Precipitates from chemical reactions

Formation of mixtures of products in these reactions can be attributed largely to the properties of the acetate group. The reactions of a number of cycloalkenes with thallium(III) salts have been investigated in some detail and the results obtained have served both to elucidate the stereochemistry of oxythallation and to underline the important role assumed by the anion of the metal salt in these oxidations. The most unambiguous evidence as to the stereochemistry of oxythallation comes from studies by Winstein on the oxythallation of norbornene (VII) and norbornadiene (VIII) with thal-lium(III) acetate in chloroform, in which the adducts (IX) and (X) could be precipitated from the reaction mixture by addition of pentane 128) (Scheme 11). Both by chemical means and by analogy with the oxymercuration... 

The structures of 123-126 were based on spectroscopic data, on combustion analyses, and chemical evidence. Compounds 124a-c showed a characteristic orange color attributed to the local push-pull system of conjugated double bonds and lone pairs. Compounds 125a-c showed a characteristic red color. The results of combustion analysis and spectroscopic data suggested the presence of 3-amino-17/-pyrazole-4,5-dicarbonitrile 126 as a precipitate from the reaction between 121c and 122. The structure of 126 was confirmed by comparison with authentic sample. 

Although all ketones with a hydrogens react with base and I2, only methyl ketones form CHI3 (iodoform), a pale yellow solid that precipitates from the reaction mixture. This reaction is the basis of the iodoform test, once a common chemical method to detect methyl ketones. Methyl ketones give a positive iodoform test (appearance of a yellow solid), whereas other ketones give a negative iodoform test (no change in the reaction mixture). 

Dissolution and precipitation are chemical reactions by which solids pass into and out of solution. A brief introduction and several examples appear in Chapter 11. These reactions involve equilibria between dissolved species and species in the solid state, and so are described by the general principles of chemical equilibrium in Chapter 14. These reactions rank alongside acid-base reactions in practical importance. The dissolution and reprecipitation of solids permit chemists to isolate single products from reaction mixtures and to purify impure solid samples. Understanding the mechanisms of these reactions helps engineers... 

Chemical precipitation is a popular method for synthesizing solid materials from solution in which a liquid-phase reaction is utilized to prepare insoluble compounds. The precipitates are composed of crystalline or amorphous fine particles. Usually, rare earth oxides are prepared by calcinations of the hydroxide or oxalate gel precipitated from a reaction of an aqueous or alcohol solution of inorganic salt (nitrate, chloride, sulfate, and ammonium nitrate, etc.) with an alkali solution (NaOH, NH4OH, and (NH2)2 H20) or an oxalic acid solution [15-21]. However, it is very difficult to obtain ultrafine particles because of growth and sintering of the particles during the calcinations. 

Chemical imidization is not widely used because it employs additional reagents. However, it has the advantage of low temperature imidization and can he nsed to directly form fine polyimide molding powder (16,50). A t5 ical chemical imidization reaction employs a 20-30% solids polyamic acid in an amide solvent with a slight molar excess of acetic anhydride and a molar equivalent of a triamine (triethyl amine, pyridine, or -picoline) (51-54). The percent conversion for chemical imidization is a function of polyimide solnhiUty. If the polymer crystallizes and/or precipitates from the reaction medium, imidization will he incomplete (16,50). Those systems that remain soluble must imdergo thermal treatment to convert any isoimide, and remove residual solvent. The mechanistic routes of chemical imidization are shown in Figure 4, and involve the use of a triamine... 

The hydrochloride salt 38 that is formed upon reduction of the nitro group precipitates from the reaction mixture. Isolating this salt by filtration makes purification easy, as this operation frees 38 from contaminants such as unchanged 35 and all by-products that are soluble in the reaction medium. The 36 that is obtained after treating 38 with aqueous potassium hydroxide is sufficiently pure for use in the next step of the sequence. On the other hand, purification of aniline (6) from the chemical reduction of nitrobenzene is more tedious (Sec. 21.2, Part A). 

Formation of a precipitate. Many chemical reactions take place between substances that are dissolved in bquids. If a soUd appears after two solutions are mixed, a reaction has likely occurred. A solid that is produced as a result of a chemical reaction in solution and that separates from the solution is known as a precipitate. A precipitateforming reaction is shown in Figure 1.2b. 

Dissolve 0-5 g. of the substance in 10 ml. of 50 per cent, alcohol, add 0-5 g. of solid ammonium chloride and about 0 -5 g. of zinc powder. Heat the mixture to boiling, and allow the ensuing chemical reaction to proceed for 5 minutes. Filter from the excess of zinc powder, and teat the filtrate with Tollen s reagent Section 111,70, (i). An immediate black or grey precipitate or a silver mirror indicates the presence of a hydroxyl-amine formed by reduction of the nitro compound. Alternatively, the filtrate may be warmed with Fehling s solution, when cuprous oxide will be precipitated if a hydroxylamine is present. Make certain that the original compound does not aflfect the reagent used. 

Techniques responding to the absolute amount of analyte are called total analysis techniques. Historically, most early analytical methods used total analysis techniques, hence they are often referred to as classical techniques. Mass, volume, and charge are the most common signals for total analysis techniques, and the corresponding techniques are gravimetry (Chapter 8), titrimetry (Chapter 9), and coulometry (Chapter 11). With a few exceptions, the signal in a total analysis technique results from one or more chemical reactions involving the analyte. These reactions may involve any combination of precipitation, acid-base, complexation, or redox chemistry. The stoichiometry of each reaction, however, must be known to solve equation 3.1 for the moles of analyte. 

Gravimetric methods based on precipitation or volatilization reactions require that the analyte, or some other species in the sample, participate in a chemical reaction producing a change in physical state. For example, in direct precipitation gravimetry, a soluble analyte is converted to an insoluble form that precipitates from solution. In some situations, however, the analyte is already present in a form that may be readily separated from its liquid, gas, or solid matrix. When such a separation is possible, the analyte s mass can be directly determined with an appropriate balance. In this section the application of particulate gravimetry is briefly considered. 

The presence of a time limitation suggests that there must be a kinetically controlled interference, possibly arising from a competing chemical reaction. In this case the interference is the possible precipitation of CaCOs. 

Boron trifluoride catalyst may be recovered by distillation, chemical reactions, or a combination of these methods. Ammonia or amines are frequently added to the spent catalyst to form stable coordination compounds that can be separated from the reaction products. Subsequent treatment with sulfuric acid releases boron trifluoride. An organic compound may be added that forms an adduct more stable than that formed by the desired product and boron trifluoride. In another procedure, a fluoride is added to the reaction products to precipitate the boron trifluoride which is then released by heating. Selective solvents may also be employed in recovery procedures (see Catalysts,regeneration). 

The holistic thermodynamic approach based on material (charge, concentration and electron) balances is a firm and valuable tool for a choice of the best a priori conditions of chemical analyses performed in electrolytic systems. Such an approach has been already presented in a series of papers issued in recent years, see [1-4] and references cited therein. In this communication, the approach will be exemplified with electrolytic systems, with special emphasis put on the complex systems where all particular types (acid-base, redox, complexation and precipitation) of chemical equilibria occur in parallel and/or sequentially. All attainable physicochemical knowledge can be involved in calculations and none simplifying assumptions are needed. All analytical prescriptions can be followed. The approach enables all possible (from thermodynamic viewpoint) reactions to be included and all effects resulting from activation barrier(s) and incomplete set of equilibrium data presumed can be tested. The problems involved are presented on some examples of analytical systems considered lately, concerning potentiometric titrations in complex titrand + titrant systems. All calculations were done with use of iterative computer programs MATLAB and DELPHI. 

Sulfur dioxide emissions may affect building stone and ferrous and nonferrous metals. Sulfurous acid, formed from the reaction of sulfur dioxide with moisture, accelerates the corrosion of iron, steel, and zinc. Sulfur oxides react with copper to produce the green patina of copper sulfate on the surface of the copper. Acids in the form of gases, aerosols, or precipitation may chemically erode building materials such as marble, limestone, and dolomite. Of particular concern is the chemical erosion of historical monuments and works of art. Sulfurous and sulfuric acids formed from sulfur dioxide and sulfur trioxide when they react with moisture may also damage paper and leather. 

The net ionic equation shows that Ag+ ions combine with Cl ions to precipitate as solid silver chloride, AgCl (see Fig. 1.5). A net ionic equation focuses our attention on the change that results from the chemical reaction. 

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