Dehydration procedures

Problem 18.2 Why do you suppose only symmetrical ethers are prepared by the sulfuric add-catalyzed dehydration procedure What product(s) would you expect if ethanol and 1-propanol were allowed to react together In what ratio would the products be formed if the two alcohols were of equal reactivity ... 

Hydroxy-( , )-a,y-dienylsulfones are prepared by an analogous one-pot dehydration procedure via an elimination reaction of the corresponding cyclic carbonate [21]... 

All zeolite samples ( 1,4g) were used after dehydration procedure at 773 K under Ar. Then, weighted amount of DPP corresponding to 1 molecule per zeolite unit cell (UC) was added under dry argon at 293 K and the powders were shaken in the dark. Powders were transferred under dry argon in quartz glass Suprasil cell or in cylindrical EPR quartz tube and sealed. 

Finally, the localizations of low-molecular-weight compounds requires special specimen preparation techniques, as these compounds are often diffusible, water- or organic-solvent soluble, and solubilized by conventional fixation and dehydration procedures. The reader is referred to ref. (12) for the processing of cells and tissues for the cytochemical and histochemical localization of these compounds. 

Reaction with acetylenic dipolarophiles represents an efficient method for the preparation of 2,5-dUiydrothiophenes. These products can be either isolated or directly converted to thiophene derivatives by dehydration procedures. The most frequently used dipolarophile is dimethyl acetylenedicarboxylate (DMAD), which easily combines with thiocarbonyl yhdes generated by the extrusion of nitrogen from 2,5-dihydro-1,3,4-thiadiazoles (8,25,28,36,41,92,94,152). Other methods involve the desUylation (31,53,129) protocol as well as the reaction with 1,3-dithiohum-4-olates and l,3-thiazolium-4-olates (153-158). Cycloaddition of (5)-methylides formed by the N2-extmsion or desilylation method leads to stable 2,5-dUiydrothiophenes of type 98 and 99. In contrast, bicyclic cycloadducts of type 100 usually decompose to give thiophene (101) or pyridine derivatives (102) (Scheme 5.37). 

Aromatic isocyanides can also be prepared conveniently by the dehydration of the corresponding formamides by phosphorus oxychloride, but much better results are obtained if the reaction is done in the presence of potassium fer/-butoxide rather than pyridine.6 Neither method of dehydrating formamides has yet been used to prepare methyl or ethyl isocyanide because their low boiling points make them difficult to isolate from the reaction mixture hence, until a suitable dehydration procedure is worked out, they are best made by reaction of the corresponding alkyl iodide with silver cyanide. ... 

Dehydration procedures have led to 2//-thiopyrans. Thus unsubstituted parent heterocycle 6 was prepared by the dehydration of hydroxy derivative 287 with potassium hydrogen sulfate.20 The same procedure applied to a mixture of isomeric hydroxy 2-methyl-5,6-dihydro-2//-thiopyrans and 3,5-dihydroxy-2,4,5,6-tetrahydrothiopyran gives all the corresponding thio-pyrans.91 3-Phenylthiopyran dioxide 289 was obtained after the dehydration of hydroxy derivative 288a with phosphoric acid at elevated temperatures.300,301 The same approach was explored for the preparation of furano-2//-thiopyran 14.42... 

NCAs are well known forms of a-amino acids which present the amino group protected and the carboxylic group activated for a peptide coupling step. The most common access to NCAs is based on dehydration procedures of a-amino acids involving carbonic acid equivalents, which has limited the method to proteinogenic... 

Microscopic examination showed that the exchange and dehydration procedures had little effect on the appearances of the crystals, all of which remained clear and colorless. 

The dehydration process may also affect antigen preservation and various special dehydration procedures have claimed improved tissue antigenicity. Similarly, certain types of paraffin, celloidin, and polyethylene glycol have been shown to be useful for immunohistology (Larsson 1993). 

Note During the dehydration procedure described in Vol 3, p C399-L, it is required to determine ethanol and water in the cylindrical block removed from the press. In these detns a piece of block (cake) is dried in an oven at 100° to obtain "total volatile content , the water is detd by Karl Fischer Method and then ale is obtd by difference. Finally ether is added to make a colloiding mixture contg 2 parts of ether to 1 part alcohol by wt. The above tests are described in the items which follow... 

Figure 5.10 H CRAMP spectra (360MHz) of AI2O3 2.O5H2O materials prepared following various dehydration procedures (a) desiccation over Drierite at ambient pressure (725Torr) for 72h, (b) heating at 110°C (in ambient pressure) forSh,...

The dehydration procedure of this preparation is stated to be imsatisfactory for substituted phenylmethylcarbinols which have a strongly electronegative group, such as the nitro or tri-fluoromethyl group, in the tmta position. 

Another dehydration procedure employs 72% aqueous magnesium nitrate (Mg(N03)2 4 H2O) as a dehydrating agent, which gets around some of these difficulties [45]. As in sulfuric acid dehydration, hot nitric acid vapor is fed... 

In hydrated CuNaH-ZSM-5 a hexa-aquo Cu complex is observed which has an isotropic ESR line at room temperature (8). The ESEM data in Figure 5a confirms the six-water coordination. This species must be located at the channel intersections since the main channels are too small. After evacuation at room temperature the CuZ becomes anchored to the zeolite framework and shows coordination to only three water molecules as shown by the ESEM data in Figure 5b. The ESR parameters still indicate approximately octahedral coordination so the Cu " is coordinated to three water and three zeolite framework oxygens. This partial dehydration procedure is totally reversible by saturating the zeolite with water vapor. 

For the n-butane-5A system both sorption and ZLC results are in substantial agreement with NMR PFG self diffusivity data. However, for all NaX systems studied the NMR self diffusivities are approximately two orders of magnitude larger than the ZLC values. This discrepancy is difficult to understand. Most of the more obvious explanations can be ruled out on the basis of the experimental evidence. For example, the possible intrusion of extraneous heat and/or mass transfer resistances is excluded by the agreement between the sorption, exchange and ZLC results. For NaX crystals both NMR and ZLC results show that differences in the origin of the sample and the initial dehydration procedure have only a relatively small effect on the diffusivity. (27) The absence of significant surface barriers (for aromatics-NaX) is... 

Dehydration of the aldol products of a Refoimatsky reaction does not normally occur under the usual reaction conditions but is often accomplished in a separate step to prepare unsaturated esters. Acid-promoted dehydration of -hydroxy esters can give significant amounts of nonconjugated unsaturated esters by either kinetic or thermodynamic control. Mirrington and cowoikers found that acetates can be prepared directly from Refoimatsky reaction mixtures by addition of acetyl chloride. Base-promoted elimination of the acetates produced conjugated esters in high yield. For the reaction shown in Scheme 14, the thermodynamic ratio of products (32) (33) is 40 60 and four different acid-promoted dehydration procedures gave at best a 68 32 ratio of products. ... 

Sample Preparation. Samples were prepared by exposing the silica gel, previously dried at 150 °C for 24 h, to either cyclohexane or pentane solutions containing selected amounts of the fluorophore. The solvent was carefully removed under vacuum when required. Complete probe adsorption of liquid-solid samples was verified with absorption spectroscopy. Less than 0.07% of the silica surface was typically covered by the probe. Immediately before data collection, dry samples were evacuated under vacuum at 125-130 °C for 30 min. The total dehydration procedure was sufficient to remove the physisorbed water while leaving the surface silanol functionality intact (12). Selected amounts of oxygen were introduced into a... 

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