Alizarine, Nitro Derivatives

Except for artificial alizarin, made from coal tar anthracene, practically all the early synthetic dyes were derived from, coal tar benzene and toluene. From the mid-1870s, however, interest turned to amino and nitro derivatives of the more abundant coal tar hydrocarbon naphthalene. These endeavours were assisted by the large body of theoretical, synthetic and analytical knowledge about aromatic chemistry that had become available during the decade following the announcement of Kekule s benzene ring theory (1865). Roussin s azo dyes were naphthalene derivatives. 

Nitro, amino and sulphonic acid derivatives of alizarin are also dyes of various colors and are known as alizarin orange, alizarin maroon, alizarin red, etc. Also there is present in the madder root another dye compound known as puipurin which is 1-2-4-tri-hydroxy anthraqui-none. Isomeric tri-hydroxy anthraquinones are dyes also but it is interesting that in all of these poly-hydroxy anthraquinones which are dyes two of the hydroxyls are always in the 1-2 positions. 

A study of the products which result from the nitration of ahzarin, shows that formula (I) is correct. In the two mononitro derivatives formed, the nitro group is in combination with the ring to which the hydroxyl groups are joined, since the two derivatives yield phthalic acid on oxidation. As a compound of the structure represented by formula (II) could not yield two such mononitro derivatives, the conclusion is drawn that the structure of alizarin is that indicated by formula (I). 

Nitrophenols and nitro-acids, (yellow to orange) salicyl-aldehyde (yellow) derivatives of alizarin JV-bromosucdn-imide phthaleins. 

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