Purity of starting materials

Careful control of chemical composition and purity of starting materials... 

As a consequence of these results the customary linear correction mentioned appears unwarranted. Preferably, the enantiomeric or optical purity of starting materials and/or reagents has to be stated explicitly. 

Analytical methods are required to assure the purity of starting materials and can be used to monitor a synthetic process to determine if it has reached completion. After synthesis, analysis is required to determine percent yield and to separate, isolate, and quantitate by-products of the reaction. 

Much attention is given in the literature to the question of cleanliness and the purity of starting materials. The importance of these precautions varies depending on the work to be undertaken but the use of doubly distilled de-ionised water as a subphase has become a standard precaution. The use of disposable surgical gloves when handling pieces of apparatus which will have contact with the subphase is also a wise precaution. 

LaNiSb was found to crystallize with the ZrBeSi type with lattice parameters of a = 0.4404, c = 0.8403 (Hartjes and Jeitschko, 1995 X-ray powder diffraction). Cold-pressed pellets of the ideal composition were arc melted in an atmosphere of argon and annealed at 1120 K for one week. Purities of starting materials were better than 99.9%. 

La-RuSb. The crystal structure of the LaRu4Sbi2 compound was studied by Braun and Jeitschko (1980). It was found to crystallize with the LaFe4Pi2 type, a = 0.92700 (X-ray powder diffraction). The purities of starting materials and experimental procedure were the same as for LaFe4Sbi2. 

Two impurities are apparent in the product, iron (III) chloride and hexachloroethane. Both are more volatile than zirconium tetrachloride. Nevertheless, if both are to be removed from the product, considerable product inevitably sublimes beyond K. The amount of hexachloroethane impurity is roughly a function of temperature, since runs made at 450° yield virtually none, whereas if the furnace is allowed to go much above 500°, appreciable quantities are formed. The amount of iron (III) chloride impurity depends on the purity of starting materials. ... 

X 10 4 M Cytochrome c—Quantities used depend on purity of starting material. The molecular weight is 13,000. Prepare 50 ml of this solution. Refrigerate. Stable for 1 week. 

All of the general precautions that hold for chemical crystal growth methods (Section III.A.l) must be observed in electrocrystallization experiments also (purity of starting materials and solvent light- and vibration-free environment). If a radical-cation salt is to be prepared, 5 mL of donor solution (1 to 5 mM) is placed in the half-cell that contains the anode. 

In spite of these reported successes, the Madelung cyclization offers the following disadvantages (i) the drastic conditions necessary limit its application to the synthesis of unsubstituted or alkyl- and aryl-substituted azaindoles (ii) the reaction is often, if not always, intractable and Hi) yields are erratic, often unreproducible, and subject to such variables as stirring and heating efficiency, purity of starting materials, quality (or manufacturer ) of reagents, and operator. 

Solubility ratios of the desirable compound over the undesirable impurities and the purity of starting material limit the maximum theoretical yield achievable with acceptable purity for the desirable compound. Continuing the previous example, let us assume that the crude starting material contains 15 wt% of one key impurity and the solubility of this impurity is identical to the solubihty of the desirable compound. If we charge 100 gm of crude solid, which has 85 gm of desired compound and 15 gm of impurity, into 1 liter of solvent, the final crystallized product will contain... 

Two research groups studied the syntheses of 36 different macrocycles (Atkins et al., 1978 Rasshofer et al., 1976). The yields averaged about 60% but varied from 7% to 90%. The yields of certain macrocycles prepared by both research groups varied by as much as 50%. These variations can be attributed to different reaction conditions, particularly in the methods used to prepare the amide salts, reaction temperatures, purity of starting materials, and reaction times. Careful drying of the solvent and prevention of atmospheric moisture from contacting the moisture sensitive amide salts generally increases the yields for the cyclization step in these reactions (Bhula et al., 1988). 

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