Solvation, carbonium ions

Evidence that the actual methylation of the anion can be divided into SnI, Eq. (3), and Sx2 types, Eq, (4), is provided by a whole series of investigations. " The terms S l and 8 2 must be taken to mean reactions with, respectively less or greater nucleophilic participation of the anion in the transition state. The importance of oriented ion pairs" in the solvents of low polarity frequently used in reactions involving diazomethanc, e.g., the ions formed by a diazoalkane and benzoic acid in ether, should be emphasized. The expression oriented ion pair means that, because of insufficient solvation, the ions are not individually solvated but exist as ion pairs within a solvent cage. The orientation within the ion pair is defined electrostatically, and this orientation fixes the path for the productdetermining step. Several indications (cf, foo otes 22-24) in the literature indicate the occurrence of carbonium ions and oriented ion pairs in Broensted-type equilibria of the type of Eq. (2). 

Carbonium ions are likewise known to be stabilized by solvation in strongly acidic media (Olah and Pittman, 1966), and consequently the reduction potential of carbonium ions, the ease of formation of a carbonium ion by the oxidation of a substrate and the products from these reactions would be expected to depend on the acidity of the electrolysis... 

It is also difficult to determine exactly the relative stabilities of vinyl cations and the analogous saturated carbonium ions. The relative rates of solvolysis of vinyl substrates and their analogous saturated derivatives have been estimated to be 10 to 10 (131, 134, 140, 154) in favor of the saturated substrates. These rate differences, however, do not accurately reflect the inherent differences in stability between vinyl cations and the analogous carbonium ions, for they include effects that result from the differences in ground states between reactants, as well as possible differences between the intermediate ions resulting from differences in solvation, counter-ion effects, etc. The same difficulties apply in the attempt to estimate relative ion stabilities from relative rates of electrophilic additions to acetylenes and olefins, (218), or from relative rates of homopropargylic and homoallylic solvolysis. 

Winstein Robinson (1958) used this concept to account for the kinetics of the salt effects on solvolysis reactions. They considered that carbonium ions (cations) and carbanions could exist as contact ion-pairs, solvated ion-pairs and as free ions and that all these forms participated in the reactions and were in equilibrium with each other. These equilibria can be represented, thus ... 

As has been suggested in the previous section, explanations of solvent effects on the basis of the macroscopic physical properties of the solvent are not very successful. The alternative approach is to make use of the microscopic or chemical properties of the solvent and to consider the detailed interaction of solvent molecules with their own kind and with solute molecules. If a configuration in which one or more solvent molecules interacts with a solute molecule has a particularly low free energy, it is feasible to describe at least that part of the solute-solvent interaction as the formation of a molecular complex and to speak of an equilibrium between solvated and non-solvated molecules. Such a stabilization of a particular solute by solvation will shift any equilibrium involving that solute. For example, in the case of formation of carbonium ions from triphenylcarbinol, the equilibrium is shifted in favor of the carbonium ion by an acidic solvent that reacts with hydroxide ion and with water. The carbonium ion concentration in sulfuric acid is greater than it is in methanol-... 

Just as interaction of the negative ion with solvent or with a catalyst can promote the ionization, the same is true of the interactions of the carbonium ion. The triaryl carbonium ions discussed so far should all be colored. Reports of colorless but conducting solutions of triarylmethyl derivatives are fairly common and may represent covalently solvated carbonium ions which are not expected to be colorless. For example, a colored solution of the ion XVI in acetic acid slowly fades when it is diluted with methanol containing enough acid to prevent the formation of any "color base. The fading is attributed to the formation of XVII.m... 

The frequently noted fading of carbonium ion colors when ether is added to the solution may also be due to the formation of similar colorless covalently solvated ions.193... 

In this respect such reactions are analogous to the S 1 or limiting reactions of compounds producing carbonium ions, although the intermediate is a solvated carbanion rather than a solvated carbonium ion. In the base-catalyzed halogenation of ketones, for example, the rate is independent of the halogen concentration and is the same for the reaction with bromine as for the reaction with chlorine.384... 

These differences were explained by solvation effects in the liquid phase. The carbenium ions are more efficiently stabilized by solvation than carbonium ions, because the former have unsaturated trivalent carbon atoms. In this way, the energy barrier between the (solvated) carbenium ion and the carbonium ion transition state increases. 

Solvation of carbonium ions has also been taken into account. The dynamics of CH5+ solvated by H2 and other molecules have been studied experimentally, and in the case of solvation by H2 through classical AIMD studies (92-94). The solvation effect on the 3c2e bonds leads to freezing of the scrambling of protons within CH5 that could be considered due to hydrogen bonding between o bonds (Figure 25). This solvation leads to a more resolved IR spectrum (93). 

Microsolvation approaches have also been considered toward understanding the role of the primary solvation shell of a carbonium ion (18,90,91,97,98). These ions tend to become transition states whenever strong solvation is taking place (Figure 27). 

An interesting feature is that, although solvated carbonium ions usually afford transition states when the anions are considered in the calculations, their chemistry could be thought of as being analogous to that of carbonium ions. In many cases, these structures could potentially be transition states with interesting properties, such as scrambling and other processes that are characteristic of weakly solvated carbonium ions. 

Attempts to obtain alkylcarbonium complexes by dissolving alkyl chlorides (bromides) in liquid Lewis acid halides (stannic chloride, titanium (IV) chloride, antimony pentachloride, etc.) as solvent were unsuccessful. Although stable solutions could be obtained at low temperature with, for example, t-butyl chloride, the observed N.M.R. chemical shifts were generally not larger than 0 5 p.p.m. and thus could be attributed only to weak donor-acceptor complexes, but not to the carbonium ions. The negative result of these investigations seems to indicate that either the Lewis acids used were too weak to cause sufficient ionization of the C—Cl bond, or that the solvating effect of the halides... 

The role of excess antimony pentafluoride can best be explained by assuming that it is capable of solvating the alkylcarbonium ion salt through interaction of the unshared electron pairs of fluorine with the vacant p-orbital of the sp -hybridized planar central carbon atom of the carbonium ion. In the system of low dielectric constant the alkylcarbonium ion is present not in the free form, but as a tightly bound ion-pair solvation affects this species rather than the free ion. 

The action of nitrous acid on a primary, aliphatic amine is still a complex process, inasmuch as it is necessary to postulate certain steps that are imperfectly understood, but it can nevertheless be considered that the process leads ultimately to a carbonium ion.26,27 The carbonium ion thus formed is termed hot, that is to say, it is non-solvated and chemically activated the nature of the products resulting from its decay is essentially determined by the conformation of the initial state the electronic factors in effect at the time of the transition state exert little influence.27... 

The charged end of a polymer and its counter-ion may recombine and form a stable covalent bond thus terminating the propagation of polymerization. Such a termination is frequently observed in carbonium ion polymerizations. For example, polymerization of a vinyl monomer if initiated by hydrochloric acid produces a carbonium ion and a chlorine-counter-ion. These two ions recombine readily forming a stable covalent C-Cl bond which does not propagate further the polymerization and forms, therefore, the dead end of a polymeric molecule. Actually, the recombination of carbonium ion with Cl- ion is such a rapid reaction that usually it follows immediately the formation of the relevant carbonium ion. This prevents the formation of a polymeric molecule and gives instead an addition product of HC1 to the reactive C=C double bond. A polymeric product can be obtained if the ions recombination is slowed down by sufficiently powerful solvation. For example, a solution of styrene in nitromethane, but not in a hydrocarbon, can be polymerized by HC1 (2), since the recombination of the solvated ions is sufficiently slow to permit the formation of a polymer. 

Polymerization of styrene initiated by trifluoro-acetic acid (3) is another example of the same phenomenon. Addition of styrene to tri-fluoroacetic acid yields polystyrene of M. W. 20,000 to 30,000 while the addition of trifluoro-acetic acid to styrene does not produce any polymer Trifluoro-acetate counter ions, formed in the first process, are strongly stabilized by the solvent (trifluoro-acetic acid) through powerful hydrogen bonds, and hence their capacity to recombine with the growing carbonium ion is greatly reduced. In the second process, the counter ions are formed in a hydrocarbon millieu (styrene) of low solvation power. Their recombination with carbonium ions remains therefore unhindered and consequently the formation of a polymer is prevented. 

It is interesting to compare this transition state in the solid with the one calculated from the HF-SbF5 system. In the liquid superacid, the ionic character is very strong and it is easier to connect the reactivity with the unusual activity of the proton even when solvated by the HF solvent. In contrast, on the solid the theoretical calculated transition state is further away from the carbonium ion type and in line with the much higher temperatures needed to activate the alkane with weaker acids. 

The rapid increase in rate, as acetic anhydride replaced acetic acid, could be explained by making the assumption that solvated, acetyl carbonium ions (Ac2OAc ) are present in solutions of perchloric acid in acetic anhydride. Thus, the rate of anomerization (R) would follow an expression of the type,... 

The rate retarding role of water in radiation induced ionic polymerizations was visualized as proton extraction by water from the growing carbonium ion and hydronium ion formation (101). The formation of solvated hydronium ions is greatly favored energetically. 

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