Question

Alizarin belongs to the class of
A. vat dyes
B. mordant dyes
C. substantive dyes
D. reactive dyes

Answer 1

Hint: Molecules of alizarin are capable of forming six-membered chelate rings with metal ions like aluminium ions. Now to find the answer we have to see which type of dye category has this characteristic.

Complete answer:
We have to understand what the options mean in order to get the answer to this particular question.
-Let us start with Option A vat dyes, these dyes are insoluble in water and thus they cannot be utilised in dyeing directly. But as their reduction is done to a leuco (colourless) form, they become soluble in alkalis and have acquired the affinity towards cellulose fibres and thus now they can be used for dyeing or printing purposes.
-Now coming to second option Mordant dyes, these are those substances that can be fixed to fibres and can be dyed later on. Mordants are metal salts that can form an insoluble complex with dye molecules like alum and ferrous sulphate. In other words, mordant dyes are those that can form complexes and impart different colours with different mordants.
-Now coming to substantive dye, these are the ones that are used in a process in which a dye molecule is attracted by physical forces at the molecular level of the textile.
-Now the last option is reactive dyes, in this particular type a new chemical compound is formed on coming in contact with a natural fibre.
-We have alizarin, as already told in the hint, these chemical substances form chelates with metal ions. Thus no new chemical compound is formed hence cannot be reactive dyes, also cannot be substantive dyes since substantive deals with physical forces and not chemicals. So we have left vat dyes and mordant. Now mordant as explained is the class that forms complex or chelates with the metal which is also the case with alizarin, hence alizarin belongs to the class of mordant dyes.
-The alizarin molecules are capable of forming six-membered chelate rings with aluminium ions. Colored lakes formed by the metal ions and dye molecules resist extraction by water and organic solvents, which readily strip similarly structured acid dyes. The sheer size of the complex may account for some of its insolubility. It is also likely that the large complexes are physically trapped within the fibre.

Therefore, the correct answer is Option (B) mordant dyes.

Additional Information: Mordant dyes have commercially been used for dyeing wool in presence of metal ions. Anthraquinone or azamethines are the only other classes of metalized dyes that have been commercially significant.

Note:
Since alizarin belongs to the anthraquinones group, it can be used as a vat dye but as far as categorising it into class is concerned, it belongs to the class of mordant dyes. Therefore one should not get confused between these two options.