The Chemistry of Dyestuff. Intermediate Compounds. V. Sulphonic Acids.

A Manual for Students of Chemistry and Dyeing
By
M. Fort, M.Sc. (Leeds) Late Lecturer in Dyeing in the Bradford Technical College and L. L. Lloyd, Ph.D. (Bern) Lecturer in Organic and Technical Chemistry in the Bradford Technical College
Cambridge: at the University Press 1919
(First edition 1917, reprinted 1919)The production of sulphonic acids has been known since 1834. For the preparation of these compounds sulphuric, fuming sulphuric and chlorsulphonic acids are mainly used; in some cases sodium bisulphite is capable of producing sulphonic acids. The manner in which the sulphonation is carried out varies with the nature of the compounds to be treated. As with nitration, the sulphonation of phenols is generally more easily accomplished than the sulphonation of amines or hydrocarbons. The temperature at which sulphonation is done plays a very important part; not only does a rise of temperature aid sulphonation but it also influences the position of the sulphonic acid groups. Sulphonic acid groups often wander from their original position when the temperature is raised. This is also controlled to a certain extent by the groups already present in the compound. In some cases the introduction of sulphonic acid groups is to provide intermediate steps in the production of other compounds, e.g., nitro bodies, but "they are generally introduced for the purpose of producing water soluble bodies or giving acid properties. The sulphonic acids are often difficult to obtain pure and therefore to identify. The commercial products . are usually amorphous bodies obtained by evaporation of solutions to dry ness (Fig. X, Appendix). The most characteristic reactions of amido and oxy-sulphonic acids are obtained with diazocompounds (see Azo Dyes). The sulphonic acids rarely melt sharply on heating or crystallise well from solutions. Titration with alkalis, boiling with dilute acid to remove sulphonic acid groups, or treatment with phosphorus pentachloride to obtain chlorine compounds of definite melting point are the chief methods available. The commercial sulphonic acids are generally sold as sodium salts.

Benzene monosulphonic acid [-] is of little importance, but on further sulphonation benzene disulphonic acid is obtained from which resorcin is manufactured. In time of war the price of phenol may be so high that it pays to manufacture it from benzene by sulphonation followed by decomposition of the sulphonate with caustic alkali.

Benzene disulphonic acid is obtained by heating benzene with fuming sulphuric acid at about 100°C. in iron vessels provided with stirring gear until the benzene has disappeared; the temperature is then gradually raised to 275°C. and the heating continued for three hours (Fig. IV, Appendix). The solution, after cooling, is diluted and treated with chalk, filtered, and the filtrate treated with potassium or sodium carbonate to prepare the potassium or sodium salt of the disulphonic acid. The three isomeric disulphonic acids of benzene are produced, but mainly the meta derivative. All on heating with caustic soda give resorcin.

The crude salt may be fused with caustic soda for the preparation of resorcin, or the solution of the potassium salt concentrated to 1,275 sp. gr. and allowed to crystallise. The potassium-meta-disulphonate separates out, leaving the potassium-para-disulphonate in solution.

Pirìa some forty years ago discovered that α-nitronaphthalene, on treating with aqueous alcoholic ammonium sulphite, gave thionaphthamic acid and naphthionic acid. With a large excess of bisulphite, α-naphthylamine 2.4-disulphonic acid is produced instead of Pirìa's compounds. The use of bisulphites in sulphonation is restricted by their simultaneous reducing action. They are used for the production of naphthylamine-3.8-disulphonic acid, and for reduction and sulphonation of 1.5 and of 1.8-dinitronaphthalene. In these cases nitrogen is not eliminated from the molecule, but with 1.8-dinitro-3.6-disulphonic acid one of the nitro groups is replaced by the hydroxy group.

Naphthalene sulphonic acids may be obtained
(a) by sulphonation of naphthalene;
(b) by the elimination of the amido group from the amido sulphonic acids by diazotising etc.;
(c) by replacement of the amido group for the thio-oxy group, and oxidation of the latter group into a sulphinic and finally sulphonic acid.

Of these methods the sulphonation of naphthalene is the most important. Unlike most substituting agents sulphuric acid gives rise to the production of β-derivatives in large amount. In no case however have two sulphonic acid groups been introduced in positions relatively 1.2, 1.4, or 1.8 to one another. The sulphonic acids and their alkali salts are readily soluble in water, but the lead, barium, and calcium salts differ in solubility and often afford a means of separating the different compounds from a sulphonation product.

From the alkali salts, crystalline amides and anil ides may be prepared which, having definite melting points, serve to characterise the acids; from the sulphonic chlorides, chloronaphthalenes may be produced which help to determine the constitution of the original acids.

By hydrolysis with caustic alkalis the naphthalene sulphonic acids give rise to hydroxy compounds, in which one or more sulphonic acid groups may be replaced by oxy groups.

1 The symbol S is here used in place of SO₃ H and this wellknown abbreviation is employed frequently where it is quite obvious what it stands for. The sulphonic acid groups are more easily eliminated from the athan from the β-positions, and in some cases further change may take place, e.g., a sulphonic acid group in the meta position to a hydroxy group may give a cresol by caustic hydrolysis, e.g.¹,  [-]

The position in which the sulphonic acid group enters the naphthalene ring depends to a large extent upon the temperature at which the reaction takes place. The sulphonic acid group may also wander from the ato the β-position by the action of heat.

The following diagrammatic scheme shows how the different sulphonic acids are obtained from one another, S representing the sulphonic acid group.

[-]

α-naphthalene sulphonic acid [-]
Finely powdered naphthalene is heated with half its weight of concentrated sulphuric acid at 40°C. until the product is completely soluble in caustic soda solution. The reaction mass on cooling separates into a lower layer of sulphuric acid and an upper layer of α-naphthalene sulphonic acid, which crystallises out with one molecule of water. M.P. 85° to 90°C.

The acid may be purified or directly converted into α-naphthol by alkali fusion.

β-naphthalene sulphonic acid [-]
Equal weights of concentrated sulphuric acid and naphthalene are heated to 180°C. for four to five hours. The product, after cooling, is run into an excess of brine, the sodium salt of naphthalene β-sulphonic acid separates out, it is filter pressed, dried, and ground. This compound is not so readily obtained pure as the α-acid. It is used for the manufacture of β-naphthol.

The introduction of a second sulphonic acid group into naphthalene also takes place at low temperatures in the aand at higher temperatures in the β-positions, the second group entering the molecule as far away as possible from the group already present.

Naphthalene-1.5-disulphonic acid (Armstrong's acid)  [-]  is obtained by sulphonation of naphthalene at 70°C. with fuming sulphuric acid.

Naphthalene-1.6-disulphonic acid is obtained by sulphonation of the naphthalene-β-sulphonic acid with fuming sulphuric acid at 100°C.

Naphthalene-1.6-, -2.6-, and -2.7-disulphonic acids are obtained by sulphonation of naphthalene with excess of sulphuric acid at 180°C. The calcium salts are formed by addition of lime, and these can be separated by means of brine. The calcium salt of the 1.6 acid is fairly soluble in cold brine, the 2.7 salt is tolerably soluble in hot but sparingly soluble in cold, and the 2.6 salt is practically insoluble in hot and cold brine.

Naphthalene trisulphonic acids are produced by the further sulphonation of naphthalene disulphonic acids or their salts by means of fuming sulphuric acid or chlorosulphonic acid. The most important are the 1.3.5, the J.3.6, the 1.3.7, and the 2.3.6 trisulphonic acids. When fused with alkali they give oxynaphthalene sulphonic acids. Tetrasulphonic acids are also produced by sulphonation of naphthalene disulphonic acids.

Amido and oxy -naphthalene sulphonic acids.

The naphthylamine sulphonic acids may be obtained by the following methods:
(a) By the sulphonation of naphthylamine.
(b) By the nitration of naphthalene sulphonic acids, followed by reduction.
(c) By the further sulphonation of the acids obtained by method (b).
(d) By the replacement of α-oxy groups of naphthol sulphonic acids by the amido group, e.g., heating with ammonia in the presence of ammonium sulphite.

As in the case of the naphthalene sulphonic acids, the sulphonic acid group may wander in the molecule. By heating naphthionic acid (sodium salt) it is converted into the sodium salt of α-naphthylamine-2-sulphonic acid.


The sulphonic acid groups in the α-positions are more easily eliminated than those in the β-positions, whether the agent be water, caustic alkali, or dilute acid at high temperature.


Naphthylamine sulphonic acids.

Naphthionic acid [-] is obtained by heating molecular quantities of concentrated sulphuric acid and α-naphthylamine at 60°C., the additive sulphate formed being then mixed with about 3 per cent, of oxalic acid and baked at 180°C. during eight hours. The oxalic acid decomposes on heating into carbon dioxide and water which causes the mass to swell and become porous, thus aiding the reaction. The free acid is sparingly soluble in water, while the commercial product, which is the sodium salt, is more soluble, cf. sulphanilic acid, p. 32.

1.5-Naphthylamine sulphonic acid (Laurent's acid)  [-] is obtained by sulphonating α-naphthyl-amine with fuming sulphuric acid at°C. or by reducing α-nitronaphthalene-5-sulphonic acid. The acid separates as a slime on pouring the product of the reaction on ice.

2.5-Naphthylamine sulphonic acid (Dahl's acid)  [-]  is obtained by mixing β-naphthyl-amine sulphate with cold concentrated sulphuric acid and allowing to stand at ordinary temperature for two or three days.
2.6-Naphthylamine sulphonic acid (Bronner's acid)  [-]  is obtained as the ammonium salt by heating 2.6-naphthol sulphonic acid with excess of ammonia in an autoclave at 190° to 200°C. for two days.

2.7-Naphthylamine sulphonic acid (F-acid)  [-] is obtained from 2.7-naphthol sulphonic acid by heating with ammonia and ammonium sulphite in an autoclave. It is obtained along with Bronner's acid by heating β-naphthylamine and concentrated sulphuric acid at 170°C.

Naphthylamine disulphonic acids are obtained by the further sulphonation of the monosulphonic acids by means of fuming sulphuric acid, or by the replacement of the oxy group in the corresponding naphthol disulphonic acids by the amido group. This is generally carried out by heating with ammonia under pressure in presence of ammonium sulphite.

α-naphthylamine-4.6 and -4.7 -disulphonic acids are obtained by sulphonating naphthionic acid with 25 per cent. (SO₃ ) fuming sulphuric acid at about 30°C. The mixed acids are converted into calcium salts, dried, and extracted with 85 per cent, alcohol, the calcium salt of the 4.6-acid being soluble.
β-naphthylamine-3.6(amido-R-acid) and -6.8-disulphonic acids (amido-Gr-acid) are obtained by the amidation of corresponding naphthol disulphonic acids with ammonia in presence of ammonium sulphite.

The naphthol sulphonic acids are obtained by the following general methods:
(1) Sulphonation of naphthol.
(2) Substitution of the amido group for the hydroxyl group by the action of bisulphite or by the action of water at a high temperature.
(3) Replacement of chlorine in α-chloronaphthalene sulphonic acids by action of caustic alkali.
(4) Replacement of a sulphonic acid in naphthalene polysul phonic acids by action of caustic alkali.

The direct sulphonation of the naphthols is of most importance for the derivatives of β-naphthol, the α-naphthol sulphonic acids being seldom prepared by this method. This is no doubt due to the ease with which sulphonation takes place in the α-position compared with the β-substitution.

The temperature at which sulphonation is carried out, and the duration of heating, are important factors in the production of these bodies on account of the alteration in the position of the sulplionic acid group brought about by heating many of these acids.

Naphthol sulphonic acids. The mono-, diand tri-sulphonic acids of alphaand betα-naphthol are very important intermediate bodies in the production of azo dyestuffs. A very interesting reaction which ia often employed in their preparation involves the interchange of the amido group and the oxy group. In presence of ammonium sulphite the oxy group is replaced by the amido group, while in the presence of sodium bisulphite the amido group is replaced by the oxy group.

α-naphthol-4-sulphonic acid ("N.W." or Neville and Winther's acid). Sodium naphthionate is heated with eight times its weight of sodium bisulphite solution of 76 Tw., and twice its weight of water, in a closed vessel, until the solution no longer gives a precipitate of uaphthionic acid on addition of hydrochloric acid.

α-naphthol-5-sulphonic acid (Cleve's acid) is obtained similarly from the corresponding amido acid.

β-naphthol-8-sulphonic acid (Crocein or Bayer's acid). β-naphthol in a finely powdered condition is mixed with twice its weight of concentrated sulphuric acid, the temperature not being allowed to exceed 20°C. The mixture is tested after about seven days' standing by boiling with an equal volume of water to find when the naphthol is no longer precipitated.

β-naphthol6 -sulphonic acid (Schäffer's acid) is obtained by sulphonating as above but at 50° to 60°C. A mixture of about equal parts of Bayer's and Schaffer's acid is obtained. About 80 per cent, of the Schaffer's acid is obtained as sodium salt by adding about 66 per cent, of the theoretical quantity of caustic soda and allowing to crystallise.

β-naphthol-6.8-disulphonic acid, "G-acid" [-] and β-naphthol-3.6-disulphonic acid, R-acid  [-], are obtained by sulphonating β-naphthol with four times its weight of sulphuric acid, the G-acid mainly at 60°C. and the R-acid mainly at 100°C. They are separated from one another as sodium salts by means of 20 per cent, salt solution, the R-salt being the more sparingly soluble.


Amidonaphthol sulphonic acids.

2.8 -Amidonaphthol-6-sulphonic acid is obtained by heating amido-G-acid with twice its weight of 50 per cent, caustic soda solution for six hours at 185°C. (Fig. VI, Appendix).

1.8 Amidonaphthol-3.6-disulphonic acid, [-]  is obtained by heating the corresponding diamido acid with 40 per cent, caustic soda to 200°C. in an autoclave. The acid and the acid salts are sparingly soluble in water.

Further information may be obtained from an excellent article in Thorpe's Dictionary qf Applied Chemistry under "Naphthalene."


Diamido-disulphonic acids.

Diamidostilbene-disulphonic acid  [-] is obtained by boiling the sodium salt of p-nitrotoluene sulphonic acid with 33 per cent, caustic soda, in this manner dinitrostilbene-disulphonic acid is obtained. The diarnido acid is obtained by reducing the alkaline solution at the boil with zinc dust until the solution no longer turns red on exposure to air, it is then filtered into an excess of hydrochloric acid.