Sulphonic acid group

The azo dyes are not of any great practical value owing to their slight solubility in water. Th4 introduction of a sulphonic acid group into the molecide has no effect upon the colour, but renders the dye water-soluble—a fact of great commercial value. The simplest way of achieving this is to employ an amine, e.g., sulphanilic acid, in which the — OjH group is already present. 

Sulphonphthaleins. These indicators are usually supplied in the acid form. They are rendered water-soluble by adding sufficient sodium hydroxide to neutralise the sulphonic acid group. One gram of the indicator is triturated in a clean glass mortar with the appropriate quantity of 0.1 M sodium hydroxide solution, and then diluted with water to 1 L. The following volumes of 0.1 M sodium hydroxide are required for 1 g of the indicators bromophenol blue, 15.0 mL bromocresol green, 14.4 mL bromocresol purple, 18.6 mL chlorophenol red, 23.6 mL bromothymol blue, 16.0 mL phenol red, 28.4 mL thymol blue, 21.5 mL cresol red, 26.2 mL metacresol purple, 26.2 mL. 

In this case the sulphonic acid group is present in a sulphon-phthalein dye namely the indicator bromophenol blue. As in the previous example, the species (R3NH + )(R S03 ) can be extracted into chloroform whilst the indicator itself is not extracted, and the colour of the extract is proportional to the quantity of surfactant in the material under test. 

In reality all carbon atoms share equally the pool of electrons which constitute the double bonds and benzene resists addition across the double bonds which would otherwise destroy its unique structure and stability. Single or multiple hydrogen atoms can be substituted to form a host of derivatives containing similar functional groups to those above, e.g. saturated and unsaturated aliphatic chains, amino, carboxylic acidic, halogeno, nitro, and sulphonic acid groups as shown in Table 3.6. 

P-Bromonaphthalene. The preparation from p-naphthylamine, which has carcinogenic properties, is avoided by the use of 2-naphthylamine-1-sulphonic acid ( 2-amino-1-naphthalenesulphonic acid ) the latter is obtained commercially by cautious treatment of p-naphthol with sulphuric acid—the SOjH group first enters the 1-position—followed by the Bucherer reaction. Diazotisation and reaction with cuprous bromide yields 2-bromonaphthalene-l-sulphonic acid heating with sulphuric acid eliminates the sulphonic acid group to give 2-bromonaphthalene. 

The polyester sizes used have a much lower average molecular mass than polyester fibres. These structures (10.69) contain sulphonic acid groups and may be water-soluble or water-dispersible types. The degree of sulphonation is low [171]. If these resins are subjected to a high pH, the sulphonate groups can be hydrolysed, giving an insoluble resin that is very difficult to remove from the fibres. 

Polymers of methacrylic acid or maleic acid, either alone or as a blend or copolymer with the sulphonated aryl-formaldehyde condensation products, have also been evaluated as stain-blocking chemicals [508,509]. An interesting development is the use of a polystyrene-maleic acid copolymer, this being unusual because of the absence of sulphonic acid groups [508,510]. Although the maleic and methacrylic acid polymers do not have the durability of the conventional syntans, they have the advantage that they are non-yellowing. 

From the various emulsifiers examined, an anionic emulsifier from the sulphonic acid group of polyglycol-ether of fatty alcohol and alkylphenol-polyglycol esters was used. The best results were achieved with the use of alkylphenol-polyglycol ester (Berol EMU27). 

Figure 10.12 Micelles of sodium dodecyl sulphate (SDS) comprise as many as 80 monomer units. The micelle interior comprises the hydrocarbon chains, and is oil like. The periphery presented to the water of solution is made up of hydrated hydrophilic sulphonic acid groups...

Many anionic dyes (section 1.6) depend on their sulphonic acid groups for their solubility in water. Dye sulphonic acids have pK values within the range of pH 1-2 and are fully ionised under dyeing conditions as either the free acid or the sodium salt. The mutual electrostatic repulsion between dye sulphonate anions ensures their uniform separation and distribution in dilute aqueous solution. At higher concentrations, however, this repulsion is counterbalanced by mutually attractive forces of various kinds operating at shorter range [3] ... 

In a variation of this procedure [19], a small amount of acid is added to a suspension of a sulphonated amine in sodium nitrite solution the sulphonic acid group on the amine generates nitrous acid and diazotisation proceeds to completion. 

Under more forcing conditions, such as the use of higher operating temperatures or of oleum (sulphur trioxide dissolved in anhydrous sulphuric acid), more than one sulphonic acid group can be introduced. This approach can lead to mixtures of isomers and, in practice, the reaction conditions must be carefully controlled if the desired isomer is to be produced in maximum yield. 

Replacement of a sulphonic acid group by a hydroxy group... 

This displacement is accomplished by heating the sulphonated derivative at a high temperature with sodium or potassium hydroxide [64]. Typical is the preparation of 2-naphthol (4.15) from naphthalene-2-sulphonic acid (Scheme 4.21). Displacement of the sulphonic acid group occurs more readily when it is located at the a- rather then at the P-position, the former requiring a fusion temperature of about 200 °C and the latter one of about 250 °C. This difference in reactivity can be exploited to prepare naphtholsulphonic acids by fusion of suitable naphthalenedisulphonic acids. 

In the above reaction mechanisms it is noteworthy that the sulphonic acid group introduced has been shown to enter at the 3-position and not the 4-position as previously postulated. A consequence of this situation is that an attempted Bucherer reaction on a naphthol (or a naphthylamine) carrying a sulphonic acid group located meta to the hydroxy (or amino) group would require a second sulphonic acid group to be introduced at this position. Since it is impossible to locate two sulphonic acid groups on the same carbon atom, these compounds cannot undergo the transformation. 

After sulphonation to 2-naphthylamine-l,5,7-trisulphonic acid, the labile 1-sulphonic acid substituent, which has now served its purpose, is eliminated by diluting the sulphonation mixture and heating. Fusion of the resulting disulphonic acid (4.42) with sodium hydroxide replaces the more labile 5-sulphonic acid group by a hydroxy group, forming ] acid. 

Monoazo dyes containing a single sulphonic acid group conveniently fall into the levelling acid group. A typical structure is Cl Acid Red 266 (4.55). 

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