Pyronin derivatives

For triarylmethyl and pyronin derivatives, the solubilities of covalent compounds, such as the carbinols, are on the order of 10-5 M in water. Thus, must be approximately — 7 kcal/mol. 

Calculated values for pyronin derivatives and the maximum pKR of the cation allowing formation of covalent solid are given in Table IV. 

The other trends in values for the various Rs in Table V are consistent with the trends in pairwise interaction contributions to heats of formation evaluated by Allen (19) and discussed by Hine (20). For example, pyronin has a C-C-X pair interaction in place of an O-C-X interaction for pyridine-4-carboxaldehyde for X = OH this difference favors the aldehyde adduct by approximately 7 kcal, although for other Xs, the difference is somewhat smaller. Thus, the values given for the aldehyde are smaller than those for pyronin. We have already pointed out that steric effects are expected in the triarylmethyl derivatives and that the comparison of these with pyronin derivatives are consistent with that expectation (1). 

For uniformity with the stmctures given in the Colourindex the ammonium radical (9) is used for the amino-substituted xanthenes and the keto form for the hydroxy derivatives. The xanthene dyes may be classified into two main groups diphenylmethane derivatives, called pyronines, and triphenylmethane derivatives (eg, (4)), which are mainly phthaleins made from phthaUc anhydride condensations. A third much smaller group of rosamines (9-phenylxanthenes) is prepared from substituted ben2aldehydes. The phthaleins may be further subdivided into the following fluoresceins (hydroxy-substituted) rhodamines (amino-substituted), eg, (6) and mixed hydroxy/amino-substituted. 

Pyronines. Pyronines are diphenylmethane derivatives synthesized by the condensation of y -dialkylarninophenols with formaldehyde, followed by oxidation of tiie xantiiene detivative (12) to the coiiesponding xanthydiol (13) which in the presence of acid forms the dye (14). If R is methyl, the dye produced is... 

Diaryl- and triaryl-methane dyes also fall into this class [(124) is known as Michler s Hydrol Blue] and a number of the heterocyclic derivatives of these dyes are well known. Introduction of a sulfur bridge into Michler s Hydrol Blue (124) results in the dye Thiopyronine (125) which absorbs at 565 nm. The analogous dye with an oxygen bridge, Pyronine, absorbs at 545 nm and that with an —NH— bridge, Acridine Orange, absorbs at 490 nm (B-76MI11201). 

In recent years, many spirochromenes such as (136) and (137) have been studied because of their photochromic or thermochromic properties, while derivatives of xanthene are useful as dyes and indicators, for example pyronine G (138), fluorescein (139) and rhodamine 6G (140). 

Di- and Triaryl Carbonium and Related Dyes. As a class, these dyes are bright and strong, hut are generally deficient in lightfastness. Consequently, they are used in outlets where brightness and cost-effectiveness. rather than permanence, arc paramount, for example, the coloration of paper. Many dyes of this class, especially derivatives of pyronines Ixamhcnes). arc among the most fluorescent dyes known. 

The phthaleins derived from di- or polyhydric phenols are all anhydrides formed by the elimination of water from two phenolic hydroxyls, attached to two different benzene rings. These anhydride phthaleins are known as pyronines, since they contain, like the pyrones, a six-membered oxygen-containing ring. 

PfUhaleins or pyronine or pyrone derivatives, usually red or violet, and related to the preceding group. 

An important group of dyes known as the phthaleins and t ified by the common indicator phenolphthalein are derivatives of tri-phenyl methane, but because they do not possess the same structure as the three preceding groups are placed in a separate series known as the pyronines. 

Color and Constitution of Phenolphthalein.—We have used phenolphthalein as an example of the pMhalein dyes in order to show their relation as tri-phenyl methane derivatives. When, however, we attempt to establish a consitution for phenolphthalein which will explain its character as a dye, in harmony with the structure of related compounds, e.g.j fluorescein and other pyronine dyesj we meet with considerable trouble and it may be said that the question is one that does not seem to be cleared up. Strictly speaking phenolphthalein is not a dye. Its well known use as an indicator is associated with the following facts, (i) In neidral or acid solution it is colorless. (2) In weak alkaline solution it is red. (3) In strong alkaline solution it is again colorless. (4) On neutralizing the excess alkali of (3) with acetic acid and boiling, the red color is restored and phenolphthalein is precipitated. 

The o-phenolphthalein of the lactone formula with two phenol hydroxyls near to each other would lose water and the resulting anhydride if written as in (C) is plainly a pyronine as well as tri-phenyl methane derivative as in (B). In this pyronine structure, however, there is no quinoid group and to form such a structure by the conversion of the phenolphthalein into the sodium salt does not appear possible. 

In a related phthalein dye, however, this condition is fully met. The dye fluorescein is resorcinol phthalein and is made from phthalic anhydride or phthalyl chloride and resorcinol just as phenol phthalein is made from phthalic anhydride and phenol (p. 750). These relationships may be expressed as follows writing the final dye salt both as a tri-phenyl methane derivative (B) and as a pyronine (C). The reactions are exactly analogous to those given for the preparation of phenolphthalein and its dye salt (p. 750), some of the intermediate steps being omitted in the present case. 

The preceding discussion of the tri-phenyl methane and pyronine dyes is by no means exhaustive but enough has been said to give the student some idea of the importance of the dye compounds which are derived from the hydrocarbon tri-phenyl methane also to give the principal facts in connection with their relation to the history of synthetic dyes and to the question of chemical constitution and color of dyestuffs. 

The Pyronines are a series of red basic dyestuffs of comparatively recent introduction. They are derivatives of diphenylmethane, and resemble the rhodamines in shade. A typical pyronine may be prepared as follows —Dimethylmetamidophenol is condensed with formaldehyde, and the resulting dioxytetramethyldiamido-diphenylmethane is treated with strong sulphuric acid, whereby one molecule of water is split off, and a leuco-base, tetramethyl-diamidodiphenylmethane-oxide, is formed, according to the equation —... 

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 ... 

A number of dyes are derived from the xanthene structure (xanthene dyes) such as fluorescein 26, eosin 27 and pyronine G 28 ... 

Acridine dyes are amino derivatives of acridine. Acridine yellow G 40 is prepared by a similar method to pyronine G (see p 266) from 2,4-diaminotoluene via the leuco compound 39 by a Bernthsen synthesis. 

The xanthene dyes can be divided into two main groups viz. the biphenylmethane derivatives (pyronins) and the triphenylmethane derivatives (mainly the phthaleins). Among the phthaleins are fluorescein (hydroxy group) and rhodamin (amino group) and the dual compounds (hydroxy and amino). Both fluorescein and rhodamin, currently used as biological dyes, are also photosensitizers. 

---