| Demonstration of Microscale Projection Experiments - Chemistry
en miniature Light Absorbance of Triphenylmethylium Salts Peter Keusch |
German version
Hazards and safety precautions:
1 mL of conc. sulfuric acid is added to the triphenylcarbinol crystals in T1.
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 Photo: Triphenylmethyl Cation crystal violet malachite green Discussion: · Triphenylmethanol is insoluble in water, but when it is treated with concentrated sulfuric acid, it dissolves to create a bright yellow solution. A colored species, the triphenylmethyl cation, is formed reversibly (1). It can be isolated with weakly nucleophilic ions (i.e. BF 4 -, SbCl 6 -).  The center of carbocation is in conjugation with three benzene rings. The positive charge is strongly de-localized. Thus the triphenylmethyl cation possesses ten resonance structures, in which the positive charge is distributed on six ortho and three para positions. · The extent of the bathochromic shift increases in the order triphenylmethyl cation < crystal violet < malachite green (2). The position of the absorption maxima of the mentioned triphenylmethanes depends on the geometry of the chromophore and on the character of he (para-) substituents on the phenyl rings.  · X-ray studies indicate that the geometrical structure of crystal violet (like triphenylmethyl cation) resembles a three-bladed propeller. The planes of the phenyl rings are twisted out of the plane defined by the central carbon atom and its three bonds. The dihedral angle between the phenyl rings and the central coordination plane is 27.7°. The twisting can be understood as a compromise between the effect of p-electron conjugation and the ortho-ortho steric repulsion involving aromatic hydrogens on adjacent rings. The p-electron conjugation (maximum resonance stabilization) favours a planar conformation. The steric interaction between the ortho-hydrogen atoms favours a non-planar structure (Fig. 2). Due to the non-planar structure, the p-electron conjugation in crystal violet is not so extensive as in a p-conjugated planar sytems.
· Unlike crystal violet, malachite green has only two out of three phenyl rings substituted with dimethylamino groups. The two substituted rings are nearly planar aligned. The degree of p-electron conjugation between the two coplanar rings (2) is at a maximum. Hence the absorption maximum of malachite green is shifted to the longer wavelengths in the visible region of the spectrum. The third aromatic ring does not enter into the resonance and is therefore turned out of the plane of the other two rings. Reference: Computer-Interfaced Experiments Absorption Maxima of Triphenylmethane Dyes Microscale Projection Experiments Light Absorption of Triphenylmethane Dye Computer-Interfaced Experiments Kinetics: Fading of Triphenylmethane Dyes - Pseudo First Order Reaction Computer-Interfaced Experiments Kinetics: Fading of Phenolphthalein in Alkaline Solution Microscale Projection Experiments Crystal Violet - a pH Indicator  Demonstration Experiment on Video Crystal violet - a pH Indicator
General experimental instructions and index of experiments 
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