Sulphonic acids, reactions preparation

The free sulphonic acids [e.g. toluene-p-sulphonic acid, for preparation see, Expt 6.37, Note (1)], as opposed to their sodium salts, may sometimes be obtained directly if the sulphonation reaction is carried out with continuous removal of the water formed in the reaction, conveniently by using a Dean and Stark water separator. p-Xylene-2-sulphonic acid (Expt 6.40) is an example of a sulphonic acid whose solubility in water is such that it crystallises directly from the reaction medium and hence it may readily be isolated. 

The sulphonic acids are usually prepared by the action of sulphuric acid upon a compound. The concentration of the acid and the temperature of reaction are varied according to the reactivity of the compound. Often oleum is used or even chiorosulphonic acid. Alternatively sulphur trioxide complexed to pyridine or dioxan can be used with reactive substrates. Aminosulphonic acids such as sulphanilic and naphthionic acids are most conveniently prepared by heating the sulphate of the amine at ISO C. 

Aromatic nitriles (or aryl cyanides) can be obtained by methods (1) and (3). but not by method (2). In addition, aromatic nitriles can be prepared by two other methods, (a) from the corresponding diazo compound by Sandmeyer s Reaction (p. 189), (b) by fusing the corresponding sulphonic acid (or its salts)... 

The oxidation of sulphones leads to either a sulphonic acid (or a sulphonic acid derivative) or to sulphate. Such reactions have rarely been used for the preparation of sulphonic acids since these are usually readily available by other well-established routes. However, polyhalogenated sulphones can be oxidized relatively easily to sulphonic acids and these reactions will be discussed here. 

The reversibility of the Bucherer reaction is utilised in the preparation of 2-p-tolylamino-5-hydroxynaphthalene-7-sulphonic acid. 5-hydroxynaphthalene-7-sulphonic acid or J acid p-toluidine and sodium bisulphite solution ... 

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. 

In some cases where it is difficult to carry out the Bucherer reaction successfully, it is easier to prepare N-arylnaphthylamines by heating together a naphthylamine and an arylamine. In particular, this reaction is useful in the preparation (Scheme 4.24) of 1-phenylaminonaphthalene-8-sulphonic acid (4.34 N-Phenyl Peri acid) and its N-4-methylphenyl analogue (Tolyl Peri acid), both of which intermediates are valuable components for the production of navy blue dyes. 

In all the above sequences, single isomers are produced by careful control of the reaction conditions combined with purification by selective isolation at various points in the synthesis. Occasionally two isomers are produced which give dyestuffs that have very similar properties in these cases it is often quite acceptable and economically beneficial not to separate the individual components but to use the total mixture in dye preparation. An example is the mixture of l-naphthylamine-6- and 7-sulphonic acids (4.46 mixed Cleve s acids), which arises by nitration and reduction of naphthalene-2 Sulphonic acid (Scheme 4.30). 

The reaction just described is carried out technically on a large scale in iron vessels provided with stirrers, for /9-naphthol, as well as the numerous sulphonic acids which can be obtained from it by the action of sulphuric acid, is widely used for the preparation of azo-dyes. Moreover, by the action of ammonia under pressure, /3-naphthylamine is produced from /S-naphthol ... 

As an alternative to the oxidation of sulphides and sulphoxides (see Chapter 10), sulphones can be prepared by the nucleophilic substitution reaction of the sulphinite anion on haloalkanes. In the absence of a phase-transfer catalyst, the reaction times are generally long and the yields are low, and undesirable O-alkylation of the sulphinite anion competes with S-alkylation. The stoichiometric reaction of the preformed tetra-n-butylammonium salt of 4-toluenesulphinic acid with haloalkanes produces 4-tolyl sulphones in high yield [1], but it has been demonstrated that equally good... 

Alcohols may be prepared (1) by hydration of alkenes (1) in presence of an acid and (11) by hydroboratlon-oxidatlon reaction (2) from carbonyl compounds by (1) catalytic reduction and (11) the action of Grignard reagents. Phenols may be prepared by (1) substitution of (1) halogen atom In haloarenes and (11) sulphonic acid group In aiyl sulphonic acids, by -OH group (2) by hydrolysis of diazonium salts and (3) industrially from cumene. 

Aryl glycosides of 2-amino-2-deoxy sugars, as might be expected, can be prepared by similar techniques as was illustrated by the synthesis of phenyl 3,4,6-tri-0-acetyl-2-acetamido-2-deoxy-p-D-glucoside and -D-galactoside by toluene -sulphonic acid catalysed reactions between phenol and the hexosamine penta-acetates These products were then anomerised using zinc chloride as catalyst to provide means of obtaining -anomers. 

Diazotise 223 g. of 2-naphtliylamine-l-sulphonic acid as detailed under fi-Bromonaphthalene in Section IV,62. Prepare cuprous cyanide from 125 g. of cupric sulphate pentahydrate (Section IV,66) and dissolve it in a solution of 65 g. of potassium cyanide in 500 ml. of water contained in a 1-litre three-necked flask. Cool the potassium cuprocyanide solution in ice, stir mechanically, and add the damp cake of the diazonium compound in small portions whilst maintaining the temperature at 5-8°. Nitrogen is soon evolved and a red precipitate forms gradually. Continue the stirring for about 10 hours in the cold, heat slowly to the boiling point, add 250 g. of potassium chloride, stir, and allow to stand. Collect the orange crystals which separate by suction filtration recrystallise first from water and then from alcohol dry at 100°. The product is almost pure potassium 2-cyanonaphthalene-l-sulphonate. Transfer the product to a 2-litre round-bottomed flask, add a solution prepared from 400 ml. of concentrated sulphuric acid and 400 g. of crushed ice, and heat the mixture under reflux for 12 hours. Collect the -naphthoic acid formed (some of which sublimes from the reaction mixture) by suction filtration... 

The following methods are available for the preparation of chloro-sulphonic acid (1) The substance can be obtained by the gradual action of moisture (e.g. atmospheric moisture) on sulphuryl chloride a similar reaction occurs with sulphuric acid and is more easily regulated ... 

A somewhat similar method of cyanide preparation is applicable in the aromatic series aromatic sulphonic acid potassium salts, on fusion with potassium cyanide or potassium ferrocyanide, yield aromatic nitriles. The reaction can be extended to derivatives of pyridine. 

This reaction is of technical importance, being applied to the preparation of some hydroxy-sulphonic acids. 

Reaction LXXV. Fusion of Aromatic Sulphonic Acids with Caustic Alkalis. (Z. Ch (1876), 3, 299 J. pr., [2], 17, 394 20, 300.)—This method is of technical importance as it is employed to prepare phenols and naphthols from the parent hydrocarbons. These phenols and naphthols are much used as intermediates in the dye industry. The method cannot easily be applied to determine structure, owing to rearrangement liable to occur at the elevated temperatures. Caustic potash is more convenient than soda, since it generally yields a more easily fusible mixture. 

General Procedure.—Dissolve 0-2—0-5 gm. of the compound to be estimated in 200 c.cs. of water, using a slight excess of hydrochloric acid for amines or a similar excess of caustic soda in the case of phenols, carboxylic- and sulphonic-acids. To the solution thus prepared 10 c.cs. of a 20% solution of potassium bromide and 5—10 c.cs. of concentrated hydrochloric acid are added the mixture is brought to the required temperature and N/5 bromate solution run in slowly until a sample withdrawn gives a reaction on starch-iodide paper. The presence of free bromine should be again tested for after 5 minutes. 

A related series of 5-substituted-2-amino-oxadiazole compounds have also been prepared in a one-pot procedure using a microwave-assisted cyclisation procedure (Scheme 6.26)164. Rapid preparation of the pre-requisite ureas from the mono acyl hydrazines and various isocyanates (or the isothiocyanate) was easily achieved by simple mixing. The resulting products were then cyclo dehydrated by one of the two procedures either by the addition of polymer-supported DMAP and tosyl chloride or alternatively with an immobilised carbodiimide and catalytic sulphonic acid. Purity in most cases was excellent after only filtration through a small plug of silica but an SCX-2 cartridge (sulphonic acid functionalised - catch and release) could be used in the cases where reactions required additional purification. 

A recent procedure for the preparation of methyl esters involves refluxing the carboxylic acid with methanol and 2,2-dimethoxypropane in the presence of toluene-p-sulphonic acid as the catalyst (Expt 5.146). The water produced in the esterification process is effectively removed by acid-catalysed reaction with the ketal to give acetone and methanol. 

Naphthylamine is no longer manufactured and its laboratory preparation should never be attempted because of its potent carcinogenic properties. For many preparative purposes (e.g. see 2-bromonaphthalene, cognate preparation in Expt 6.72, and 2-naphthoic acid, Expt 6.154), 2-naphthylamine-l-sulphonic acid may be used. This is obtained commercially by cautious treatment of 2-naphthol with sulphuric acid - the sulphonic acid group entering the 1-position - followed by a Bucherer reaction. 

These compounds are suggested if sulphur is present. If nitrogen is also present the compound may be an aminosulphonic acid. The infrared spectrum will show absorption at 3400-3200 cm -1 (OH str.) and 1150 and 1050 cm-1 (S=0 str. in a sulphonic add) or at 1090cm-1 (S=0 str. in a sulphinic acid). For derivative preparations for sulphonic acids see Section 9.6.26, p. 1284. The presence of an aromatic sulphinic add may be further confirmed by dissolving in cold concentrated sulphuric add and adding one drop of phenetole or anisole when a blue colour is produced (Smiles s test), due to formation of a para-substituted aromatic sulphoxide. The reaction is ... 

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