Strong acid systems

Over a decade of research, we were able to show that practically all conceivable carbocations could be prepared under what became known as stable ion conditions using various very strong acid systems (see discussion of superacids) and low nucleophilicity solvents (SO2, SO2CIF, SO2F2, etc.). A variety of precursors could be used under appropriate conditions, as shown, for example, in the preparation of the methylcyclopentyl cation. 

Until the late 1950s chemists generally considered mineral acids, such as sulfuric, nitric, perchloric, and hydrofluoric acids, to be the strongest acid systems in existence. This has changed considerably as extremely strong acid systems—many billions or even trillions of times stronger than sulfuric acid—have been discovered. 

A mixture of methanesulfonic acid and P Oj used either neat or diluted with sulfolane or CH2CI2 is a strongly acidic system. It has been used to control the rcgiosclcctivity in cydization of unsymmetrical ketones. Use of the neal reagent favours reaction into the less substituted branch whereas diluted solutions favour the more substituted branch[3]. 

An unusual solvent system was chosen for the intramolecular reductive alkylation of the masked amino ketone (15). The purpose of the strongly acid system was to prevent cyclization of the deblocked amino ketone to 16, further hydrogenation of which gives the unwanted isomer 17 by attack at the convex face. The desired opposite isomer can be obtained by reduction of 16 with UAIH4 (52). 

Goheen and Bennett showed that regular nitric acid could be used, in about two molar excess, for the oxidation of dimethyl sulphoxide to dimethyl sulphone in 86% yield. The reaction temperature was 120-150°C with a reaction time of about 4 hours. The mechanism for this reaction was postulated to involve initially a protonated sulphoxide species (which has been shown to be present in other strongly acidic systems ) followed by nucleophilic attack by nitrate, and the loss of nitrogen dioxide as shown in equations (4) and (5). 

The formation of a proton addition complex can take place either in a binary system (equation (6)) or in a ternary system (equation (6)). Correspondingly, one obtains the basicity constants and K respectively, according to (7) and (8), and these are related to one another through the equilibrium (9) and the relationship (10). It is in the nature of these strongly acid systems that conventional methods for analysing the equilibrium composition can only be used with difficulty. 

Before we turn to other investigations of the stabilized ion it is interesting briefly to compare the rates of hydride shifts found in the strong acid systems with rates obtained under solvolytic conditions. Collins and Lietzke (1967) and Berson et al. (1967) have deduced the rates of 3,2- and 6,2-hydrogen migration relative to the rate of solvent capture for norbornyl ions in acetic acid, formic acid and other common solvents from elaborate C scrambling studies due to Roberts and Lee (1951) and Roberts et al., (1954) and tritium labelling studies of Lee and Lam (1966). From the ratio of these... 

Quinoline is much more reactive towards electrophilic substitution than pyridine, but this is because substitution occurs on the benzene ring, not on the pyridine. We have already seen that pyridine carbons are unreactive towards electrophilic reagents, with strongly acidic systems protonating the nitrogen... 

The reacting species is a rather weak electrophile and, therefore, only particularly reactive aromatics are suitable substrates. Consequently, the reaction is restricted for polyhydric phenols and their ethers, and reactive heterocycles. With strong acidic systems, such as AICI3 in halogenated benzene solvents, alkylben-zenes can react at more elevated temperatures (>50°C). Trichloroacetonitrile works also well with nonactivated aromatics. The 12 chloroimine, the protonated nitrile (13), or the nitrile coordinated with the Lewis acid are possible involved electrophiles. 

Oxidation of several nonmetals in strongly acidic systems produces polyatomic cationic species of the general type Y" +.w Among those characterized are S4+, Se +,... 

Safety Limits for Hydrogen Peroxide/Organics/Strong Acid Systems , Solvay Interox Technical Information. 

The sequestrant approach to stabilization is usually better than the scavenger approach, as it is based on prevention rather than cure. It can, however, only act on metal-based catalytic decomposition, and is poor in strongly acidic systems. 

Strongly acidic systems (TaFs/HF or SbFs/HF) have also been used for hydrogenation of aromatic compounds at 50 °C under 50-1 ()0 atm of H2. ° ... 

Because of the high stability of the tertiary ions, these are preferentially formed in the strong acid systems from both tertiary, secondary, and even primary precursors115. ... 

The dependence of the shape of the titration curve and of the equivalence point pH on concentration is shown in /Figure 8.6 for different concentrations of HOAc and NaOH. Obviously, phenolphthalein could not be used as an indicator for solutions as dilute as 10" M (curve 3). Note that the equivalence point pH decreases as the weak-acid system becomes more dilute (which doesn t happen in the strong-acid system). 

In these strong acid systems one can often ob rve diprotonation of aromatic compounds. Thus, bimesityl and decamethylbiphenyl in HSO3F—SbFj attach the protons at 3- and 3 -positions while 1,2,3,5,6,7-hexamethyl- and octamethyl-naphthalenes, at the 1- and 5-positions from octamethylnaphthalene two isomeric ions are formed corresponding to cis- and trans-diprotonation ... 

As a first approximation this equation serves to estimate the reaction constant for the protonation not only of methylbenzenes, but also other substituted benzenes. The q value for strong acid systems is relatively little dependent on the medium. 

Preparation 3-1 is one of the early examples of the polymerization in a strongly acid system. This polymer evidently seemed to contain only 40% of the expected carboxylic acid groups, and its elementary analysis deviated considerably from calculated values [96]. 

Louie, D.K., 2012. Strong acid system— troubleshooting—stickiest. Retrieved from http //www. sulphuric-acid.com/techmanual/strong%20acid/sa ts stick test.htm (accessed 05.10.12). 

---