Alcohol, generally precipitation

Solubility is a very important criterium for the different uses of polysaccharides in pharmacy. Table 4 lists the solubility criteria of some polysaccharides in water. Solutions containing sugars and alcohol generally depress the solubility of polysaccharides. Polysaccharides containing carboxyl groups, i.e., pectins, alginates, and carboxy-methylcellulose, are insoluble at low pH values. They will be precipitated when the pH is lowered below 3. 

Precipitation also can be induced by additives, a process generally called salting out because salts with ions common to those whose precipitation is desired are often used for this purpose. For instance, ammonium chloride is recovered from spent Solvay liquors by addition of sodium chloride and the solubility of BaCl2 can be reduced from 32% to 0.1% by addition of 32% of CaCl2. Other kinds of precipitants also are used, for instance, alcohol to precipitate aluminum sulfate from aqueous solutions. 

Polyamide (PA) powders are generally precipitated from solution. Ethylene-vinyl alcohol copolymer (EVOH) powders are precipitated by hydrolyzing ethylene-vinyl acetate copolymer solutions. 

The solubihty of alkylphenols in water falls off precipitously as the number of carbons attached to the ring increases. They are generally soluble in common organic solvents acetone, alcohols, hydrocarbons, toluene. Solubihty in alcohols or heptane follows the generalization that "like dissolves like." The more polar the alkylphenol, the greater its solubihty in alcohols, but not in ahphatic hydrocarbons likewise with cresols and xylenols. The solubihty of an alkylphenol in a hydrocarbon solvent increases as the number of carbon atoms in the alkyl chain increases. High purity para substituted phenols, through Cg, can be obtained by crystallization from heptane. 

Aqueous formaldehyde, known as formalin, is usually 37 wt % formaldehyde, though more concentrated solutions are available. Formalin is the general-purpose formaldehyde of commerce suppHed unstabiLized or methanol-stabilized. The latter may be stored at room temperature without precipitation of soHd formaldehyde polymers because it contains 5 —10% methyl alcohol. The uiiinhibited type must be maintained at a temperature of at least 32°C to prevent the separation of soHd formaldehyde polymers. Large quantities are often suppHed in more concentrated solutions. Formalin at 44,... 

The nitrites aie most conveniently prepared from the corresponding alcohols by treatment with nitrosyl chloride in pyridine. The crude nitrites can be precipitated by addition of water and recrystallized from appropriate solvents. However nitrites prepared from carbinols in which the adjacent carbon is substituted by halogen, free or esterified hydroxyl or a carbonyl function are very readily hydrolyzed and must be recrystallized with great care. In general the photolysis gives higher yields if purified and dried nitrites are used which do not contain acids or pyridine, although occasionally the addition of small amounts of pyridine is recommended in order to prevent hydrolysis of the nitrite. Traces of acids do in fact catalyze the thermal decomposition of secondary nitrites to equimolar amounts of alcohol and ketone. ... 

B. General Oxidation Procedure for Alcohols. A sufficient quantity of a 5% solution of dipyridine chromium (VI) oxide (Note 1) in anhydrous dichloromethane (Note 7) is prepared to provide a sixfold molar ratio of complex to alcohol. This excess is usually required for complete oxidation to the aldehyde. The freshly prepared, pure complex dissolves completely in dichloromethane at 25° at 5% concentration to give a deep red solution, but solutions usually contain small amounts of brown, insoluble material when prepared from crude complex (Note 8). The alcohol, either pure or as a solution in anhydrous methylene chloride, is added to the red solution in one portion with stirring at room temperature or lower. The oxidation of unhindered primary (and secondary) alcohols proceeds to completion within 5 minutes to 15 minutes at 25° with deposition of brownish-black, polymeric, reduced chromium-pyridine products (Note 9). When deposition of reduced chromium compounds is complete (monitoring the reaction by gas chromatography or thin-layer chromatography analysis is helpful), the supernatant liquid is decanted from the (usually tarry) precipitate and the precipitate is rinsed thoroughly with dichloromethane (Note 10). 

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