Colour quinonoid theory

The quinonoid theory, however, was not entirely satisfactory because, for example, di-iminoquinone is colourless. An interesting observation was made by Gomberg in 1899 when he demonstrated the existence of tri-phenyl methyl, which is a yellow colour. This radical contained one trivalent carbon atom, and isolated unsaturated atoms were added to the list of chromophores. 

Following Graebe and Liebermann (1868, see p. 789) Armstrong proposed a quinonoid theory of colour. He proposed a theory of benzene substitution, and a centric formula of benzene (see p. 804). His work with F. P. Worley on reaction velocities in the catalytic hydrolysis of cane sugar and esters by acids ... 

In complete accordance with Willstatter s theory, fuchsone is only yellowish-orange in colour. But if, in addition, one of the two free benzene rings has an OH-group in the p-position, we have the dye benzaurin already mentioned on p. 328. The o-carboxylic acid of this dye is the quinonoid form of phenolphthalein. In fact, the colour tones of these two substances are very similar. Phenolphthalein is decolorised by concentrated alkali NaOH (KOH) is added on and the trisodium (or potassium) salt of the benzenoid carbinol form is produced. (Try these reactions with phenolphthalein.)... 

As we have already seen these lone pairs can form part of the system of n electrons. The difference between chromo-phoric and auxochromic groups is in this way of secondary importance. Also the much discussed question whether a ben-zoid (benzene-like) or a quinonoid (quinone-like) structure should be attributed to dyestuffs becomes, in the light of the resonance theory, an incorrectly chosen alternative. It is the possibility of resonance which is reflected in the multiplicity of the valence structure that forms the true basis for light absorption. An isolated benzoid configuration is just as little a colouring matter as a quinonoid structure compare the uncoloured hydroquinone and the very weakly coloured quinone. 

Green originated the important technical ingrain process of developing a dye on a fabric, and worked on sulphur and stilbene dyes, aniline black, oxonium compounds, and sulphanilamide. Green and A. G. Perkin criticised Ostwald s theory of indicators and explained the red colour of alkaline phenol-phthalein as due to a quinonoid form of the undissociated molecule. 

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