Malachite Green - Structures and Properties

Structures and Properties

Malachite green is classified in the dyestuff industry as a triarylmethane dye. Formally, Malachite green refers to the chloride salt Cl, although the term Malachite green is used loosely and often just refers to the colored cation. The oxalate salt is also marketed. The chloride and oxalate anions have no effect on the color. The intense green color of the cation results from a strong absorption band at 621 nm (extinction coefficient of 105 M−1cm−1).

Malachite green (first transition) (pH indicator)
below pH 0.2 above pH 1.8
0.2 ↔ 1.8
Malachite green (second transition) (pH indicator)
below pH 11.5 above pH 13.2
11.5 ↔ 13.2

Malachite green is prepared by the condensation of benzaldehyde and dimethylaniline to give leuco malachite green (LMG):

C6H5CHO + 2 C6H5N(CH3)2 → C6H5CH(C6H4N(CH3)2)2 + H2O

Second, this colorless leuco compound, a relative of triphenylmethane, is oxidized to the cation that is MG:

C6H5CH(C6H4N(CH3)2)2 + HCl + 1/2 O2 → Cl + H2O

A typical oxidizing agent is manganese dioxide.

Hydrolysis of MG gives the carbinol form:

Cl + H2O → C6H5C(OH)(C6H4N(CH3)2)2 + HCl

This alcohol is important because it, not MG, traverses cell membranes. Once inside the cell, it is metabolized into LMG. Only the cation MG is deeply colored, whereas the LMG and carbinol derivatives are not. This difference arises because only the cationic form has extended pi-delocalization, which allows the molecule to absorb visible light.

Read more about this topic:  Malachite Green

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