Schaeffer’s acid

The preparation of 2-naphthol by high-temperature sulphonation of naphthalene followed by alkali fusion of the resulting naphthalene-2-sulphonic acid has been mentioned previously. Further sulphonation of 2-naphthol yields several useful naphtholsulphonic acids and conditions can be chosen to make one or other of these compounds the main product. The initial product is the unstable 2-naphthol-l-sulphonic acid, which readily rearranges to 2-naphthol-6-sulphonic acid (4-36 Schaeffer s acid). Further sulphonation leads to 2-naphthol-6,8-disulphonic acid (4-37 G acid) at low temperature and 2-naphthol-3,6-disulphonic acid (4.38 R acid) at higher temperature. 

Pio. 0. Dependence of the logarithm of the retention factor of six dyettuCb and inter mediateli upon cetrimide (cetylirimelhylaaimonium bromide). The data were obtained on 7 /tm SAS silica (WoUson Unit) with water-propranol (5 2, v/v) containing cetrimide at the indicated concentration as the mobile phase. The ehiites are as follows (1) Ponceau 4R (2) Ponceau MX (3) taitrazine (4) Sunset Yellow (5) Schaeffer s acid (6) sulfonilic acid. Re printed with permission from Knox and Laird (5i). 

Schaeffer s acid is obtained along with the latter aeid by heating a-naphthol with concentrated sulphuric acid on the water-bath. It differs from the a a-acid by the sparing solubility of its sodium-salt in alcohol, and more especially by the dyestuffs it yields. These are analogous to the corresponding a-naphthol derivatives, and therefore very probably para-compounds. 

One of these (the a-sulphonic acid or crocein-acid) is best obtained at a low temperature. By further heating with sulphuric acid it goes over into the )9- or Schaeffer s acid, which was prepared by Schaeffer a long time ago. These acids are distinguished... 

From the facts that azo-dyes from Schaeffer s acid differ but little in shade and solubility from those obtained from -naphthol, and that simple /3-naphthol azo-compounds on sulphonation always yield derivatives of Schaeffer s acid, and never those of crocein-acidj it may naturally be concluded that the azo-group enters in the same position both in /S-naphthol and in Schaeffer s acid. This is the a-y position. 

The behaviour of the o-acid is totally different. The dyestuffs do not resemble those from Schaeffer s acid and /3-naphthol in the least, either as regards shade or solubility. Further, this acid may easily be converted into a dinitro-compound with nitric acid, and this is not the case with either y8-naphthol or Schaeffer s aeid. Another striking property of the a-acid is that its combination with diazo-compounds takes place with much more diflSculty than is experienced with Schaeffer s acid. Its constitution is therefore... 

Schaeffer s acid yields sulphophthalic acid on oxidation, showing that the hydroxyl and sulpho-groups are in different rings. 

The acid soluble in alcohol and salt-solution (G-acid of Meister, Lucius, and Briining, 7-acid of Cassella and Co.) behaves very like, /3-naphthol-a-monosulphouic acid (crocein acid), and must be regarded as a derivative of the latter, as it is formed quantitatively on further sulphonation of crocein acid., It may also be obtained from Schaeffer s acid (this is contradicted by some chemists), and Armstrong gives it the following formula —... 

This R-acid is best obtained by further sulphonation of Schaeffer s acid, and it is also stated that G-acid is converted into R-acid on heating with sulphonic acid. The constitution of the R-acid is expressed by the formula —... 

The sulphonic acid is prepared by acting on Schaeffer s acid with nitrous acid. The iron compound of this acid is soluble in water, and is fixed on animal fibres like the acid dyestuffs. 

SCHEME 7. Conversion of (J-naphthol to G and R adds via Schaeffer s acid... 

Amination of Schaeffer s acid (2-naphthol-6-sulfonic acid) (50) as its salt at 150-160 °C yields 2-aminonaphthalene-6-sulfonic acid, Bronner s acid (46c) (Scheme 11). 

R R acid (2R acid) Schaeffer s acid Schoelkopf s acid sultam acid Tobias acid... 

Naphthols. Of the four important naphtholmonosulfonic acids, only two are produced by direct sulfonation of the naphthol. 2-Naphthol-l-sulfonic acid (Armstrong s acid) is formed by treatment with 97 per cent sulfuric acid at low temperature with a short reaction time or with SO3 using a solvent, w hile 2-naphthol-6-sulfonic (Schaeffer s acid) is produced at 120-125 C with 78 per cent sulfuric acid. The two major naphtholdisulfonic acids [2-naphthol-6,8-disulfonic acid (G acid) and 2-naphthol-3,6-disulfonic acid (R acid)] are formed together by direct sulfonation of 2-naphthol, using sulfuric acid followed by oleum and raising the temperature in steps from 15 to 80 C. They are isolated as the potassium and sodium salts, respectively, by differential salting out. 

Salicylic acid, 11, 42 14, 48, 52 Schaeffer s acid, 16, 16 Schaeffer s salt, 16, 13 Schotten-Baumann acylation, 12, 41 ... 

Two important monosulfonic acids result ing from the sulfonation of 2-naphthol are Schaeffer s acid and Crocein acid. At 100°C, Schaeffer s acid is preponderant at lower temperatures more Crocein acid is formed. 

FD C Red 40 is made from diazotized cresidine sulfonic acid (4-amino-5-methoxy-2-toluenesulfonic acid) and Schaeffer s acid. Acid Red 337 is made from diazotized o-aminobenzotrifluoride and Gamma Acid. The coupling in this case is acidic, and the resulting dye is used for the coloration of nylon. 

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