Sample preparation organic syntheses

Before the actual sample preparation procedure is described some general observations should first be made. However excellent the sample preparation and however sophisticated the equipment, the accuracy of the analysis will only be as good as the quality of the sample that is taken. If the sample is that of a reaction mixture from an organic synthesis laboratory, it is likely to be taken from a single bottle or container, by a professional chemist, and is likely to be truly representative of the bulk of the material. 

SYNTHESIS A solution of 0.67 g 5-hydroxyindole (indol-5-ol) in 10 ml dry MeOH was treated with a solution of 0.30 g NaOMe in MeOH, followed by 0.70 g benzyl chloride. The mixture was heated on the steam bath for 0.5 h, and the solvent removed under vacuum. The residue was suspended between H20 and CH2CI2, the organic phase separated and the aqueous phase extracted once with CH2CI2. The combined organics were stripped of solvent under vacuum, and the residue distilled. A colorless fraction came over at 170-190 °C and spontaneously crystallized in the receiver. There was obtained 0.90 g (80%) 5-benzyloxyindole with a mp 81-86 °C which increased, on recrystallization from toluene / hexane, to 94-96 °C. A sample prepared from the decarboxylation of 5-benzyloxyindole-2-carboxylic acid has been reported to have a mp of 102 °C from benzene. 

In contrast to single-mode reactors, dedicated multimode instruments allow scale-up to be performed in multivessel rotor systems utilizing various types of sealed vessels. In these systems, reactions can be carried out in batch to synthesize multiple gram quantities (< 250 g) of material in typically up to 1 L processing volume. Most of the multimode instruments available for organic synthesis have been derived from closely related sample preparation equipment [39-41]. The MARS Microwave Synthesis System (Fig. 4) is based... 

Impurities. Of course, the presence of impurities in a sample will have a dramatic effect on the XRD characteristics. Zeolite preparations, as synthesized, can contain both organic and inorganic impurities. After washing and calcination, many impurities become amorphous, and the resulting XRD powder pattern will clearly show changes from the as-synthesized material. Some impurities, however, are stable to calcination and can make identification and characterization of the material (especially a new material) rather difficult. This is particularly true for cases where only a small number of samples, prepared in a narrow synthesis regime, are available for XRD examination. Common impurities found in zeolite preparations are the stable silicates, quartz and cristobalite. 

The absence of commercial equipment for US-assisted sample preparation clearly reflects that the analytical chemists give little importance to this way of accelerating the operations. For this reason, most applications in this area have been developed by using standard laboratory materials such as round-bottomed flasks or even beaker or precipitation vessels. Some authors have implemented US-assisted sample preparation in commercially available cells specially designed for organic synthesis or have designed and constructed their own custom devices. 

The book comprises 8 chapters. The first provides background, introduces the topic of asymmetric synthesis, outlines principles of transition state theory as applied to stereoselective reactions, and includes the glossary. The second chapter details methods for analysis of mixtures of stereoisomers, including an important section on sample preparation. Then follow four chapters on carbon-carbon bond forming reactions, organized by reaction type and presented in order of increasing mechanistic complexity Chapter 3 is about enolate alkylations. Chapter 4 nucleophilic additions to carbonyls. Chapter 5 is on aldol and Michael additions (2 new stereocenters), while Chapter 6 covers rearrangements and cycloadditions. The last two chapters cover reductions and oxidations. 

Aluminum azide, A1(N3)3 is a white, crystalline solid which is chemically not very stable. Freshly prepared samples deflagrate in the match test with sputtering, and explode on impact. When exposed to atmospheric moisture, the compound decomposes hydrolytically within minutes to yield insoluble products with little or no explosive sensitivity [233]. Similarly, aluminum hydroxide is immediately precipitated when sodium azide is added to solutions of aluminum salts [135]. In tetrahydrofuran, A1(N3)3 dissolves with solvation such solutions are useful as azidation agent in organic synthesis [234]. 

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