Mixtures, melting-point determinations

Amino-4-r-butylphenol and 2-amino-4-chlorophenol behaved similarly under identical conditions. A solid product was obtained in each case which melted at 205.5-207 C (4) and 206-207 C (5), respectively. A mixture melting point determination of a mixture of the two materials melted over a range (179-186 C) to confirm the individual identities. Spectral data for both products are in the Experimental. 

In several runs individual subunits were further degraded to locate the position of labeling in the subunits. The routes employed are indicated in Fig. 4. Periodate oxidation was essentially complete within a few minutes and all runs were stopped (by addition of barium hydroxide) after hour in order to avoid overoxidation. Formic acid was absorbed on Dowex-1 (carbonate form) and was later removed from the resin by washing with Q. N sulfuric acid, and ultimately isolated as barium formate. Formaldehyde, which was not absorbed by the resin, was isolated as methylene-bis-methone and identified by melting point and mixture melting point determinations. 

In the experience of the present author, minor deviations from this procedure may result in decreased yields. Oxalacetic acid of high quality is essential, and this should be verified by a melting-point determination prior to use. Decarboxylation of oxalacetate has been reported111 to occur rapidly at pH 7, and it should be kept to a minimum by maintaining the pH as close to 10 as possible when dissolving the oxalacetic acid. A modification of the Comforth reaction is the co-balt(II)-ion-catalyzed condensation of D-erythrose 4-phosphate with oxalacetate to give 3-deoxyheptulosonic acid 7-phosphate112 (as a mixture of the arabino and ribo isomers). Other procedures for the preparation of KDO will be discussed in subsections 3 and 4 of this Section. 

The procedure known as the mixed melting point determination, whereby the melting range of a solid under test is compared with that of an intimate mixture of equal parts of the solid and an authentic specimen of it, may be used as a confirmatory identification test. Agreement of the observations on the original and the mixture usually constitutes reliable evidence of chemical identity. 

Mix 0.05 mole of benzaldehyde with the theoretical quantity of acetone, add one-half the mixture to a solution of 5 g of sodium hydroxide dissolved in 50 mLofwaterand40 mL ofethanol at room temperature (<25°C). After 15 min add the remainder of the aldehyde-ketone mixture and rinse the container with a little ethanol to complete the transfer. After one-half hour, during which time the mixture is swirled frequently, collect the product by suction filtration on a Buchner funnel. Break the suction and carefully pour 100 mL of water on the product. Reapply the vacuum. Repeat this process three times in order to remove all traces of sodium hydroxide. Finally, press the product as dry as possible on the filter using a cork, then press it between sheets of filter paper to remove as much water as possible. Save a small sample for melting point determination and then recrystallize the product from ethanol using about 10 mL of ethanol for each 4 g of dibenzalacetone. Pure dibenzalacetone melts at IIO-IITC, and the yield after recrystallization should be about 4 g. 

When all the nitric acid has been added, warm the mixture to room temperature and after 15 min pour it on 50 g of cracked ice in a 250-mL beaker. Isolate the solid product by suction filtration using a small Buchner funnel and wash well with water, then with two 10-mL portions of ice-cold methanol. A small sample is saved for a melting point determination. The remainder is weighed and crystallized from an equal weight of methanol. The crude product should be obtained in about 80% yield and with a mp of 74-76°C. The recrystallized product should have a mp of 78°C. 

The first total synthesis of ( + )-dauricine was reported by Kametani and Fukumoto in 1964 (18,19). Arndt-Eistert reaction of homoveratryl-amine with the acid chloride XIX afforded the amide XXII. Bischler-Napieralski cyclization of the above amide gave the dihydro isoquinoline derivative XXIV, the methiodide of which when reduced with zinc dust and ethanol-hydrochloric acid afforded + )-dauricine. The identity of the synthetic product with ( )-dauricine w as concluded through a comparison of its physical properties (spectra and chromatographic behavior) with those of an authentic sample of the alkaloid. Melting-point determination of a mixture of derivatives of the two specimens is not recorded. 

A synthetic route to ( )-0,f>-dimethyltubocurarine iodide (CXXV), via the racemate of 0,0-dimethylbebeerine (CXXIII), was announced in 1959 by Tolkachev and his collaborators (94). It started by the condensation of 3-methoxy-4-hydroxyphenethylamine with 4-benzyloxy-phenylacetic acid to give the amide CXXVI. Reaction of the potassium salt of the latter with the methyl ester of 3-bromo-4-methoxyphenyl-acetic acid in the presence of copper powder gave compound CXXVII. This on condensation with 3-methoxy-4-hydroxy-5-bromophenethyl-amine afforded compound CXXVIII, which was methylated to CXXIX. The latter compound was cyclized with phosphorous oxychloride to the dihydroisoquinoline derivative CXXX. Debenzylation of CXXX followed by intramolecular Ullmann condensation yielded compound CXXXI. The latter was converted to racemic dimethylbebeerine (CXXIII) by reduction with zinc dust in acetic acid followed by methyla-tion. Finally, treatment of ( + )-CXXIII with methyl iodide furnished the dimethyl ether of ( + )-tubocurarine iodide, identified by comparison of its UV-spectrum with that of the dimethyl ether of natural tubo-curarine iodide and by melting-point determination of a mixture of the two specimens. 

In the Phillips process, polyphenylene sulfide (PPS) is obtained from the polymerization mixture in the form of a fine white powder, which, after purification, is designated Ryton V PPS. Characterization of this polymer is complicated by its extreme insolubility in most solvents. At elevated temperatures, however, Ryton V PPS is soluble to a limited extent in some aromatic and chlorinated aromatic solvents and in certain heterocyclic compounds. The inherent viscosity, measured at 206°C in 1-chloronaphthalene, is generally 0.16, indicating only moderate molecular weight. The polymer is highly crystalline, as shown by x-ray diffraction studies (9). The crystalline melting point determined by differential thermal analysis is about 285°C. 

The experimental conditions of DTA necessitate that the AT parameter be plotted as a function of sample temperature, while in DSC the differential power curve is recorded as a function of time. For melting-point determinations, the sample may be encapsulated in the case where the thermocouple must be inserted into the sample, a sample diluent mixture must be employed. The encapsulations are usually in a tightly sealed metal container of high thermal conductivity. In the case of boiling-point determinations, provision must be made for (1) equilibration of liquid and vapor and (2) control of atmospheric pressure. Barrall (39) described in detail how these two criteria can be met. 

In Experiment 3D of this experiment, you will determine the identity of an unknown using the melting point technique. The mixture melting point technique is introduced in this part. 

Finally, if (X) is a solid, an intimate mixture of it with an authentic specimen is subjected to a melting point determination (mined melting point). If the melting points of (X) and (P) separately do not differ by more than 2° from the mixed melting point, it can be concluded that (X) is identical with (P). 

If a solution of the tested substance forms a precipitate on the addition of a 2,4-dinitrophenylhydrazine solution in 2N HCl, this represents a detection of the presence of a carbonyl compormd. If the precipitate is isolated and purified by crystallization and its melting point determined, identification has been made, because on comparison with the published data or on measuring the mixture melting point with an authentic sample we can determine the identity of the original substance. 

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