Dehydration, strain effect

Sulfur Dioxide and Sulfites. Sulfur dioxide [7446-09-5], SO2, sodium bisulfite [15181-46-1], NaHSO, and sodium metabisulfite [23134-05-6] ate effective against molds, bacteria, and certain strains of yeast. The wine industry represents the largest user of sulfites, because the compounds do not affect the yeast needed for fermentation. Other appHcations include dehydrated fmits and vegetables, fmit juices, symps and concentrates, and fresh shrimp (79). Sulfites ate destmctive to thiamin, and cannot be used in foods, such as certain baked goods, that ate important sources of this vitamin. 

In the gut, many pathogens adhere to the gut wall and produce their toxic effect via toxins which pervade the surrounding gut wall or enter the systemic circulation. Vibrio cholerae and some enteropathic E. coli strains localize on the gut wall and produce toxins which increase vascular permeability. The end result is a hypersecretion of isotonic fluids into the gut lumen, acute diarrhoea and consequent dehydration which may be fatal in juveniles and the elderly. In all these instances, binding to epithelial cells is not essential but increases permeation ofthe toxin and prolongs the presence of the pathogen. 

The cause of the promotional effect is not clear. Probably the chromium attaches somewhat differently to a dehydrated surface than to one which is fully hydroxylated. A higher concentration of dichromate, instead of chromate, is one possible cause since the remaining hydroxyls are more widely spread. The promotional effect could also be due to a strained attachment which is frozen to the surface by the low binding temperature. Heating these anhydrously prepared catalysts at 900°C destroys the promotional effect, leaving only an ordinary aqueous type catalyst. Probably the increased mobility at 900°C relaxes the strain or rearranges the Cr(VI) back to its usual mode of attachement. 

Cubylcarboxonium ions have been also studied by Prakash, Olah, and co-workers.579,580 The parent cation 281 prepared under superacid conditions was stable at low temperature but decomposed to cubylacylium cation 282 as a result of further protonation and dehydration [Eq. (3.71)]. In addition to cation 281, di- and tetra-carboxonium ions and the corresponding protonated methyl esters were also observed as long-lived species stable under superacidic conditions. Experimental evidence and theoretical data indicated that the strained cubyl system effectively stabilizes the carbocationic centers through C—C bond hyperconjugation (283). On the basis of 13C data, three conformers of protonated dimethyl cubane-l,4-dicarboxylate (284-286) could be identified. 

For concrete at high temperature the most important is the effect of cracking and dehydration process on the material properties, e.g. porosity n=f(Thydr)> intrinsic permeability k=/(T/,(/,/r. /7. T), and its deformations. Irreversible part of strains and so called thermal creep are expressed as functions of thermochemical damage parameter V, [8],... 

Nozaki reaction (12, 137 14, 96 15, 95). An intramolecular version of this reaction can effect cyclization even to a strained nine-membered ring. Thus 1 on treatment with CrCl2 at a low temperature cyclizes mainly to 2, a mixture of epimers in which a cyclononadiyne ring is fused to a cyclopentene ring. After acetylation of the secondary hydroxyl group, the mixture is dehydrated to 3, with a nine-membered enediyne ring. 

These results seemed to be inconsistent with any simple electronic theory of hydrogenation catalysis they were, however, relevant to the general concept that dehydration of oxide catalysts should leave the surface in a strained, catalytically active condition ( , S). A systematic study was therefore undertaken of the activation of pure y-alumina for ethylene hydrogenation and hydrogen-deuterium exchange the effects of pretreatment, drying conditions, and rehydration were investigated. 

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