SYNTHETIC ORGANIC PRODUCTS 477 



Dye-stuffs. Natural dye-stuffs have now been almost 

 entirely superseded by manufactured products, which can be 

 prepared more cheaply and are of superior quality. Logwood, 

 still used as a black mordant dye for silk, is the only important 

 exception. The total value of natural dyes imported into the 

 United States in 1919 was only $1,250,000, while the artificial 

 dyes made in the country in the same year were worth over $70,- 

 000,000. 



The vast bulk of synthetic dyes are built up from ring hydro- 

 carbons (p. 352) and their derivatives, extracted from coal tar 

 (p. 424). By substituting suitable groups into the molecule 

 of the simpler colorless products, more complex derivatives with 

 great brilliancy of color are obtained. Thus indigo, formerly 

 the most extensively used of all natural dyes, is now manufactured 

 most conveniently with aniline (p. 353) as a starting-point. Ali- 

 zarin (Turkey red), once extracted from madder root, is obtained 

 from anthracene. 



By varying the position of the substituted groups in the mole- 

 cule, the most delicate variations in color can be effected. The 

 most precious of all dyes in ancient times was Tyrian purple, 

 obtained from certain species of sea snails (Murex). The secret 

 of preparing this substance was lost for centuries, but in 1909 

 Friedlaender gathered 12,000 of these mollusks and succeeded in 

 isolating 1.5 grammes of the coloring material for analysis. He 

 showed it to be a derivative of indigo, containing two bromine 

 atoms in place of two of the hydrogens. This identical substance 

 had been prepared synthetically five years earlier, but found to 

 be inferior to another dye containing the bromine atoms in differ- 

 ent positions in the molecule! 



Preparation of Typical Dyes. The student is recommended 

 to attempt the following preparations, which are at the same 

 tune easy and instructive. 



(a) Phenolphthakin. Take 0.1 gram of phthalic anhydride (the 



