Compound separation, solvents

When these benzoyl compounds separate in the course of the Schotten-Baumann reaction, they frequently occlude traces of unchanged benzoyl chloride, which thus escapes hydrolysis by the alkali it is advantageous there fore to recrystallise the benzoyl compounds whenever possible from ethanol or methylated spirit, since these solvents will esterify the unchanged chloride and so remove the latter from the recrystallised material. 

Thujone is best identified by its tribromo-compound, CjflHjgBrgO, melting at 121° to 122°. It is obtained by adding 5 c.c. of bromine (at once) to a solution of 5 grams of thujone in 30 c.c. of petroleum ether. The tribromo-compound separates on evaporation of the solvent and is washed with alcohol and recrystallised from boiling acetic ether. 

In this context, the enantiomeric pair containing the eutomer of cyclothiazide can be resolved by HPLC on cellulose-derived coated CSPs. Nevertheless, the poor solubility of this compound in solvents compatible with this type of support makes this separation difficult at preparative scale. This operation was achieved with a cellulose carbamate fixed on allylsilica gel using a mixture of toluene/acetone as a mobile phase [59]. 

Consequently, due to preferred cis-cis orientation a dimeric structure is observed for the indium complex and an unprecedented cis-trans arrangement in the thallium structure leads to a polymeric aggregate. Further N-NMR spectroscopic studies show that the aluminum and gallium complexes are stable contact ion pairs even in solution whereas the indium and thallium compounds are solvent-separated ion pairs in THE solution. 

An inductively coupled plasma formed by passing argon through a quartz torch is widely used for the mass spectroscopic analysis of metal compounds separated by online HPLC.6 Samples are nebulized on introduction into the interface. Plasma impact evaporates solvent, and atomizes and ionizes the analyte. Applications include separation of organoarsenic compounds on ion-pairing F4PLC and vanadium species on cation exchange. 

Further important results of compound separation (two-component solvent mixtures) using hosts 47 and 48 are taken from Table 9 and are as follows (A 47 allows... 

Figure 7.2 Diagram of a TLC plate. The plate is suspended vertically in the solvent containing the compounds to be separated (the solvent level or origin), and over time the compounds and solvent migrate up the plate to different heights and are separated. The retention factor (Rf) for compound 1 is calculated as AB/ AD, and for compound 2 AC/AD.

Figure 1.5 Thermodynamic cycle for the dissolution of compound C in solvent S. For dissolution to occur, the attractive forces between solute and solvent must outweigh the free energy required to separate solvent-solvent and solute-solute attractive forces...

For estimating the contribution of volatile compounds to bread aroma Rothe and coworkers (S) defined "aroma value" as the ratio of the concentration of some volatile compounds to the taste threshold value of the aroma. This concept was further developed by Weurman and coworkers (9) by introducing "odor value", in which aroma solutions were replaced by synthetic mixtures of volatile compounds in water. These mixtures showed the complexity of the volatile fractions of wheat bread, because none of them resembled the aroma of bread. Recently two variations of GC-sniffing were presented (10-11), in which the aroma extract is stepwise diluted with a solvent until no odor is perceived for each volatile compound separately in the GC effluent. The dilution factors obtained indicate the potency of a compound as a contributor to the total aroma. 

The oxidation of benzyl alcohol la to benzaldehyde 2a is representative of the general procedure employed. Benzyl alcohol la (0.108 g, 1 mmol) and IBD (0.355 g, 1.1 mmol) doped on neutral alumina (1 g) are mixed thoroughly on a vortex mixer. The reaction mixture is placed in an alumina bath inside an unmodified household microwave oven and irradiated for a period of 1 min. On completion of the reaction, followed by TLC examination (hexane-AcOEt, 9 1, v/v), the product is extracted into dichloromethane and is neutralized with aqueous sodium bicarbonate solution. The dichloromethane layer is separated, dried over magnesium sulfate, filtered, and the crude product thus obtained is purified by column chromatography to afford pure benzaldehyde 2a in 94% yield. Alternatively, the crude products are charged on a silica gel column that provides io-dobenzene on elution with hexane followed by pure carbonyl compounds in solvent system (hexane-ethyl acetate, 9 1, v/v). 

The identification of phenolic compounds separated by TLC is somewhat challenging. The most common strategy is to include a set of reference compounds on the TLC plate. These compounds are applied individually, and if the mixture contains any of the reference compounds, they can be identified based on the i /-value. Note that this approach always leaves some room for uncertainty, because two different compounds can have the same /< /-value. Further characterization is necessary to establish compound identity with more confidence. This can be achieved by scraping off the area on the TLC plate where the compound of interest has migrated to, followed by solvent extraction of the matrix, and more detailed chemical analyses, such as, for example gas chromatography-mass spectrometry or mass spectrometry (see Section 1.5 and Chapter 5). 

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