Melting point determination: technique

The technique for the removal of solids by filtration with suction has already been described (Section 2.19). The same technique will of course be applied to the collection of recrystallised compounds. Additionally, however, it should be noted that the mother-liquor from a recrystallisation is often of value for the recovery of further quantities of product, and should be transferred to another vessel after the crystals have been drained and washed with solvent. The mother-liquor may be then subsequently concentrated (Section 2.24 suitable precautions being taken, of course, if the solvent is flammable), and a further crop of crystals obtained. Occasionally yet another crop may be produced. The crops thus isolated are generally less pure than the first crystals which separate, and they should be combined and recrystallised from fresh solvent the purity is checked by a melting point determination. 

If the compound dissolved in the hot solvent and crystallized on cooling, the crystals should be obtained and their melting point determined in order to see whether purification has been effected. This may be done by collecting the crystals on a Hirsh or other type of microfunnel. However, the following technique is more convenient. A circle of filter paper about 13 mm in diameter is cut out with a well-sharpened cork borer. This is placed over the mouth of the test tube, and a hollow glass tear (Fig. 2-20) is placed over the filter paper. A retaining wire made from 20-gauge nichrome wire is placed over the tear, and a... 

DSC, however, is not complete in the characterization of the thermal properties of the material. TGA is required to confirm transitions such as volatilizations and decompositions, and a visual technique, such as capillary melting-point determinations or HSM, is required to confirm transitions such as melts and recrystallizations. 

The purity of protected amino acids is especially important for the synthesis of longer peptides. Standard techniques such as melting point determination, nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry, and optical rotation are effective means of characterization. The optical purity can also be evaluated by high-performance liquid chromatography (HPLC) after derivatization with Marfey s reagent [216,217]. The advanced Marfey method refers to analysis by mass spectrometry after derivatization with Marfey s reagent [218-221]. Purification of side-chain protected amino acids by recrystallization is usually sufficient. 

It should be pointed out that melting points determined using the diluent technique are more likely to be equilibrium melting points since there is mobility in the presence of diluent. 

Combine the fractions containing only 2-bromonitrobenzene and then remove the solvent by simple distillation. Alternatively, use rotary evaporation or other techniques to concentrate the solution. Characterize the residue obtained by either melting point determination, TLC, or spectroscopic analysis. 2-Bromonitrobenzene has a low reported melting point and thus may be isolated as an oil. Obtain IR and NMR spectra of your product and compare them with those of an authentic sample (Figs. 15.23 and 15.24). 

Electrical measurements are useful for determining the total content of electrically active impurities in conductors and semiconductors. The shape of the freezing curve gives considerable information on total impurities in a material with a suitable melting point. Special techniques must be used for most metals that melt at high temperatures or for materials that decompose on melting. 

The physical properties of the pyridopyrimidines closely resemble those of their nearest A-heteroeyclie neighbors the quinazolines and the pteridines. Thus, in common with the pteridines, the presence of groups capable of hydrogen-bonding markedly raises the melting point and lowers the solubility. - The acid dissociation constants (pif a values) and ultraviolet absorption spectra of all four parent pyridopyrimidines have been determined by Armarego in a comprehensive study of covalent hydration in these heterocyclic systems. The importance of these techniques in the study of covalent hydration, and... 

However, for most studies, DTA has been mostly used in a qualitative sense as a means to determine the characteristic temperatures of thermally induced reactions. Owing to the experimental conditions used for its measurement, the technique is most useful for the characterization of materials that evolve corrosive gases during the heating process. The technique has been found to be highly useful as a means for compound identification based on the melting point considerations, and has been successfully used in the study of mixtures. 

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