Beckmann method

BECKMANN METHOD. A method of measuring elevation of the boiling point or depression of the freezing point of a solution, It may be used to measure concentration if the nature of the solute is known, or the molecular weight of the solute if the volume concentration is known. 

There arc several methods for measuring this depression In the Beckmann method the freezing point of pure solvent and that of solution is measured by a special type of thermometer, the "Beckmann thermometer. The solvent or solution is contained in a double-walled glass apparatus and placed in a freezing mixture not more than S°C below the freezing point of the solvent. By rapid stirring when the liquid has supercooled about i. crystallization is induced and the temperature rises to the freezing point. 

Before describing the chief methods by which the molecular weight of an organic compound in solution may be determined, a description is given of the Beckmann thermometer, and of the Tabloid Press, both of which are frequently used in the above determinations. 

If a small Beckmann thermometer, similar to that used in the Sucharda-Bobranski method (p. 442) is available, the above method can be readily modified by replacing the tube T (Fig. 79) by an ordinary small boiling-tube without the side-arm A. The thermometer B and the stirrer S are fitted as in Fig. 79. The volume of solvent (usually ca 5 ml.) must of course cover the bulb of the thermometer. 

To meet the needs of the advanced students, preparations have now been included to illustrate, for example, reduction by lithium aluminium hydride and by the Meerwein-Ponndorf-Verley method, oxidation by selenium dioxide and by periodate, the Michael, Hoesch, Leuckart and Doebner-Miller Reactions, the Knorr pyrrole and the Hantzsch collidine syntheses, various Free Radical reactions, the Pinacol-Pinacolone, Beckmann and Arbusov Rearrangements, and the Bart and the Meyer Reactions, together with many others. 

The ready availability and low cost of A -20-keto steroids from the degradation of sapogenins has led to intensive study of methods for the preparation of androstanes via these intermediates. The simplest, most practical and most widely used method on a production scale is the Beckmann rearrangement of -20-oximinopregnenes ... 

Statical Method.—The most convenient form of apparatus for determining molecular weight by this method is Beckmann s boiling-point apparatus shown m Fig. 32. 

A modification of this method, related to the Beckmann rearrangement, entails treatment of a ketoxime with one equivalent of CDI, then four to five equivalents of a reactive halide such as allyl bromide or methyl iodide (R3X) under reflux in acetonitrile for 0.5-1.5 h. Quatemization of the imidazole ring effectively promotes the reaction by increasing the electron-withdrawing effect. The target amides then are obtained by hydrolysis. High yields, neutral conditions, and a very simple procedure make this modification of the synthesis of amides by azolides a very useful alternative. 1243... 

Fe3+, La3+ and Ce3+ ion-exchanged MAPO-36 was prepared by wet ion-exchange method. The materials were characterized by XRD, TGA and TPD (ammonia). Lewis acid metal ions are suggested to remain as charge compensating MO+ species after calcination in ion-exchanged MAPO-36. Beckmann rearrangement of cyclohexanone oxime was studied over these catalysts in the vapour phase. 

Of course, other reaction types have been also investigated more recently, such as the Beckmann rearrangement [247,277,278] or ethylbenzene disproportionation [279, 280], just to name a couple. In situ NMR methods are expected to play a vital role in the future science of heterogeneous catalysis. 

Yokomatsu, Y., Yoshida, Y., Nakabayashi, N., and Shibuya, S., Simple and efficient method for preparation of conformationally constrained aminome-thylene gem-diphosphonate derivatives via Beckmann rearrangement,. Org. Chem., 59, 7562, 1994. 

The various methods are not described here since, as a rule, they are learned in the practical course in physics or physical chemistry. The cryoscopic process is much to be preferred to the ebullioscopic. The most frequently used solvents are benzene and glacial acetic acid, and the best apparatus is the closed one of Beckmann, with electromagnetic stirring. 

References in parentheses refer to the preparation of intermediates or to older methods. Assuming that the acetamido compound is prepared via Beckmann rearrangement from 2-acetyldibenzothiophene. s... 

The acid, protonating the hydroxylamine, prevents the reverse reaction and thus causes rapid and complete hydrolysis distillation of the final solution then drives off the aldehyde or ketone, and the hydroxylamine sulfate remains behind. This method must be used with care, however, as the acid may cause a Beckmann rearrangement to occur. 

As mentioned previously, the Beckmann reaction remains one of the most reliable transformations to incorporate efficiently a nitrogen atom in acyclic or cyclic systems, providing a powerful synthetic method (Scheme 8). 

Amines 297 and/or carboxylic acids 298 may be obtained by amide hydrolysis and the latter compound may be produced from a ketone by a Beckmann rearrangement (equation 103). Therefore, the Beckmann rearrangement of a ketone oxime followed by amide hydrolysis provides a synthetic method to cleave the C—C bond between the carbon... 

Several different reaction conditions were tested to perform the Beckmann rearrangement of the substituted thiophene 323 (equation 118). The reaction of the tosyl oxime derivative was found to be the best available method, and 324 was obtained in high yield °. ... 

The production of the intricate carbon framework of Lycopodine 407 by Grieco and Dai included an azepine formation by the classical Beckmann rearrangement method (equation 169). 

Recently, racemic 2-azabicyclo[3.2.2]nonanes 437 were synthesized to be tested as /3-glycoside inhibitors (equation 185). Once again the Beckmann rearrangement was selected as the method to introduce selectively the nitrogen atom into the carbocyclic structure. 

Although the Beckmann fragmentation may be regarded as a limitation for the application of the Beckmann rearrangement, it provides a synthetic method to produce nitrile compounds. Some examples from the recent literature are collected in the next section. 

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