Primary carbenium ion

Figure 13.19 Protonation mechanism for /-butylene over a Bronsted acid site-primary carbenium ion.

The unimolecular mechanism involves formation of a protonated cyclopropane ring first, which avoids the formahon of a primary carbenium ion until after skeletal rearrangement has taken place. Such reachon intermediates were first... 

Alkylation of methane, ethane, propane, and n-butane by the ethyl cation generated via protonation of ethylene in superacid media has been studied by Siskin,148 Sommer et al.,149 and Olah et al.150 The difficulty lies in generating in a controlled way a very energetic primary carbenium ion in the presence of excess methane and at the same time avoiding oligocondensation of ethylene itself. Siskin carried out the reaction of... 

Isobutyl species formed on the catalyst in the initiation steps do not react in unimolecular processes since such reactions (e.g., the formation of n-butyl species) involve primary carbenium ion transition states. Therefore, propagation steps in the conversion of isobutane include oligomerization reactions... 

Preparatively, it is important that mineral acids, carboxylic acids, and terf-carbenium ions can be added to alkenes via carbenium ion intermediates. Because of their relatively low stability, primary carbenium ions form more slowly in the course of such reactions than the more stable secondary carbenium ions, and these form more slowly than the even more stable tertiary carbenium ions (Hammond postulate ). Therefore, mineral and carboxylic acids add to unsymmetrical alkenes regioselectively to give Markovnikov products (see Section 3.3.3 for an explanation of this term). In addition, these electrophiles add most rapidly to those alkenes from which tertiary carbenium ion intermediates can be derived. 

A Wagner-Meerwein rearrangement can be part of the isomerization of an alkyl halide (Figure 14.4). For example, 1 -bromopropane isomerizes quantitatively to 2-bromopropane under Friedel-Crafts conditions. The [l,2]-shift A — B involved in this reaction again is an H atom shift. In contrast to the thermoneutral isomerization between carbenium ions A and B of Figure 14.3, in the present case an energy gain is associated with the formation of a secondary carbenium ion from a primary carbenium ion. Note, however, that the different stabilities of the carbenium ions are not responsible for the complete isomerization of 1-bromopropane into 2-bromopropane. The position of this isomerization equilibrium is determined by thermodynamic control at the level of the alkyl halides. 2-Bromopropane is more stable than 1-bromopropane and therefore formed exclusively. 

This cation can be drawn either as an oxonium ion or as a primary carbenium ion. The oxonium ion structure is the more realistic. Primary carbenium ions are not known in solution, let alone as isolable intermediates, and the proton NMR spectrum of the cation compared with that of the isopropyl cation (this is the best comparison we can make) shows that the protons on the CH2 group resonate at 9.9 p.p.m. instead of at the 13.0 p.p.m. of the true carbenium ion. 

A primary carbenium ion is less stable than a secondary and a tertiary ion is most stable. The interaction of various carbenium ions with the zeolite wall decreases in the same order. A primary carbenium ion forms covalently bonded ethoxy species with the zeolite wall. As illustrated in Fig. 4.64, when an ethylene molecule approaches a proton or weak n hydrogen bond is formed initially. Upon proton transfer a stable a ethoxy species is formed. 

Carbenium ions have been shown to isomerize readily (Fig. 4.71). As long as no primary carbenium ions are involved, isomerization of carbenium ions occurs with low activation energy and at low temperatures. Isoalkane formation occurs by reaction with another alkane and transfer of a hydride ion (Fig. 4.72). 

In addition to the transfer reactions already discussed, propagating carbe-nium ions also react with nucleophilic hydride and methide anions. This reaction may be bimolecular, or it may occur by an intramolecular hydride shift to form a more stable carbenium ion. The activation energy of hydride transfer is usually higher than that of propagation, and therefore occurs only at elevated temperatures. Nevertheless, hydride transfer is the dominant reaction when a-methylstyrene is initiated by triphenylcarbenium ions. That is, steric hindrance prevents initiation by direct electrophilic addition of the carbenium ion to a-methylstyrene. Instead, it occurs by hydride transfer from monomer to yield triphenyl methane and the primary carbenium ion of a-methylstyrene [cf., Eq. (40)]. 

This reaction has been the subject of many experimental, " and theoretical studies. The reaction, which initiates from a propylene physisorbed to the acidic proton, leads to the formation of a more stable chemisorbed propylene, or alkoxy species. Such reaction has been shown experimentally to occur readily at room temperature within an acidic zeolite. Whereas this reaction in principle can produce two different alkoxy species (viz. a primary and a secondary alkoxy species), experiment reports that only the secondary alkoxy species can be formed. This is explained by the fact that the formation of a transient primary carbenium ion is energetically more demanding than the formation of a secondary carbenium ion. " As already... 

Cyclopropyl ring opening at the C3 —C4 carbon bond results in the formation of a primary carbenium ion ... 

The highly unstable primary carbenium ion rearranges rapidly to give the relatively stable tertiary butyl cation, which gives isobutylene by proton abstraction ... 

The mechanism of skeletal isomerization of n-butenes may be rationalized in terms of the steps presented previously the key reaction intermediate is the 5-butyl cation. The predominent structure of the adsorbed intermediate was recently considered to be an alkoxy 50), which cither adds to one butene molecule and cracks into C3, C4, or C5 fragments (the bimolccular mechanism) or rearranges into isobutylene (the monomolecular mechanism) via a primary carbenium ion. 

To overcome the formation of a primary carbenium ion intermediate, it has been proposed for an aged ferricrite catalyst, which is highly selective for isobutylene, that a carbenium ion trapped within the carbonaceous residues formed in or on the zeohte could be the active site. The authors suggested that the structure of such an active site could be... 

The proposed pathway will be more favorable kinetically than that suggested for the true monomolecular process, whereby a primary carbenium ion is formed. To further test the idea that carbonaceous residues are the active and selective sites for the skeletal isomerization of n-butenes, the authors reported results showing that the rate of isobutylene formation catalyzed by ferrierite passed through a maximum as the conversion continuously decreased (Fig. 12) (51). 

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