Malachite green carbinol

Malachite Green (carbinol) [510-13-4] M 346.4, m 112-114°. The oxalate was recrystd from hot water and dried in air. The carbinol was ppted from the oxalate (lg) in distd water (100ml) by adding M NaOH (10ml). The ppte was filtered off, recrystd from 95% EtOH containing a little dissolved KOH, then washed with ether, and crystd from pet ether. Dried in a vacuum at 40°. An acid soln (2 x 10 5M in 6 x 10 5M H2SO4) rapidly reverted to the dye. [Swain and Hedberg JACS 72 3373 1950]. 

The central carbon atom is derived from an aromatic aldehyde or a substance capable of generating an aldehyde during the course of the condensation. Malachite green is prepared by heating benzaldehyde under reflux with a slight excess of dimethyl aniline in aqueous acid (Fig. 2). The reaction mass is made alkaline and the excess dimethylaniline is removed by steam distillation. The resulting leuco base is oxidized with freshly prepared lead dioxide to the carbinol base, and the lead is removed by precipitation as the sulfate. Subsequent treatment of the carbinol base with acid produces the dye, which can be isolated as the chloride, the oxalate [2437-29-8] or the zinc chloride double salt [79118-82-4]. 

Michler s ketone is converted into the carbinol base of malachite green by the action of phenyl magnesium bromide (the Grignard reaction)... 

Benzo[6]thienylketones are reduced by lithium aluminum hydride to secondary alcohols.465, 526 The carbinol bases of a series of benzo-[6]thiophene analogs of malachite green have been prepared.621... 

The resultant tetra-methyl-diamino-triphenyl methane is then oxidised to the carbinol, which is converted to malachite green by hydrogen chloride ... 

It is prepared as follows Di-methyl aniline, two molectdesy and benzaldehyde, one molecule are heated with zinc chloride or hydrochloric acid. Condensation with the loss of water takes place and the leuco base of malachite green is obtained. The oxidation of the leuco base to the carbinol base is accomplished with lead peroxide, PbOa. 

Accepting the former value, the deamination enthalpy of liquid bis(p-aminophenyl) methane is ca —7 kJ mol-1 per NH2 group compared to the —14 kJ mol-1 for deamination of p-toluidine56. Reference 5 reports a study of the related tris(p-aminophenyl)methane, where the triple p-derivative is assumed given the rich and colorful triphenylmethane/ carbinol dye chemistry of the era (e.g. rosaniline, malachite green). The enthalpy of formation given is 200 kJ mol 1 for the solid. This value is hard to reconcile with the 171.1 1.4 kJ mol 1 for solid triphenylmethane itsell4 57 why should the tris-aniline have a more positive enthalpy of formation than the parent hydrocarbon ... 

Anyone who has worked with triarylmethyl or pyronin systems has probably been struck by the distinctions between the solid states of various derivatives. Malachite Green chloride, for example, is a highly colored crystalline ionic material, and the carbinol is a colorless, reasonably low nelting [mp 163 °C (14)] solid. This distinction between ionic and covalent iolids can be considered in another thermodynamic cycle ... 

Much of the work in this area has focussed on polymeric dyes which can participate in redox reactions. Early work by Manecke and Kossmehl on vinyl Malachite Green (VMG) is specially significant as it established a simple method of entry into the polymerizable triphenylmethane class of dyes. Thus the carbinol base form (ill) of VMG was synthesized by a Grignard reaction between vinyl-phenyl magnesium chloride (l) and Michler s ketone (ll). 

N,N-dimethyl-p-phenylenediamine by the method of Bernthsen33 to give poly( -vinyl Methylene Blue), This polymer acts as a hydrogen acceptor in the enzymatic dehydrogenation of ethanol. The carbinol form (XVIl) of polymeric Malachite Green may be prepared by the. nucleophilic attack of poly(p-lithium styrene) on Michler s ketone 3 acidification gives the colored dye (XVIIl). 

Emerald Powder (Brit). One of the varieties of Cooppal Powder See Vol 3 of Encycl, p C512-R), manufd in England ca 1889. It contd NC, Ba nitrate resins and was colored green by malachite (oxalate of tetramethyl-diamido triphenyl carbinol)... 

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