Sulphonic acids Character

The number and position of the sulfonic acid substituents in zinc(II) phthalocyanine have an appreciable effect on bioactivity. Partial sulfonation gives the greatest activity, and the S2a derivatives (two sulfonic acid substituents on adjacent benzene rings) appear to be the most active.254,255 While this can be readily understood in general terms of developing amphiphilic character (Section 9.22.3), the detailed mechanism responsible for this result is not yet clear. The aggregation of zinc(II) sulphonic acid derivatives in solution and in tissue has been studied in relation to PDT.256... 

Sulphonic acids and their salts are analysed by GC after esterification with diazomethane or after chlorination with thionyl chloride or phosgene [119]. Reaction with thionyl chloride proceeds according to Scheme 5.14. A 0.5-g sample of sulphonic acid or its salt is placed into a round-bottomed flask fitted with a magnetic stirrer and a reflux condenser, 0.5 ml of dimethylformamide and 20 ml of thionyl chloride are added and the mixture is refluxed for several minutes up to 2 h (according to the character of the sample) until the evolution of gas from the reaction mixture ceases (detection with the aid of a bubbler filled with chlorobenzene). If a salt is chlorinated, solid chloride produced in the reaction mixture must be removed by dilution with dichloromethane and by careful filtration through a fine glass filter. Excess of thionyl chloride and solvent is evaporated carefully under decreased pressure. The residue is dissolved in a suitable solvent (CCU) and analysed by GC (silicone stationary phase, temperature 160°C). 

The introduction of phases that have mixed mode character has allowed the use of low-pH buffers in the mobile phase without any loss in EOF. Theses phases are a combination of reversed phase and sulphonic acid chemistries. Fig. 4.6 shows an EOF vs. pH plot on a capillary packed with a Cfi/SCX mixed mode phase (Hypersil. Runcorn). This provides a good EOF over the whole pH range and was also found to have a more predictable retention mechanism based on revensed-phase retention characteristics. 

The sulphonic acids, it will be recalled, are strong acids, their acid character being due to the remaining acid hydroxyl left in the compound. Sulphuric acid is di-basic and only one of the two hydroxyls is eliminated by the substitution in the ring. Nitric acid, however, is mono-basic and possesses only one acid hydroxyl. 

Naphthyl blue is probably a salt of a sulphonic acid of amido-isonaphthylrosinduline, and naphthyl violet is a dyestuff of similar character. They are principally intended for silk, on which they show a red fluorescence. 

In the above example,it happens, as in many cases, that the sulphonic acid group enters in the para-position to the amido-(NH2) group. The amido sulphonic acids are colourless crystallisable compounds melting with decomposition they possess add properties, i.e. in dissolving in alkalies. The basic character of the amine is so greatly weakened by the introduction of the negative sulphonic add group that the amido sulphonic adds cannot form salts with adds. They differ... 

Fischer-Speier esterification to give a salt of an amino acid ester (by refluxing an alkanol with anhydrous HC1 or hot benzyl alcohol with toluene-/>-sulphonic acid) is straightforward. The fact that the nearby amino group is protonated when the carboxy group reacts to give a-amino acid esters does not slow the reaction down unduly a nearby positive site might have been expected to reduce the electrophilic character of the carboxy carbon atom. Without an acid catalyst, Walkylation can accompany esterification (Equation 4.6). 

To maximise the electrostatic repulsion between pigment particles, it is necessary to incorporate a high concentration of ionic groupings. These are usually anionic in character which are introduced using either caiboxylic or sulphonic acid groupings. 

PPP-type calculations, based on simplified models, of the sulphonic acids (23) and (24) have been carried out. In these models conjugation between the sulphonic group and the tt electrons of the benzene ring is considered to be due to the presence of a vacant orbital centred on the sulphur atom in the sulphonic group. CNDO/2 calculations for these compounds suggest that the SO3H group can rotate nearly freely. The C—S bond has a very weak ir character with only a small contribution from the d-orbitals. 

Figure 7.7 Zeta potentials (calculated from electrophoretic mobility data) relating to particles of different ionogenic character plotted as a function of pH in acetate-veronal buffer at constant ionic strength of 0.05 mol dm 3, (a) Hydrocarbon oil droplets, (b) Sulphonated polystyrene latex particles, (c) Arabic acid (carboxylated polymer) adsorbed on to oil droplets, (d) Serum albumin adsorbed on to oil droplets...

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