Superacid solvents

All these data could be obtained by means of two techniques, namely n.m.r. spectroscopy and the use of superacid solvent systems (such as HF—BF3, HF—SbFj, FHSO3—SbFs, SbFs—SOj). As will become evident in this article, this is equally true for the data of the carbonyl-ation and decarbonylation reactions (3). With less acidic systems the overall kinetics can, of course, be obtained but lack of knowledge concerning the concentrations of the intermediate ions prevents the determination of the rate constants of the individual steps. ... 

However, with the advent of the superacid solvents, a multitude of long-lived stable carbonium ions have been made available for extended study. Clearly, then, the door is open to quantum 5ueld determinations, kinetic treatment, studies on the effects of various solvents and sensitizers and quenching experiments (compare the very recent study by Bethell and Clare, 1972). In short, the photochemistry of carbonium ions is still in its infancy and there is a wealth of information yet to be gained. 

Superacid solvent systems have recently been reviewed by Gillespie and Peel (1971), who report Hq data for a number of superacid mixtures. 

The protonation of esters in superacid solvents (HSO3F—SbFs and HF—BF3) also occurs on the carbonyl oxygen (cation [202])... 

More recently, development of the superacid solvent systems has permitted the preparation at low temperature of stable solutions of carbocations of many structural types. The solvents ordinarily used consist of the strong Lewis acid antimony pentafluoride with or without an added protonic acid, usually hydro-... 

HSO3F has a wide liquid range (mp=— 89.0°C, bp =+162.7°C), making it advantageous as a superacid solvent for the protonation of a wide variety of weak bases. 

The direct observation of stable penta(or higher)-coordinate species with eight electrons around the carbon center in solutions was not reported until recent studies of long-lived nonclassical ions in superacid solvent systems. 

The secondary butyl and amyl cations can be observed only at very low temperatures, and they rearrange readily to the more stable tertiary ions. Generally, the most stable tertiary or secondary carbocations are observed from any of the isomeric alkyl fluorides in superacidic solvent systems. 

There are numerous ways to determine experimentally pK values of chemical compounds (205). Classical methods are potentiometric titration and ultraviolet (UV) spectroscopy, among others. These techniques have been widely applied for nucleobases and also for metal-nucleobase complexes. For the extremes such as negative pK values (pK < —2) of singly or multiply protonated nucleobases, or very high pK values (pK >15) for deprotonation of exocyclic amino groups of nucleobases (C, G, A), modifications have to be employed. These include the consideration of the Hammett acidity function in superacidic solvents or solvent mixtures (206), as well as extrapolative techniques according to Bunnett-Olsen and Marziano-Cimino-Passerini to be applied in polar, aprotic solvents (45, 207). 

A very early application of a superacid solvent was the use of 100% H2S04 for the preparation of stable solutions of carbonium ions, such as the triphenylcarbonium ion (Hantzsch, 1908c) (C6H5)8C+, e.g.,... 

This definition is principally concerned with ionic solvents. A few important organic species may be synthesised in the superacid solvents to which this definition has most application. 

Generally, the most stable tertiary or secondary carbocations are observed from any of the isomeric alkyl fluorides in superacidic solvent system. 

---