Alkylcarbonium ions

The two main reasons for studying the reversible reaction (3) were (a) to complete the picture of the Koch reaction in terms of quantitative information and (b) to set up a scale of reactivity towards a neutral nucleophile for carbonium ions of different structure. The first item is important from a practical point of view because there are reactions competing with the carbonylation step (3), which can be divided into intramolecular and intermolecular processes. Rearrangement of the intermediate alkylcarbonium ion, e.g. 

In discussing the elFect of structure on the stabilization of alkyl cations on the basis of the carbonylation-decarbonylation equilibrium constants, it is assumed that—to a first approximation—the stabilization of the alkyloxocarbonium ions does not depend on the structure of the alkyl group. The stabilization of the positive charge in the alkyloxocarbonium ion is mainly due to the resonance RC = 0 <-> RC = 0+, and the elFect of R on this stabilization is only of minor importance. It has been shown by Brouwer (1968a) that even in the case of (tertiary) alkylcarbonium ions, which would be much more sensitive to variation of R attached to the electron-deficient centre, the stabilization is practically independent of the structure of the alkyl groups. Another argument is found in the fact that the equilibrium concentrations of isomeric alkyloxocarbonium ions differ by at most a factor of 2-3 from each other (Section III). Therefore, the value of K provides a quantitative measure of the stabilization of an alkyl cation. In the case of R = t-adamantyl this equilibrium constant is 30 times larger than when R = t-butyl or t-pentyl, which means that the non-planar t-adamantyl ion is RT In 30= 2-1 kcal... 

Secondly, the stabilization of alkylcarbonium ions can be conveniently determined by measuring the equilibrium constants of the carbonylation-decarbonylation reactions. For some cases the rates of carbonylation are used as a kinetic criterion for stabilization. 

Acid solutions, strong, spectroscopic observation of alkylcarbonium ions in, 4, 305... 

Spectroscopic observations of alkylcarbonium ions in strong acid solutions, 4, 305 Spectroscopy, l3C NMR, in macromolecular systems of biochemical interest, 13,279 Spin alignment, in organic molecular assemblies, high-spin organic molecules and, 26,179 Spin trapping, 17, 1... 

SPECTROSCOPIC OBSERVATION OF ALKYLCARBONIUM IONS IN STRONG ACID SOLUTIONS... 

Simple alkylcarbonium ions that have been discussed in the chemical literature were considered until very recently only as transient entities. Their existence had been inferred from the study of the course of certain reactions. No reliable physical measurements other than electron impact measurements were reported until recently for any of the simple alkylcarbonium ions. 

Measurements by electron impact methods have led to experimental stabilization energies of a number of alkylcarbonium ions. Muller and Mulliken (1958) compared these with calculated values obtained by a procedure based on the LCAO-MO approximation, the large stabilization energies found for carbonium ions being attributed to the combined effects of hyperconjugation and charge redistribution (Table 1). 

The heat of formation, AHj, of a series of alkylcarbonium ions from... 

Stabilization Energies of Alkylcarbonium Ions in kcal mole i... 

The transitory existence of alkylcarbonium ions in alkyl halide-Lewis acid halide systems has been inferred from a variety of observations, sueh as vapour-pressure depressions of OHsCl and C2H5CI in the presence of gallium chloride (Brown et al., 1950), the electric conductivities of aluminium chloride in ethyl chloride (Wertyporoch and Firla, 1933) and of alkyl fluorides in boron trifluoride (Olah et al., 1957), as well as the... 

The boron trifluoride alkyl fluoride (chloride) complexes gave no evidence of alkylcarbonium ion formation. It must, however, be emphasized that (a) the physical investigation of the binary system was carried out at such low temperatures (generally below —100°) that ionization of the halides could hardly be expected (with exception of highly reactive tertiary halides) (b) the methods used could not he relied on to detect a small ionization equilibrium even if it existed. 

Olah and co-workers in 1963 flrst observed the formation of stable alkylcarbonium ion complexes when t-butyl fluoride was dissolved in excess antimony pentafluoride (serving as both the Lewis acid and the solvent). 

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. 

Using assumed molecular models and force constants based on the force constants derived from the paraffin series, normal co-ordinate calculations for the simple alkylcarbonium ions were carried out. These calculations were made in order to predict the vibrational spectra. Comparison with the experimentally obtained infra-red spectra show that the main observed features can indeed be reasonably explained in terms of the modes calculated for the planar models of the ions and allowed an assignment of the fundamentals (Table 11). 

The exceptional features can be reasonably interpreted in terms of the planar structure of the alkylcarbonium ions,... 

Other possible alkylcarbonium ion models such as isoelectronic trialkylboranes (Davies et al., 1959) and (CH30)3C (Taft et al., unpublished) also do not absorb above 220 my. 

Mono- and di-arylalkylcarbonium ions will be considered here as alkylcarbonium ions with aryl substituents. The diphenylethyl-carbonium ion 19 and the phenylmethylethylcarbonium ion 20 are representative examples. The N.M.R. spectra of 19 and 20 have been obtained in SbF5-FS03H-S02 systems (Olah et al., 1965), and are shown... 

The last years have seen a rapid development of the direct spectroscopic observation of stable carbonium ion intermediates in strongly acidic solutions. Thus many alkylcarbonium ions, previously suggested... 

Also other Type B and C series from Table II are consistent with the above elimination mechanisms. The dehydration rate of the alcohols ROH on an acid clay (series 16) increased with the calculated inductive effect of the group R. For the dehydrochlorination of polychloroethanes on basic catalysts (series 20), the rate could be correlated with a quantum-chemical reactivity index, namely the delocalizability of the hydrogen atoms by a nucleophilic attack similar indices for a radical or electrophilic attack on the chlorine atoms did not fit the data. The rates of alkylbenzene cracking on silica-alumina catalysts have been correlated with the enthalpies of formation of the corresponding alkylcarbonium ions (series 24). Similar correlations have been obtained for the dehydrosulfidation of alkanethiols and dialkyl sulfides on silica-alumina (series 36 and 37) in these cases, correlation by the Taft equation is also possible. The rate of cracking of 1,1-diarylethanes increased with the increasing basicity of the reactants (series 33). 

---