Melting point of derivatives

The melting points of derivatives of selected alcohols are collected in Table 111,27. 

Table 111,74 lists the melting points of derivatives of some selected aliphatic and cycloaliphatic ketones. 

Capillary gc/ms, hplc, nmr, ir, and uv are all analytical methods used by the terpene chemist with a good Hbrary of reference spectra, capillary gc/ms is probably the most important method used in dealing with the more volatile terpenes used in the davor and fragrance industry (see Flavors and spices). The physical properties of density, refractive index, boiling point, melting point of derivatives, and specific rotation are used less frequendy but are important in defining product specifications. 

Once the unknown is determined to be a ketone or an aldehyde. the melting point of the imine derivative is determined. The melting point and other physical characteristics (i.e.. solubility of unknown and boiling point or melting point of the unknown) are used to determine the unknown s identity. The information is compared to tables in books which contain the melting points of derivatives and other physical data for organic compounds. 

TABLE 1 Melting Points of Derivatives of Chapter 30 Aldehydes and Ketones Some Aldehydes and Ketones 309... 

Suppose, for example, that we are trying to identify an unknown liquid of formula CgHio and boiling point 137-139 that we have shown. in other ways to be an alkylbenzene (Sec. 12.22). Looking in Table 12.1 (p. 375), we find that it could be any one of four compounds o-, w-, or p-xylene, or ethylbenzene. As shown below, oxidation of each of these possible hydrocarbons yields a different acid, and these acids can readily be distinguished from each other by their melting points or the melting points of derivatives. 

Exhaustive tables of physical constants—for example, melting points of derivatives —should be an integral supplement to a handbook on organic analysis. However, the size of this book does not allow us to insert these tables, and lists of melting points of derivatives are only occasionally given as examples. We plan to publish these tables in a separate volume. 

From the list of melting points of derivatives of halo acids given in Table 10 it is evident that for the identification of halo acids, amides, anilides and /7-toluides are suitable / -bromophenacyl esters were prepared only in a few cases. If satisfactory results cannot be obtained by the application of procedures given for the preparation of carboxylic acid derivatives (see p. 249), original literature should be consulted. Identification of aromatic, halo acids is easy because they are well-crystallized solids, and, in addition the preparation of derivatives, for example, p-bromophenacyl esters, causes no difficulty. 

In view of the indefinite melting-points of many sugars and of their readily obtainable derivatives e.g., osazones, cf. p. 139), their identification should be based primarily on their chemical properties. Their rotatory power can often be used for identification purposes, but is not considered in this book. 

With some acids (e.g., succinic acid and sulplianilic acid) more satisfactory results are obtained by reversing the order of mixing, i.e., by adding the solution of the so um salt of the acid to the reagent. It should be pointed out that the melting points of the derivatives as determined on the electric hot plate (Fig. II, 11, 1) may differ by 2-3° from those obtained by the capillary tube method. In view of the proximity of the melting points of the derivatives of many acids, the mixed m.p. test (Section 1,17) should be applied. 

Add 0 -5 ml. of phenyl isothiocyanate to the distillate and shake the mixture vigorously for 3-4 minutes. If no derivative separates, crystallisation may be induced by cooling the flask in ice and scratching the walls with a glass rod. Filter off the crude product, wash it with a little 50 per nent. ethanol, and recrystaUise from hot dilute alcohol. (See Table 111,123 for melting points of phenylthiourea derivatives of amines.)... 

The melting points of the derivatives of a number of amino acids are collected in Table 111,132. Most a-amino acids decompose on heating so that the melting points would be more accurately described as decomposition points the latter vary somewhat with the rate of heating and the figures given are those obtained upon rapid heating. 

The melting points of a few derivatives of selected polyhydric alcohols are collected in the following table. 

The melting points of the derivatives of a number of selected primary and secondary aromatic amines are given in Tables IV, 100A and IV, 1OOB respectively. 

The melting points of some 0-aryl saccharin derivatives are phenol, 182° o-cresol, 163° m-cresol, 146° p-cresol, 172° o-nitrophenol, 236° p-nitrophenol, 192°. 

The melting points of the various derivatives of a number of typical aromatic aldehydes are collected in Table IV,135,... 

The derivative selected in any particular instance should be one which clearly singles out one compound from among all the possibilities and thus enables an unequivocal choice to be made. The melting points of the derivatives to be compared should differ by at least 5-10°. Whenever possible, a derivative should be selected which has a neutralisation equivalent as well as a melting point (e.g., an aryloxyacetic acid derivative of a phenol. Section IV,114,4, or a hydrogen S nitrophthalate of an alcohol. Section 111,25,5). 

The methods of preparation of some of the more important derivatives of a number of classes of organic compounds are described in the various Sections dealing with their reactions and characterisation. These Sections conclude with tables incorporating the melting points and boiling points of the compounds themselves, and also the melting points of selected derivatives. For convenience, the references to the various tables are collected below. 

The normal boiling point of 2-methylthiazole is 17 0= 128.488 0.005°C. The purity of various thiazoles was determined cryometrically by Handley et al. (292), who measured the precise melting point of thiazole and its monomethyl derivatives. Meyer et al. (293, 294) extended this study and, from the experimental diagrams of crystallization (temperature/degree of crystallization), obtained the true temperatures of crystallization and molar enthalpies of fusion of ideally pure thiazoles (Table 1-43). 

Acetaldehyde can be isolated and identified by the characteristic melting points of the crystalline compounds formed with hydrazines, semicarbazides, etc these derivatives of aldehydes can be separated by paper and column chromatography (104,113). Acetaldehyde has been separated quantitatively from other carbonyl compounds on an ion-exchange resin in the bisulfite form the aldehyde is then eluted from the column with a solution of sodium chloride (114). In larger quantities, acetaldehyde may be isolated by passing the vapor into ether, then saturating with dry ammonia acetaldehyde—ammonia crystallizes from the solution. Reactions with bisulfite, hydrazines, oximes, semicarb azides, and 5,5-dimethyl-1,3-cyclohexanedione [126-81 -8] (dimedone) have also been used to isolate acetaldehyde from various solutions. 

Table 1. Melting Points of Some Hydantoin Derivatives ...

Phosphonic Acid and P(III) Derivatives. Phosphonic or phosphorous acid is a white dehquescent crystalline compound having a melting point of 73.6°C. As evidenced by its stmcture,HP(=0)(0H)2, phosphonic acid is dibasic. The first hydrogen is strongly ionized, pR 1.3—1.7, and... 

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