Arenesulphonic acids

The yields of arenesulphonic acids (-80%) via the reaction of activated haloarenes with potassium sulphite under phase-transfer catalytic conditions [62, 63] are no better than conventional non-catalytic methods, although reaction conditions are less severe. There is evidence that indicates the initial attack by the sulphite anion is at C-5. Surprisingly, tri-n-butylamine is a better catalyst, producing higher yields (>90%). 

A different type of a-sulphonyl radical is obtained when arenesulphonic acids or their sulphonyl fluorides are reacted with Pb02 in HS03F at ca 200 K. Under these conditions, electron transfer takes place to generate the radical cations of the parent compound (equation 17)40. 

Table 9 contains the data for radical cations of arenesulphonic acids and arenesulphonyl fluorides40,41. 

Alkene- and arenesulphonic acids have been studied in strong or super acid media by NMR spectroscopy. Olah and coworkers64 in 1970 examined sulphonic acids in fluorosulphuric acid-antimony pentafluoride-sulphuryl chloride fluoride solution at low temperature ( — 60 °C). In S02 solution methanesulphonic acid has a chemical shift of <5 3.1 ppm for the methyl group. In the superacid medium there are two singlets at S 4.15 and 4.07, with ratio 60 40, for the methyl protons. It was suggested that this indicated the presence of two isomers, possibly 27 and 28, due to restricted rotation about the SO bond. 

TABLE 1. Isomer distribution for the rearrangement of some arenesulphonic acids... 

SCHEME 1. Mechanism for the isomerization of substituted arenesulphonic acids... 

The electrochemical behaviour of other substituted arenesulphonic acids can be related to the cathodic or the anodic properties of substituents prior to the cleavage of C—S bonds as mentioned above. For example, an aromatic nitro group will be reduced55 before the S03H group ... 

Patents are still appearing on further substitution reactions of naphthalene disulphonic acids by both kinetically and thermodynamically controlled processes, which have been of great commercial importance for many years. Isomer distributions for the sulphonation of other arenesulphonic acids and some alkyl derivatives have been compiled142. 

The arenesulphonic acid salt of emetine has been prepared for qualitative analysis. 

A useful practical study has shown that arenesulphonic acids can be extracted from aqueous solutions into immiscible organic solvents as ion-pairs with amines. 

Properties of Sulphonic Acids.—In the course of a study of the kinetics of nitration of arenesulphonic acids in H2SO4 it was shown that the sulphonate anion is a deactivating substituent even though it carries a negative charge. 

The discovery that an arenesulphonic acid is in equilibrium with its fluoride in FSO3H solution (ArSOgH + FSO3H ArS02p + H2SO4) may open up a new field of study because of its implications in mechanistic and synthetic aspects. 

Hydrogenation of arenesulphonic acids to ciy-cyclohexanesulphonic acids is feasible using Rh-Al20a or Ru-AlgOa catalysts. ... 

Preparations from sulphinic acids, SO2, other simple sulphur compounds, and sulphonyl halides are reported in the papers that are listed (with brief details) at the end of this section. The AlClj-catalysed rearrangement of an aryl arenesulphonate gives the corresponding diaryl sulphone. ... 

Methyl- and 2-phenyl-[l,2,4]triazolo[3,2-/)]benzothiazoles are formed when 2,3-diaminobenzothiazolium arenesulphonates are treated with acetyl or benzoyl chloride. " The intermediate 2-A -acylated compounds are isolable and may be cyclized with polyphosphoric acid. Analysis of the n.m.r. spectrum of [6- H][l,2,4]triazolo[3,4-6]benzothiazole allows all the H chemical shifts in the parent compound to be assigned. ... 

Sulphonates.—Novel syntheses are reported for methyl arenesulphonates (activated benzene + Me03SF at 100 C) and more generally," employing R C0P(0)(0R 2 for esterification of sulphonic acids. A photo-Fries rearrangement (76) (77) is notable." ... 

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