Syntans: abbreviation for synthetic tannins. 
It covers substances which are manufactured to replace partially or completely, the natural vegetable extracts, to accelerate production and to make it cheaper.
Introduction of new functional groups which do not occur in vegetable tannins such as sulfonic groups opened new prospects for tanning. Syntans are frequently used in retanning chrome leather. There are replacement syntans and auxiliary (assisting to the tanning process) syntans.
Sizes of 800 or less, which correspond to 2-4 aromatic rings are of practical importance. Molecules of greater sizes do not tan or tan only superficially.
Bonds between aromatic rings in syntans are essential for their properties. For example extension of a molecule by a CH₂ member weakens tanning ability of syntan. Replacement of such a bridge by a sulfonyl group one increases tanning and improves resistance to light. Resistance to light is also improved by a -SO₂-NH- sulfonamide sequence. Bonding groups can be put in the following order of increasing light resistance:

-CH₂-<-CHR-<-CH₂NH-CO-NHCH₂-<-CR₂-<-CO₂-<-SO₂-NH-

Apart from the bridges connecting Ar rings, the amount, kind and position of functional groups built into these rings are significant. Only hydroxyls, sulfo groups and amino groups have been applied.
Thus factors characterizing a syntan are:
1)  number and kind of rings in the molecule
2)  number and kind of functional groups
3)  molecular weight
Syntans may contain chromium components as well. Syntans produced in industry generally are not chemical individua, chemical species, generally one speaks of an average of the properties mentioned.
Hydroxyls: attached to aromatic rings,have phenolic character; it is desirable to introduce them into the molecule in greatest possible amount. Regular distribution of hydroxyls is adventageous since maximum possible conjugated bonds can be formed. 
It is known that derivatives of a-napthol are better tanning agents than those of  b-napthol. 
Presence of other substituents in the ring influences reactivity. R substituents decrease tanning ability of syntans; sulfo and carboxylic groups in orto and para position improve the tanning ability, whereas they decrease this ability in meta position. As  rule hydroxylic groups do not occur in syntan side chains.
Sulfonic group: as a substituent on the aromatic ring, it is strongly dissociated, and gives acidic character to the compound.It affects the tanning ability negatively but increases its water solubility. As a rule compound which have one sulfonic group per 3 to 4 rings are applied.
Amino groups: occur sometimes in syntans, may act as hydrophiles, however their action is limited to acidic medium only. If the amino groups are the only functional groups they participate in the binding of collagen. They probably form H-bonds with oxygen from the peptide group. The solubility of syntans containing only amino groups decreases with the acidity of the solution, whereas under the same conditions its ability to bind to hide increases.
Among synthetic tannins of special importance are chromium salts, complexed with condensation products of benzene and napthalene derivatives.For example, Rotanin CR is a product of this kind containing 12-13% of Cr₂O₃ and derivatives of napthalenesulphonic, hydroxybenzenesulfonic and o-pthalic acids.It is a tanning agent that may be applied alone; it gives very bright leather of Ts about 100 C, of properties similar to those of chromium leather.
Auxiliary syntans may be dispersing agents,bleaching agents,resin tannins etc. This group leads us to macromolecular products produced in industry of many classes(novolacs, diene and acrylic resins etc.).