The reaction between phenols and aldehydes has been further studied by Michael, [Footnote: Amer.Jour., 5, 338; 9, 130.] who prepared a condensation product from phenol and resorcinol with benzaldehyde, and Russanow, [Footnote: Ber.9 1889, 22, 1944.] who also employed benzaldehyde and phenol. Lipp [Footnote: Diss., Bern., 1905.] investigated the action of benzaldehyde and piperonal on phenols, anisoles, cresols, cresylic ether, resorcinol, and the ether of the latter and phenol, and showed that when free phenols are condensed with benzaldehyde the hydroxyls occupy the same position as by the interaction between benzaldehyde and the corresponding phenolic ethers. The resulting dihydroxytriphenylmethane derivatives form beautiful crystals, which on oxidation are converted into benzaurines, the constitution of the latter probably being—
O= ^=_____ ^ OH
| | | |
| |== __| |
=v v v
C
|
C_6H_5
In alkalies, the hydroxylated triphenylmethanes dissolve without imparting any colour to the solution; by concentrated sulphuric acid they are taken up with intense coloration.
If the hydroxyls occupy the ortho-position to methyl, they may form xanthenes by splitting off water—
O
^ ^ ^
| | | |
| | | |
CH_3 v v v CH_3
CH
|
C_6H_5
In the benzene series this reaction is difficult to establish, and has to be induced by distilling the particular dihydroxy-diphenylmethane at ordinary pressure. In the naphthalene series, on the other hand, the ring closes up by, for instance, the condensation of [Greek: b]-naphthol with benzaldehyde or paraldehyde, and yields the following compounds:—
C_10H_6 C_10H_6 C_6H_5-CH{ }O CH_3-CH{ }O C_10H_6 C_10H_6
These xanthenes are white, silk-glossy needles, which are soluble in water and in alkalies. In concentrated sulphuric acid, they are taken up with beautiful fluorescence.
6. Summary
From the qualitative reactions of the different condensation products described it may be seen that their tannoid properties are not dependent on whether they precipitate gelatine or are adsorbed by hide powder or not. Hydroxynaphthylmethanesulphonic acid, for instance, precipitates gelatine but does convert pelt into leather; on the other hand, sodium dicresylmethanesulphonate does not precipitate gelatine, and neither does it tan pelt; nevertheless it is adsorbed by hide powder as "tanning matter". The author discovered that o-nitrophenol does not precipitate gelatine, but has some tanning action on both hide powder and pelt.