Question

Alizarin dye obtained from the root of a madder plant is anthraquinone derivative. Its structure corresponds to:
A.1,2 – dihydroxy anthraquinone
B.2,3 – dihydroxy anthraquinone
C.1,4 – dihydroxy anthraquinone
D.1 – hydroxy anthraquinone

Answer 1

Hint: Alizarin is a red dye which is originally obtained from the root of the common madder plant, Rubia tinctorum, in which it occurs combined with the sugars xylose and glucose.

Complete answer:
Alizarin, also written as Alizarine, is a red dye which is originally obtained from the root of the common madder plant, Rubia tinctorum, in which it occurs combined with the sugars xylose and glucose. The cultivation of madder and the use of its ground root for dyeing by the complicated Turkey red process were known in ancient India, Persia, and Egypt; the use spread to Asia Minor about the 10th century and was introduced into Europe in the 13th.
The alizarin molecules are capable of forming six-membered chelate rings with aluminum ions. It is an anthraquinone derivative. The ortho dihydroxy structure in the hydroxyanthraquinone molecules could greatly enhance the chelation. Alizarin's structure corresponds to 1,2 -dihydroxy anthraquinone \[\left( {{C_{14}}{H_8}{O_4}} \right)\] .

Therefore, the correct answer is option (A).

Note: The method of preparing alizarin by laboratory methods from anthraquinone were discovered in 1868, and, upon commercial introduction of the synthetic dye in 1871, the natural product disappeared from the market for textile dyes, though natural rose madder is still occasionally used, as a lake, for artists’ colours. The use of alizarin to cotton, wool, or silk requires prior impregnation of the fibre with a metal oxide, or mordant. The shade produced depends on the metal aluminium present which yields a red colour and iron which yields a violet and chromium which yields a brownish red colour.