Q.What is Malachite green ? How is it prepared ?

A useful dye must have a suitable colour, be capable of being fixed to the fabric, and be fast (resistant to light, washing, water, acids and alkalis).

Malachite green is a triphenylmethane dye made in two stages: acid-catalysed condensation of benzaldehyde with two equivalents of N,N-dimethylaniline to a colourless leuco base, followed by oxidation to the coloured dye cation.

Identity: Malachite green is a triphenylmethane dye, a brilliant bluish-green basic (cationic) dye, widely used to dye silk, wool, leather and paper, and as a biological/microscopy stain.

Preparation — Step 1 (condensation): Benzaldehyde is condensed with two moles of N,N-dimethylaniline in the presence of concentrated sulphuric acid (or anhydrous $ZnCl_2$), which acts as a condensing/dehydrating agent:

$C_6H_5CHO + 2\,C_6H_5N(CH_3)_2 \xrightarrow{conc.\ H_2SO_4} \big[(CH_3)_2N-C_6H_4\big]_2CH-C_6H_5 + H_2O$

This colourless product is 4,4′-bis(dimethylamino)triphenylmethane, called the leuco base (leuco malachite green) — it is not yet coloured because it lacks an extended conjugated cationic chromophore.

Preparation — Step 2 (oxidation): The leuco base is oxidised with an oxidising agent such as lead dioxide ($PbO_2$) or $MnO_2$ in the presence of a trace of acid; this converts the central $CH$ to a $C^+$ carbocation, generating an extended, cross-conjugated system over all three aromatic rings, which is chromophoric (coloured): …

In para-hydroxyazobenzene the azo linkage is the chromophore that confers colour, the aromatic azobenzene framework carrying it is the chromogen, and the hydroxyl substituent is the auxochrome that deepens/intensifies the colour.

Structure: Para-hydroxyazobenzene is $HO-C_6H_4-N=N-C_6H_5$ — a benzene ring bearing a $-OH$ group at the para position, joined through an azo ($-N=N-$) linkage to a second, unsubstituted benzene ring.

Chromophore: A chromophore is an unsaturated group capable of exciting electrons (via $\pi \rightarrow \pi^*$ or $n \rightarrow \pi^*$ transitions) with visible/near-UV light, thereby imparting colour to the compound. In this molecule, the chromophore is the azo group, $-N=N-$, which conjugates the two aromatic rings and is responsible for the compound being coloured.

Chromogen: A chromogen is an aromatic compound containing a chromophore (the colour-bearing group is present, but the compound need not yet be a true dye). Here, the azobenzene system — the two benzene rings joined through the $-N=N-$ chromophore — constitutes the chromogen.

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