Tanning materials Reactions

The principal reactions and tests used for the identification of different tanning materials and for the detection of adulterations are described briefly below. 

Group III comprises tanning materials containing tannins derived from pyrogallol, and are not precipitated by bromine water and give a bluish-black coloration with ferric alum. These also are subdivided into (a) those which give the above characteristic reaction with nitrous acid, and (6) those which give no reaction or only a brown colour. 

The reactions involved in the chrome-tanning process are those of coordination complexes. They involve the interaction between charged carboxyl groups on the collagen macromolecule and polynuclear chromium(III) coordination compounds. The most widely used chrome-tanning material is 33% basic chromium(III) sulfate produced industrially by reducing sodium dichromate with sulfur dioxide. 

Sihcon nitride occurs in two forms, a-Si N and P-Si N. Pure Si N is white, but the colors of commercial materials may be tan, gray, or black because of residual siUcon or impurities. Si N may be prepared by nitriding siUcon powder at 1200—1400°C or, for extremely fine-grained Si N, by the reaction of SiCl or SiH and N2 or NH (see also Advanced ceramics). 

The Rheometric Scientific RDA II dynamic analy2er is designed for characteri2ation of polymer melts and soHds in the form of rectangular bars. It makes computer-controUed measurements of dynamic shear viscosity, elastic modulus, loss modulus, tan 5, and linear thermal expansion coefficient over a temperature range of ambient to 600°C (—150°C optional) at frequencies 10 -500 rad/s. It is particularly useful for the characteri2ation of materials that experience considerable changes in properties because of thermal transitions or chemical reactions. 

Melting points were taken at Intervals in order to gain an idea of the extent of reaction. The final residue was boiled with alcohol but since the solid exhibited insufficient solubility in the hot solvent, the mixture was filtered. The residue consisted of tan crystals, MP about 220°-221 G, and the filtrate on cooling gave an additional crop of tan crystals, MP about 219°-221°C. The two materials were identical and consisted of diphenimide. 

Fig. 21. Loss tangent, tan 3, for five partially crosslinked PDMS samples at different extents of reactions. For the liquid, a negative slope is observed. At the LST, tan 3 is independent of frequency. The loss tangent of the solid material exhibits a positive slope...

Note 12). After a total reaction time of 30 minutes (Note 13), during which the mixture has become almost solid and is very difficult to stir, the material is filtered hot (Note 14). The yield of 3,5-dibromosulfanilamide is 85-90 g. (90-94%), and the crude tan product (Note 15) melts over a range of 1-2° in the region 230-237° (Note 16). 

A. Benzenediazonium-2-carboxylate. A solution of 34.2 g. (0.25 mole) of anthranilic acid (Note 1) and 0.3 g. of trichloroacetic acid (Note 2) in 250 ml. of tetrahydrofuran (Note 3) is prepared in a 600-ml. beaker equipped with a thermometer and cooled in an ice-water bath. The solution is stirred magnetically, and 55 ml. (48 g., 0.41 mole) of isoamyl nitrite (Note 4) is added over a period of 1-2 minutes. A mildly exothermic reaction occurs, and the reaction mixture is maintained at 18-25° and stirred for a further 1-1.5 hours. A transient orange to brick-red precipitate may appear (Note 5) which is slowly converted to the tan product. When the reaction is completed, the mixture is cooled to 10°, and the product is collected by suction filtration on a plastic Buchner funnel and washed on the funnel with cold tetrahydrofuran until the washings are colorless. (Caution The filter cake should not be allowed to become dry.) The benzene-diazonium-2-carboxylatc is then washed with two 50-ml. portions of 1,2-dichloroethane to displace the tetrahydrofuran, and the solvent-wet material is used in the next step (Notes 6, 7, and 8). 

C. N-Hydroxy-4-(p-chlorophenyl)thiazole-2(3H)-thione. O-Ethyl S-[2-oximino-2-(p-chlorophenyl)ethyl]dithiocarbonate (56.0 g, 0.19 mol) is placed in a 500-mL round-bottomed flask that is equipped with a magnetic stir bar. Diethyl ether (120 mL) is added and the slurry is treated at 0°C in small portions with solid anhydrous zinc chloride, ZnClj, 79.1 g, 0.58 mol) at such a rate that the solvent does not boil constantly (Note 8). After the addition is complete, the flask is stoppered with a drying tube (CaCl2) and stirring is continued for 48 hr at 20°C. The reaction mixture turns into a dear, dark brown solution that solidifies toward the end of the reaction. The flask is immersed in an ice bath and treated dropwise with 5.5 M hydrochloric add (140 mL, Note 9). The precipitate dissolves immediately. Stirring is continued for 30 min at 0°C whereupon a tan-colored solid separates. This material is collected by filtration. It is washed with small portions of diethyl ether (total of 110 mL) and dried to afford 39.8 g (86%) of N-hydroxy-4-(p-chlorophenyl)thiazole-2(3H)-thione (Note 10). The crude material is transferred to a 2-L, round-bottomed flask equipped with a reflux condenser. 2-Propanol (760 mL) is added and the reaction mixture is heated to reflux. Once a dear solution is obtained the heat source is immediately removed (Note 11). The solution is allowed to cool to room temperature. Precipitation of N-hydroxy-4-(p-chlorophenyl)thiazole-2(3H)-thione is completed by immersing the flask for 30 min in an acetone-dry ice bath (-78°C). The product is collected by filtration and dried to afford 21.9 g (53.5%) of N-hydroxy-4-(p-chlorophenyl)thiazole-2(3H)-thione as tan crystals (Notes 12,13). 

Indeed, the Na-HMPA route consistently provided the cleanest products and has been the only synthesis to provide solutions of Na3[M(CO)4]. It is often important to use solutions rather than slurries of trianion salts to minimize the formation of side products during the reactions of these materials with electrophiles. Until recently, product separation from the viscous and high-boiling HMPA has always been a problem (and remains so in some cases). For example, addition of excess THF to solutions of Na3[M(CO)4] in HMPA invariably resulted in the formation of sticky solids that contained HMPA and did not analyze satisfactorily (14). But recently, it was discovered that addition of these HMPA solutions to excess liquid ammonia resulted in practically quantitative precipitation of tan to pale yellow brown solids, which provided satisfactory elemental analyses of unsolvated Na3[M(CO)4] (M = Mn, Re). Virtually all impurities remained in the HMPA-NHj filtrate [Eqs. (4) and (5)]. 

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