Malachite, illustration

The most versatile route to lactone color formers is based on the elegant synthesis of Malachite Green lactone described by Haller and Guyot in 1899.24 Scheme 3 illustrates the application of this route for the preparation of CVL. 

The valence-bond (resonance) description of the triphenylmethine dye Malachite Green (125) is illustrated in Figure 6.5. Comparison with Figure 6.4 reveals their structural similarity compared with cyanine dyes. Formally, the dye contains a carbonium ion centre, as a result of a contribution from resonance form II. The molecule is stabilised by resonance that involves delocalisation of the positive charge on to the p-amino... 

Figure 19.3 illustrates the floatability of pure malachite with different amines. It should be pointed out that there are several varieties of malachite, Cu4(P04)2(0H)4-0H. Pseudomalachite is difficult to float, and it is well known that pseudo-malachite can be floated with anionic collectors, but responds poorly to the sulphidization method. 

Equation (47) is of significance in mechanistic studies of nucleophiles with carbenium ions and it fits a wide range of anion-cation combination reactions over a wide range of reactivity contrary to the reactivity-selectivity principle (see Chapter 6). The N+ values in Appendix 3, Table 4 are obtained from reaction of either malachite green or 4-nitromalachite green. Figure 11 illustrates plots of log(A 7/Cwater) for a number of carbenium ions (1-4). 

The methods used to prepare the dyes of this class may be illustrated by a consideration of pararosaniline, rosaniline, methyl violet, crystal violet, and malachite green. 

Figure 4.11 (a) Schematic illustration of a self-assembled monolayer of malachite green-terminated aUcanethiol (MG-CN-SH) with pH- and ultraviolet-responsive wetting properties, (b) Curves A and B represent the ideal model for two kinds of pH-responsive surfaces with completely different pH responses. 

A second method for the synthesis of triphenylmethane dyes is illustrated with Basic Green 4 (Malachite Green). Benzalde-hyde (one mole) is condensed with two moles of Af,Af-dimethylaniline to form a leuco base, which is oxidized with lead peroxide and treated with hydrochloric acid to give the dye. 

---