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Concept
Malachite green
Summary
Malachite green is an organic compound that is used as a dyestuff and controversially as an antimicrobial in aquaculture. Malachite green is traditionally used as a dye for materials such as silk, leather, and paper. Despite its name the dye is not prepared from the mineral malachite; the name just comes from the similarity of color.
Malachite green is classified in the dyestuff industry as a triarylmethane dye and also using in pigment industry. Formally, malachite green refers to the chloride salt [C6H5C(C6H4N(CH3)2)2]Cl, although the term malachite green is used loosely and often just refers to the colored cation. The oxalate salt is also marketed. The 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 ).
Malachite green is prepared by the condensation of benzaldehyde and dimethylaniline to give leuco malachite green (LMG):
C6H5CHO + C6H5N(CH3)2 -> (C6H5N(CH3)2)2C6H5 + H2O
Second, this colorless leuco compound, a relative of triphenylmethane, is oxidized to the cation that is MG:
C6H5CH(C6H4N(CH3)2)2 + HCl + O2 → [C6H5C(C6H4N(CH3)2)2]Cl + H2O
A typical oxidizing agent is manganese dioxide.
Hydrolysis of MG gives an alcohol:
[C6H5C(C6H4N(CH3)2)2]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 leuco and alcohol derivatives are not. This difference arises because only the cationic form has extended pi-delocalization, which allows the molecule to absorb visible light.
The leuco form of malachite green was first prepared by Hermann Fischer in 1877 by condensing benzaldehyde and dimethylaniline in the molecular ratio 1:2 in the presence of sulfuric acid.
Malachite green is traditionally used as a dye. Kilotonnes of MG and related triarylmethane dyes are produced annually for this purpose.
Official source
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