocenter. Like the reaction of oxacyclopropanes with anionic nucleophiles, the acid-catalyzed process includes backside displacement—in this case, on a highly polarized cyclic alkyloxonium ion. 

Considering the Woodward-Hoffinann rules, if the conrotatory six-electron ring closure is allowed under photochemical conditions, it must be forbidden under thermal conditions. In consequence, the cyclization of 1 to tra 5-dihydronaph-thalene 3 in the presence of triflic acid should not be a concerted process. Therefore, in this case, it is more reasonable to think of a stepwise acid-promoted mechanism for the cyclization, possibly involving carbocations as intermediates. 

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