Direct sulphonation

Aromatic hydrocarbons may be monosulphonated by heating with a slight excess of concentrated sulphuric acid for benzene, oleum (7-8% S03) gives somewhat better results. The reaction is usually complete when all the hydrocarbon has dissolved. 

The mechanism of aromatic sulphonation is broadly analogous to that previously described for aromatic nitration and halogenation and may be represented in the following way, the neutral sulphur trioxide molecule functioning as the electrophilic species. Sulphonation differs from nitration and halogenation, however, in that the overall reaction is reversible. 

Because of their high solubility in water the sulphonic acids are not usually isolated from aqueous solution in the free state, but are converted into and isolated as their sodium salts. The simplest procedure is to partially neutralise the reaction mixture (say, with sodium hydrogen carbonate) and then to pour it into water and add an excess of sodium chloride, when the following equilibrium is established. 

The high sodium ion concentration results in crystallisation of the sodium salt. This process of salting out with common salt may be used for recrystallisation, but, for example, sodium benzenesulphonate (and salts of other acids of comparable molecular weight) is so very soluble in water that the solution 

The sulphonation of toluene (Expt 6.37) with concentrated sulphuric acid at 100-120°C results in the formation of toluene-p-sulphonic acid as the chief product, accompanied by small amounts of the ortho and meta isomers these are easily removed by crystallisation of the sodium salt of the para isomer in the presence of sodium chloride. Sulphonation of naphthalene at about 160°C yields largely the 2-sulphonic acid (the product of thermodynamic control) (Expt 6.38) at lower temperatures (0-60 °C) the 1-sulphonic acid (the product of kinetic control) is produced almost exclusively. In both cases the product is isolated as its sodium salt. In anthraquinone the carbonyl groups deactivate the aromatic nucleus towards electrophilic attack and vigorous conditions of sulphonation are required, i.e. oleum at about 160 °C. The product is largely sodium anthraquinone-2-sulphonate (Expt 6.39). 

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A further difference between aliphatic and aromatic hydrocarbons is that only the latter are capable of direct sulphonation. Thus benzene when heated with concentrated sulphuric acid gives benzenesulphonic acid, a reaction which proceeds more readily, however, if chlorosulphonic acid is used instead of sulphuric acid an excess of chlorosulphonic acid however may convert the sul phonic acid into the sulphonyl chloride (c/. p. 181). 

This direct sulphonation should be compared with the indirect methods for the preparation of aliphatic sulphonic acids, e.g., oxidation of a thiol (RSH -> RSOjH), and interaction of an alkyl halide with sodium sulphite to give the sodium sulphonate (RBr + Na,SO, -> RSO,Na + NaBr). 

Electrophilic substitution at the anthraquinone ring system is difficult due to deactivation (electron withdrawal) by the carbonyl groups. Although the 1-position in anthraquinone is rather more susceptible to electrophilic attack than is the 2-position, as indicated by jt-electron localisation energies [4], direct sulphonation with oleum produces the 2-sulphonic acid (6.3). The severity of the reaction conditions ensures that the thermodynamically favoured 2-isomer, which is not subject to steric hindrance from an adjacent carbonyl group, is formed. However, the more synthetically useful 1-isomer (6.7) can be obtained by sulphonation of anthraquinone in the presence of a mercury(II) salt (Scheme 6.4). It appears that mercuration first takes place at the 1-position followed by displacement. Some disulphonation occurs, leading to the formation of the 2,6- and 2,7- or the 1,5- and 1,8-disulphonic acids, respectively. Separation of the various compounds can be achieved without too much difficulty. Sulphonation of anthraquinone derivatives is also of some importance. 

The presence of at least two sulphonic acid groups in the triarylmethane ring system permits the derivatives to be applied as acid dyes. Although sulphonic acid groups are often present in the intermediates used, acid dyes can also be obtained by direct sulphonation of the basic dye itself or at the leuco stage. 

The chief advantage in the use of this acid is its selective property, whereby certain sulphonic acids are formed, which could not be formed by direct sulphonation with sulphuric acid or oleum, or which might be formed only in presence of other isomers, the separation of which might be difficult. For example, naphthalene sulphonated with oleum at the ordinary temperature gives a mixture of 1 5- and 1 6-disulphonic acids, while chlorosulphonic acid yields only the 1 5-acid. Similarly, with toluene, chiefly the ortho acid is formed. With excess of chlorosulphonic acid a sulphonyl chloride is formed, except in the case of phenols or naphthols, which give the free sulphonic acid. 

Sulphinic acids are unstable liquids passing readily into sulphonic acids on oxidation with alkaline permanganate. Where a mixture of isomeric sulphonic acids is formed by direct sulphonation the individual sulphonic acids may be prepared by this method from the corresponding sulphonic acids. For example, o-toluenesulphonic acid may be prepared from o-toluidine. 

The preparation of arylsulphonic acids by direct sulphonation (Expts 6.37 to 6.40). 

The former are true aromatic sulphonic acids prepared by direct sulphonation, and reacting like benzene sulphonic acid. The latter are aliphatic sulphonic acids both in methods of preparation and reaction. 

At ordinary temperatures the sulphonation of phenol yields mostly the ortho compound with some of the para. At raised temperatures the para compound only is obtained, the first formed ortho compound being converted into the para. The meta compound is not formed by direct sulphonation of phenol. As previously stated (p. 522), the alkali fusion of di-sulphonic acids yields the di-phenols. By careful fusion... 

From Sulpho Benzoic Acid.— The most common method of preparing phenols is by the alkali fusion of the sulphonic acids, Sulpho benzoic acids will thus yield hydroxy benzoic acids. In case the sulpho benzoic acid has been made by direct sulphonation of benzoic acid the meta compound will result. If, however, we start with tolmne and sulphonate it we will obtain the ortho and para compounds. 

By the action of nitric acid on a-naphthol mono- and di-sulphonic acids (obtained by direct sulphonation), the sulpho-groups are replaced by nitro-groups, while on nitration of a-naphthol trisulphonic acid, one sulpho-group remains intact [8], the other two being replaced by nitro-groups. The compound formed is a monosulphonic acid of dinitronaphthol. This acid forms long yellow needles, which are easily soluble in water [9]. 

They are produced technically by two methods, either by direct sulphonation of the naphthylamines or by heating the naphthol-sulphonic acids with ammonia (he., a compound yielding ammonia on heating). 

These bodies are in considerable demand, especially for wooldyeing, and numerous products are met with in commerce. They are generally prepared by sulphonation of the leuco-bases, and subsequent oxidation as the bases themselves do not give good results on direct sulphonation. The sulpho-group enters especially easily into benzylated bases j and probably enters the benzene ring of the benzyl group. 

By far the most common method for the industrial preparation of sulphonic acids and their salts is by direct sulphonation. Consequently, there are many hundreds of patents covering the industrial applications of these reactions, especially with reference to aromatic sulphonic acids, and the reader is referred to the reviews of Knaggs, Nunfaum and Schultz2 and of Gilbert5 for key references to this patent literature. 

Miscellaneous Methods of Preparing Phosphines.- The preparation, and uses in catalysis, of the various types of water-soluble phosphines, have been reviewed. The direct sulphonation of diphos gives a complex mixture of sulphonated products, from which the water-... 

A new solid acid catalyst is developed by the direct sulphonation of the ethene bond of a pure trans ethene bridged Periodic Mesoporous Organosilica. The catalytic activity of this mesoporous material is evaluated in an esterification reaction and compared with p-toluenesulphonic acid. The sulphonated ethene PMO can compete with a homogeneous catalyst and maintains its porosity. 

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