Dehydration methods

Secondary amines cannot form imines, and dehydration proceeds to give carbon-carbon double bonds bearing amino substituents (enamines). Enamines were mentioned in Chapter 7 as examples of nucleophilic carbon species, and their synthetic utility is discussed in Chapter 1 of Part B. The equilibrium for the reaction between secondary amines and carbonyl compounds ordinarily lies far to the left in aqueous solution, but the reaction can be driven forward by dehydration methods. 

The synthesis of a-chiral isocyano amides has also been reported using various dehydration methods [47, 53-66]. Because of the lower acidity of the a-proton in a-isocyano amides, high stereoinductions can be obtained using relatively harsh dehydration methods (POCls/EtsN, —20°C) [62]. However, the triphosgene/NMM protocol remains the method of choice [53]. 

Direct interactions of aqueous metal salt solutions with bases failed, as a rule, to yield uniform particles thus, it was necessary to design some other approaches to achieve such dispersions. The fundamental ideas were based on the fact that seldom, if ever, can a metal ion in an aqueous solution be directly converted to an oxide. Instead, the initial steps involve the formation of metal hydroxides or oxyhydroxides. Either these products remain stable as precipitated, or they convert to oxides on aging by processes that may take different lengths of time but that can be accelerated by various dehydration methods. Consequently, metal ion hydrolysis... 

Although this method proved successful for 4-keto-L-proline methyl ester 48, it was decided to investigate the conventional azeotropic dehydration method. Derivative 48 and pyrrolidine (1.2 equiv) were heated under reflux in benzene, collecting the water produced using a Dean and Stark apparatus. A quantitative yield of the corresponding enamine 52 was obtained after only 30 min (Scheme 19). Again, full assignment of NMR spectra was complicated by the apparent presence of rotameric structures. 

Madalageri, B.B., Mahadev, and Hiremath, S.M. (1996) Dehydration methods, oil extraction and flavour components detection in curry leaf (Murraya koenigii Spreng) and detection of flavour components. Karnataka Journal of Agricultural Sciences 9, 284-288. 

When compression and cooling alone is inefficient to achieve the desired level of dehydration, then some additional dehydration is required in order to reduce the water content to the desired level. The commonly employed dehydration methods are applicable to acid gas dehydration, but there are several important considerations that apply when dehydrating a stream like this. 

The focus of this chapter will not be dehydration in general, but rather the specific problems with dehydrating acid gas streams. However, some brief introduction to the common dehydration methods will be presented. 

The pXr values of diarylmethyl cations [22C ] were basically those determined according to the same method as for the triarylmethyl cations in aqueous sulphuric acid solutions by Deno and coworkers (Deno et ai, 1955 Deno and Schriesheim, 1955 Deno and Evans, 1957). For reasons of solubility, a wide set of pXr+ values in aqueous acetic acid solutions were determined by Mindl and coworkers (Mindl and Vecera, 1971, 1972 Mindl, 1972). Unfortunately, however, the pXr+ values, determined by means of the protonation-dehydration method for diarylmethanols, could only be obtained for more stable derivatives and not for those less stable than the unsubstituted... 

The condensation of aldehydes and ketones with halo esters may lead directly to olefinic acids and esters by dehydration of the intermediate /S-hydroxy compounds (Reformatsky). ° More often, the hydroxy esters are isolated and purified prior to dehydration (method 103). 

Anhydrous lanthanide trihalides, particularly the trichlorides, are important reactants for the formation of a variety of lanthanide complexes, including organometallics. Routes for the syntheses of anhydrous lanthanide trihalides generally involve high temperature procedures or dehydration of the hydrated halides.The former are inconvenient and complex for small scale laboratory syntheses, while dehydration methods may also be complex and have limitations, for example, use of thionyl chloride. - Moreover, the products from these routes may require purification by vacuum sublimation at elevated temperatures. Redox transmetalation between lanthanide metals and mercury(II) halides was initially carried out at high temperatures. However, this reaction can be carried out in tetrahydrofuran (THF, solvent) to give complexes of lanthanide trihalides with the solvent. These products are equally as suitable as reactants for synthetic purposes as the uncomplexed... 

Chugaev Reaction. The conversion of an alcohol to an olefin by the thermal decomposition of its xantliate ester is called the Chugaev reaction. The unique feature of the reaction is that alcohols which frequently undergo rearrangements by usual dehydration methods can be dehydrated with no change in the carbon skeleton. Thus pinacolyl alcohol (XXXIV) leads to terf-butylethylene (XXXV),22 and optically... 

Also examples of extranuclear dehydroxylation by other dehydrative methods 1239,1377 Qj. jjy reduction have been reported. 

As mentioned earlier, one of the partial dehydration methods is ICF. Binary brine solutions are generally used, most often sodium chloride (eutectic -20°C) or calcium chloride (eutectic == -50°C), although ternary solutions (e.g., water + NaCl + ethanol, or sugars) have also been studied (Lucas and Raoult-Wack, 1996). The ICF... 

In general, these two reactions are definitely not suitable for the preparation of appreciable quantities of pure isonitriles indeed, they have been largely replaced by the more widely applicable dehydration method [lc,d,fj. This was first discovered by Hagedorn in 1956 and entails transformation of primary amines into for-mamides, followed by dehydration either with phosgene (or its precursors) and triethylamine, or phosphorus oxychloride and di-wo-propylamine [If]. 

Eastern European farmers produce a variety of fruit juices, which they wish to dehydrate to prolong shelf-life and facilitate transportation. One very simple dehydration method is to put the juice in a plastic bag and drop the bag into brine at 283 K. If the bag is permeable to water but not to salt or juice components, then osmotic flow will concentrate the juice. Is the osmotic pressure generated in this way significant Assume that the juice contains a solids concentration equivalent to 1.0 wt% sucrose (342 g/mol molecular weight) and that the brine contains 35 g of NaCl per 100 g of water. For simplicity, assume that the juice and the brine are ideal solutions. 

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