Melting point of a mixture

We are now in a position to understand why the melting point of a mixture is lower than that of the pure host. Previously, when we considered the melting of a simple single-component system, we framed our thinking in terms of the molar Gibbs function Gm. 

Actually, taking a mixture melting point, the melting point of a mixture, is more correct. But I have seen this expressed both ways. 

Eutectic melting is a well-known phenomenon that lowers the melting point of a mixture below the melting point of each pure compound in the mixture [4]. 

A 2.0-gram sample of the oil was boiled with 50 ml. of N sodium hydroxide for 4 hours under reflux, and then the mixture was distilled to yield 20 ml. of distillate. Ethanol was identified in the distillate by gas chromatography. The alkaline aqueous residue was acidified with dilute sulfuric acid and extracted with ether. The ether was evaporated to yield a solid mass, which was recrystallized from water to yield colorless crystals, melting at 199°-201°C. and not depressing the melting point of a mixture with authentic o-phthalic acid. An infrated spectrum of this compound was identical with that of authentic o-phthalic acid. 

We must now consider what is meant by the melting point of a mixture. Assuming that the mixture will not form a solid solution and that the impurity is present in relatively low concentration, a plot of the temperature of a slowly cooling mixture against time as shown in Fig. 2-11 is obtained. As the temperature of the solution drops, one will... 

Various reactions of obaberine, such as the Hofmann degradation and reductive cleavage, suggested that it is identical with 0-methyloxy-acanthine (LXXIII) (46). The identity was confirmed when, in the course of this work, 0-methyloxyacanthine was obtained for the first time in crystalline form and the melting point of a mixture of the two specimens was determined. 

Wilhelm Heinrich Heintz (Berlin, 4 November 1817-Halle, i December 1880), D.Phil. Berlin, was at first an apothecary. He was associate (1851) and full (1857) professor in Halle, and director of the Pharmaceutical Institute. He published much work on organic chemistry. In his work on fatty acids he paid attention to J. Gottlieb s discovery that the melting-point of a mixture of fatty acids is lower than the mean or even of one of the acids. 

The Nef reaction was used solving O.lg (0.0005 mol) of 6A in 5 water ethanol) and after stirring hour, the pH of the solution was adjusted to 7 with 50% HCl. A yellowish white solid (65%) precipitated out and was filtered, dried and recrystallized from ethanol-water, m.p. 140-46 C. 4A obtained from oxidation of 3A (see Scheme 1) melted at 146-48°C. There was no depression in the melting point of a mixture of the two samples. The infrared spectrum was identical to that of 4A prepared by oxidation of 3A. 

The general case of two compounds forming a continuous series of solid solutions may now be considered. The components are completely miscible in the sohd state and also in the hquid state. Three different types of curves are known. The most important is that in which the freezing points (or melting points) of all mixtures lie between the freezing points (or melting points) of the pure components. The equilibrium diagram is shown in Fig. 7, 76, 1. The hquidus curve portrays the composition of the hquid phase in equihbrium with sohd, the composition of... 

The attitude we adopt in this discussion is that only those chain segments in the middle of the chain possess sufficient regularity to crystallize. Hence we picture crystallization occurring from a mixture in which the concentration of crystallizable units is Xj and the concentration of solute or diluent is Xg. The effect of solute on the freezing (melting) point of a solvent is a well-known result T j, is lowered. Standard thermodynamic analysis yields the relationship... 

A quasi-racemate or pseudo-racemate is a true racemate like molecular compound formed between optical antipode of different (but related) compounds. The quasi-racemate also has a melting point ciin c resembling the curve of a true racemate but with quasi-racemic compounds the curves are unsymmetrical, because the melting points of the components are different as shown in Fig. (9.3). The curve A represents the melting point of a true-racemate formed by mixing (+) mandelic acid XXII and (-) hexahydromandelic acid XXIII while B represents that of a mixture of (+) XXII and (+) hexa hydro-mandelic acid XXIII. 

In contrast with the classical synthesis, this reaction occurs in one step, in the absence of a solvent and the formed oxadiazole is easily isolated from the other reaction products. Even in the case of non- or mono-substituted oxadiazoles, the new reaction affords comparable or improved 3delds. The process is quite general. It concerns all kinds of amides and amidoxime salts and is generally carried out at the melting point of the mixture (100—200° C). 

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