Stable carbenium ion

The second step is a -scission, the breaking of a carbon—carbon bond P to the charged carbon. The sum of the two reactions is the stoichiometry of the overall cracking reaction R H — RH + olefin. R+, a relatively stable carbenium ion such as the /-butyl cation, is a chain carrier. The role of the catalyst is to donate the proton to start the chain. This is a greatiy simplified representation. 

The initial step is the coordination of the alkyl halide 2 to the Lewis acid to give a complex 4. The polar complex 4 can react as electrophilic agent. In cases where the group R can form a stable carbenium ion, e.g. a tert-buiyX cation, this may then act as the electrophile instead. The extent of polarization or even cleavage of the R-X bond depends on the structure of R as well as the Lewis acid used. The addition of carbenium ion species to the aromatic reactant, e.g. benzene 1, leads to formation of a cr-complex, e.g. the cyclohexadienyl cation 6, from which the aromatic system is reconstituted by loss of a proton ... 

In the initial step one hydroxy group is protonated, and thus converted into a good leaving group—i.e. water. Subsequent loss of water from the molecule proceeds in such a way that the more stable carbenium ion species 2 is formed. The next step is a 1,2-shift of a group R to the tertiary carbenium center to give a hydroxycarbenium ion species 4 ... 

In the case of an appropriate substrate structure, the carbenium ion species can undergo a 1,2-alkyl shift, thus generating a different carbenium ion—e.g. 4. The driving force for such an alkyl migration is the formation of a more stable carbenium ion, which in turn may undergo further rearrangement or react to a final product by one of the pathways mentioned above—e.g. by loss of a proton to yield an alkene 3 ... 

In the copolymerization of isopropenylferrocene with a-methyl-styrene at 0°C, it appears that the stable carbenium ion of isopropenylferrocene acted as an inhibitor for the polymerization. 

FIGURE 3.8 Deprotection of carboxyl groups by acid-catalyzed hydrolysis (A) of amides and (B) of esters. Protonation generates a relatively stable carbenium ion that usually requires heat to fragment it. 

Aroyl and related salts (ArCO, AlkCO ) and stable carbenium ion salts (e.g. Ar3C ) of complex anions MtX . i can be synthesised essentially by two routes, silver salt double decomposition, and direct combination ... 

The regioselectivity in the fluorination of phenyl-substituted alkenes 9 has been reported and in the case of an electron-withdrawing group (C02Me, Ac) and a donating group (4-OMe) a more stable carbenium ion is formed and complete regioselectivity is observed, but it is completely lost in the reaction with the chloro derivative.2... 

A few examples of uncommon hydrolyses, corresponding formally to an SN(Ca) reaction should also be mentioned (reaction 176). Indeed with particularly stable carbenium ions such as diphenylcyclopropenium580 or tropylium581,582, spontaneous dissociation of the phosphonium ion into the phosphine and the carbenium ion occurs before reaction with the hydroxide anion can take place. 

Attempts to cyclize the keto sulfide (282) with polyphosphoric acid at temperatures greater than 100 °C gave as the major product the 2-isopropyl-3-methylthiophene (284). When the cyclization was conducted at lower temperatures, the thiochromene (283) was the major product. Since 4-phenylthio-2-butanone cyclized to produce a stable thiochromene, the quaternary carbon in (283) which can form a stable carbenium ion (284a)... 

Theoretical and Experimental NMR techniques provide powerful tools for the investigation of heterogeneous catalysis. Recent advances in in situ NMR techniques are summarized, as are advances in theoretical methods. The utility of our combined theoretical/experimental approach is illustrated by studies of the pentamethylbenzenium cation and the 1,3-dimethylcyclopentenyl cation in zeolite HZSM-5, acetylene adsorption on MgO, and the isopropyl cation on frozen SbF5. We also discuss the role of the basicity of adsorbates in the formation of stable carbenium ions on zeolites. 

The. S n 1 hydrolysis of 5-methoxyacenaphthylene 1,2-oxide was found to occur preferentially via the 7.73 kcal mol-1 more stable carbenium ion,8 as confirmed by B3LYP/6-31G calculation. The predominant formation of the less stable c/s-diol is a consequence of kinetic control and explained by calculations at the MP2/6-31G // MP2/6-31G level of theory, which reveal the stabilizing influence of the hydrogen bonding that occurs between the attacking water molecule and the /3-OII group on the carbenium ion in the transition state. 

The proton adds first to the carbon-carbon double bond. The carbon bearing more substituents forms a more stable carbenium ion attack of bromide ion follows in a second step ... 

Any substrate capable of generating a stable carbenium ion is a suitable starting material primary alcohols do not react under these conditions, with exception of benzylic alcohols ... 

As a measure of the rate of formation of this carbenium ion, one can take the free energy of heterolysis of the alkylating agent from which it arises. Low free energies of heterolysis are related to the formation of a stable carbenium ion and thus high SN1 reactivity. Because SN1 reactions are carried out in solution, the free energies of heterolysis in solution are the suitable stability measure. These values are not available, but they can be calculated from other experimentally available data (Table 2.1) by means of the thermodynamic cycle in Figure 2.15. 

Fig. 14.15. Regioselectivity of the pinacol rearrangement of an unsymmetrical glycol. The more stable carbenium ion is formed under product development control. Thus, the benz-hydryl cation B is formed here, while the tertiary alkyl cation D is not formed.

The concept of the generation of silylenium ion by hydride abstraction with a stable carbenium ion has been explored in designing the synthesis of saturated and unsaturated cyclic silaethers (104). For this purpose the incipient silylenium ion is intramolecularly trapped by a properly located ethereal nucleophile, which leads to cyclization [Eq. (21)]. If the triphenyl-... 

The C8 carbenium ions formed may isomerize via hydride transfer and methyl shifts to form more stable carbenium ions ... 

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