Relatively to the possibilities of forming condensation products possessing tannoid properties, the following may be stated:—
All mono- and polyhydric phenols may be converted into true tanning matters by either condensing them as such, or after their conversion into the corresponding sulphonic acids, by substances capable of eliminating the elements of water. It makes no difference to the final product whether the condensation is the first step followed by sulphonation and consequent solubilisation of the intermediary insoluble product, or whether, vice versa, the sulphonic acid is subjected to condensation. Alkaline solution of phenols may also be condensed, the reaction products, when condensed, constituting tanning matters soluble in water.
Among the substitution products of the phenols, the thio-, chloro-, bromo-, nitro-, and aminophenols as a rule yield tanning matters similar in character.
The quinones are as such—i.e., without being condensed—substances possessing tannoid properties.
The aromatic dihydric alcohols are easily condensed with the different sulphonic acids and yield valuable tanning matters.
Of aromatic acids all those which yield water-soluble sulphonation products seem suitable for the industrial production of tanning matters. If the acids themselves do not yield water-soluble sulphonation products, the alkali salts of the latter may be condensed with formaldehyde, and the resulting products then constitute tanning matters provided their solutions can be neutralised or faintly acidified without the solute being thrown out of solution in insoluble form.
The diphenyl derivatives of the above groups often possess tannoid properties.
The same holds good of those compounds with condensed nuclei (naphthalene, anthracene, etc.), and all their derivatives which satisfy the above conditions.
The choice of condensing agent is, as a rule, of little significance. Elimination of the elements of water by the mere application of heat succeeds in few cases only, since the high temperature required to induce reaction in many cases causes decomposition of the substances. This difficulty is overcome by heating in vacuo. Condensation with formaldehyde always succeeds, with acetaldehyde and benzaldehyde only partly.
The action on hide powder, pelt, and gelatine by these characteristic
substances is tabulated below:—
Relative Behaviour towards
Substance. Gelatine. Hide Powder Pelt
Formaldehyde … … Tanning
Phenol Ppte. … …
Chlorophenol " … …
Surface
Tribromophenol Slight ppte Tanning tanning
o Nitrophenol No ppte " "
Bromonitrophenol Slight ppte " "
Trinitrophenol Ppte " Tanning
Bromotrinitrophenol Slight ppte " "
p Aminophenol Ppte … …
m Dihydroxybenzene " … …
Orcinol " … …
p Dihydroxybenzene " Tanning Tanning
Monochloro p
dihydroxybenzene " … …
o Dihydroxybenzene " … …
Pyrogallic acid " … …
Surface
Tribromopyrogallic acid " Tanning Tanning
Gallic acid No ppte Not tanning Not tanning
Bromophloroglucinol Ppte Tanning "
Gallotannic acid " " Tanning
Galloflavine Slight ppte " Not tanning
Quinone " " Tanning
Bromosalicylic acid " " Not tanning
Dinaphthylmethanedisulphonic acid Ppte " Tanning
Diphenylmethanedisulphonic acid " " "
Dicresylmethanedisulphonic acid " " "
Sodium
dicresylmethanedisulphonate acid No ppte " Not tanning
Dixylylmethanedisulphonic acid Ppte " Tanning
Naphtholdisulphonic acid " Not tanning Not tanning
Methylenedinaphthol " Tanning Tanning
Hydroxyphenylmethanesulphonic " " "
acid Not tanning
Hydroxynaphthylmethanesulphonic Slight ppte " "
acid
Diaminonaphthylmethanedisulphonic Ppte Tanning Not tanning
acid
Dihydroxynaphthylmethanedisulphonic
acid " " "
Dichloronaphthylmethanedisulphonic
acid " " Surface tanning
Dinitronaphthylmethanedisulphonic
acid " " "
Dithionaphthylmethanedisulphonic
acid " " Tanning
Bromo [Greek: b] naphthol [1] Slight ppte " "
Rosolic acid_ [1] Ppte " "