Carbonium equivalents

Two other important properties of silicon-carbon bonds are that carbonium ions fl and carbanions (or metalloid equivalents) a to silicon are favoured over alternatives, i.e. that situations involving Si—C—C+ and Si—C are thermodynamically relatively good. 

Figure 3 illustrates a conductivity experiment with four successive additions of styrene whenever more styrene was added, the equivalent conductance (referred to the total perchloric acid concentration) fell abruptly, but not quite down to its original value during the first latency period. This is not because some carbonium ions remained after the styrene addition, but because the free acid present after the end of the polymerisation reacted with the tungsten leads, as mentioned above. This was confirmed by a blank experiment without styrene which gave the dotted base-line in Figure 3. 

At the end of the polymerisation, when species IA has disappeared and ions are present, the addition of styrene makes the ions vanish instantaneously and they remain absent whilst polymerisation proceeds. Moreover, this polymerisation has the same rate constant as the first. This means that it cannot have been initiated only by the acid that was free at that time and that the acid bound as ions must also have become available. These facts are represented by the reaction paths leading to esters IB and IC, which complete the cycle whereby eventually ions are formed again, and can be destroyed again by addition of more monomer. Of course, reaction of the freshly added monomer with the then free acid leads to formation of ester IA. The maximum concentration of carbonium ions increases after each addition because of the increasing double bond concentration, as the polymer concentration increases. Thus the final value of the equivalent conductance and... 

In the case of benz[a]anthracene the positions 7 and 12 are approximately equivalent. Mackor and collaborators therefore discussed the effect of the position of methyl groups on the spectrum of the proton addition complex benz[a]anthracene in connection with the investigation of the basicity of methyl-benz[a]anthracenes (Mackor et al., 1956). The carbonium ions A and B are present in solution ... 

Aromatics are in a sense unique in their catalytic cracking reactions. The aromatic ring contains the equivalent of six double bond or pi electrons, which are, however, mutually stabilized by strong resonance energy. We may postulate an association between a carbonium ion and these electrons in a generalized sense ... 

Tphe excellent catalytic activity of lanthanum exchanged faujasite zeo-A lites in reactions involving carbonium ions has been reported previously (1—10). Studies deal with isomerization (o-xylene (1), 1-methy 1-2-ethylbenzene (2)), alkylation (ethylene-benzene (3) propylene-benzene (4), propylene-toluene (5)), and cracking reactions (n-butane (5), n-hexane, n-heptane, ethylbenzene (6), cumene (7, 8, 10)). The catalytic activity of LaY zeolites is equivalent to that of HY zeolites (5 7). The stability of activity for LaY was studied after thermal treatment up to 750° C. However, discrepancies arise in the determination of the optimal temperatures of pretreatment. For the same kind of reaction (alkylation), the activity increases (4), remains constant (5), or decreases (3) with increasing temperatures. These results may be attributed to experimental conditions (5) and to differences in the nature of the active sites involved. Other factors, such as the introduction of cations (11) and rehydration treatments (6), may influence the catalytic activity. Water vapor effects are easily... 

It is subsequently interesting to enquire the degree of additivity of effects in electrophilic substitution and carbonium ion reactions of substituted five-membered rings, using aj, for 4-substituents and at for 5-substituents. As reasoned previously, the p values for such correlations compared with those for the equivalent reactions in benzene indicate the status of the o+ values for the heteroatoms. If the p values are the same... 

Roberts suggested that a set of charge-delocalized, rapidly equilibrating carbonium ions, which he called bicyclobutotiium ions, areThe first-formed ions from all three systems, In Scheme 3 are shown the bicvclobntnninm inns fm-mad fmm the deamination of 14C-labeled cyclopropylcarbinyl amine (Equation 6.32). According to Roberts, there would be two equivalent first-formed rarhonium... 

If the norbornyl cation is formed on solvolysis of < xo-norbornyl derivatives, C2 should become equivalent to C and C7 to C3. In a most elegant tracer experiment, Roberts and Lee synthesized w-2-norbornyl-[2,3-14C] brosylate (85), solvolyzed it in acetic acid, and degraded the product. Equation 6.39 shows the product distribution expected if the symmetrical carbonium ion (82) were formed. The label was found not only at Cx, C2, C3, and C7, but also at C5 and... 

Tri-0-acetyl-2-amino-2-deoxy-a-D-glucosyl bromide hydrobromide (LXVa)169 170 is more stable than the V-acyl analogs, possibly because heterolysis of the C—Br bond with the generation of a carbonium cation proceeds less readily as a result of the presence of the neighboring—NH3 center. /3-u-Glycoside formation proceeds normally with alcohols,169 and treatment with an equivalent of sodium ethoxide under anhydrous conditions liberates170 the free base (LXVb). 

It was correctly assumed and later determined unequivocally (64) that in each of these two reactions the BH vertex that was removed was always one of the two equivalent vertices found as the nearest neighbors of the two equivalent CH vertices. The fact that the carbon atoms in polyhedral surfaces of C2Bb 2H carboranes are electronic counterparts of boron atoms in the corresponding isoelectronic B H 2 polyhedral ions (62) requires these carbon atoms to resemble C+, a species present in carbonium ions. Consequently, the BH vertices which are nearest neighbors of two such carbon atoms will be activated for nucleophilic attack by base through the advent of a strong inductive effect. 

The concepts of electron and ligand transfer can be applied to the oxidation and reduction of organic substrates by metal complexes,61-64 since one-equivalent changes in the oxidation states of metals in inorganic redox reactions also have analogies in organic chemistry. Thus, the interconversion of the series of species carbonium ion (R+), free radical (R ), and carbanion (R-) results from one-equivalent changes, namely,... 

Aside from questions of feasibility (i.e. would the less stable carbonium ions envisioned as intermediates in the third and fourth equations undergo the rearrangement steps ), there were other potential limitations of the proposed transformations. In the third equation, there are two apparently equivalent regiochemical possibilities. In the fourth equation, transannular loss of HORj might be expected to compete with rearrangement. 

We have already seen that the one-electron deficit in the corresponding carbonium-sulfonium structures of the 1,2-dithiolium ion (1) (Section II, B, 1) is distributed over two equivalent carbonium structures. The electrophilic character of these two equivalent C-3 and C-5 positions should therefore be weaker than that of C-2 in 2, in spite of the fact that C-2 is flanked by two sulfur atoms which tend to reduce the charge. 

In the case of normal heptane it is immediately observed that there is something wrong stoichiometrically - whereas the moles of propane and butane produced are nearly equivalent, there is a substantial discrepancy between the moles of methane and hexane as well as between the moles of ethane and pentane. It is inevitable that the first step is to check the analysis - any departure from stoichiometry is initially inconceivable. But the analysis is correct and this is where the interdisciplinary forces must come to the rescue - ability to understand what is happening on the surface and what is the effect of the composition of the surface upon the series of reactions that must take place. What is happening has been very aptly named by Professor Burwell as organometallie Zoo . This is shown in figure 1. It is the intermediate formation of butyl and propyl carbonium ions on the surface which can then react with adjacent heptyl carbonium ions to produce C and carbonium ions. These can then split to... 

It should be explicitly pointed out that the mechanism proposed for the formation of CH3T requires the additional assumption that the bond between the T atom and the rest of the protonated carbonium ion is not significantly weaker than the bonds involving the protium atoms, since the preferential transfer of T" " in reaction (45) would prevent the formation of tritiated methane. Either a rapid scrambling of the five hydrogen atoms within a structure containing non-equivalent C—H bonds, or the equivalence of the five C—H bonds within the methanium ion, represent in fact a necessary condition for the formation of CH3T via the reactions (42) and (45). 

FIGURE 7. Cleavage of Hj by a carbenium ion in superacid solution. The reaction involves a pentavalent carbonium ion, formed by transferring a hydride equivalent to the carbenium ion (Olah et al., 1995). 

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