Dehydration, formation

Various possible steps are involved in the transfer of an adsorbate to the adsorption layer. Transport to the surface by convection or molecular diffusion, attachment to the surface, surface diffusion, dehydration, formation of a bond with the surface constituents. 

Oxiranes cannot be prepared directly from 1,2-diols by dehydration. Formation of the oxirane intermediate has been studied in connection with the mechanism of the pinacolic rearrangement. Oxiranes can be prepared stereoselectively from the acetals and ketals of 1,2-diols. D-(+)-2,3-epoxybutane has been obtained from an optically active diol via conversion of the ketal 64 to a halohydrin ester (Eq. 52). ... 

Surface nucleation diffusion of adsorbed ions partial dehydration formation of two-dimensional nucleus growth to three-dimensional nucleus. 

Polysaccharides are partially lost (ca 30%) by roasting, via various and not well-known mechanisms, dehydration, formation of condensation complexes linked, for example, with polysaccharides, protein or protein fragments, and chlorogenic acids (Trugo, 1985). 

In both dehydrogenation and dehydration formate ions on the surface thus seem to play a central role. It is therefore of interest to study the decomposition of bulk formates, although it is obviously to be expected that there will always be differences between two-dimensional surface compounds and bulk compounds. The decomposition reactions of various formates will be discussed in the following section. Knowledge of the properties of these compounds will enable us to deal more extensively with the reaction mechanisms of the dehydration and the dehydrogenation of formic acid. 

Medullary cells are loosely packed, and during dehydration (formation), they leave a series of vacuoles along the fiber axis. Medullary cells are spherical and hollow inside and are bound together by a cell membrane complex type material (see Figure 1-43). Because the medulla is believed to contribute negligibly to the chemical and mechanical properties of human hair fibers [131] and is difficult to isolate [122,134], it has received comparatively little scientific attention. The chemical composition of medullary protein derived from African porcupine quill has been reported by Rogers [118] and is described in Chapter 2. 

Reoxidation by take up of oxygen from the gas phase dehydration formation of oxide ions diffusion to the vacant lattice sites. 

A Dakin-West reaction on plant scale is demonstrated by elaboration of a modified procedure avoiding uncontrolled release of carbon dioxide. It seems to be generally accepted that the mechanism involves dehydrative formation of an azlactone (oxazolinone), which is then C-acylated (in equilibria with G-acylation), undergoes ring-opening hydrolysis, and then decarboxylates to form the acylamino ketone. Pyrimidine ring formation is achieved in a simple one-pot reaction, which is followed by a simple isomerization. Laboratory experiments had made clear that the mechanism... 

I PAM 220-335 Major reactions imidisation followed by nitrile formation minor reaction dehydration formation of imide, nitrile, C=C and imine... 

Dehydration formation of most snbstituted catalytic acid (H+, e.g., coned... 

If produced gas contains water vapour it may have to be dried (dehydrated). Water condensation in the process facilities can lead to hydrate formation and may cause corrosion (pipelines are particularly vulnerable) in the presence of carbon dioxide and hydrogen sulphide. Hydrates are formed by physical bonding between water and the lighter components in natural gas. They can plug pipes and process equipment. Charts such as the one below are available to predict when hydrate formation may become a problem. 

Dehydration can be performed by a number of methods cooling, absorption and adsorption. Water removal by cooling is simply a condensation process at lower temperatures the gas can hold less water vapour. This method of dehydration is often used when gas has to be cooled to recover heavy hydrocarbons. Inhibitors such as glycol may have to be injected upstream of the chillers to prevent hydrate formation. 

When a mixture of aniline, nitrobenzene, glycerol and concentrated sulphuric acid is heated, a vigorous reaction occurs with the formation of quinoline. It is probable that the sulphuric acid first dehydrates the glycerol giving acrolein or acraldehyde (A), which then condenses at its double bond with the amino group of the aniline to give acrolein-aniline (B), The latter in its enol... 

Molisch s Test. Dissolve about 01 g. of the carbohydrate in z ml. of water (for starch use 2 ml. of starch solution ), add 2-3 drops of a 1 % alcoholic solution of i-naphthol (ignoring traces of the latter precipitated by the water) and then carefully pour 2 ml. of cone. H2SO4 down the side of the test-tube so that it forms a heavy layer at the bottom. A deep violet coloration is produced where the liquids meet. This coloration is due apparently to the formation of an unstable condensation product of i-naphthol with furfural (an aldehyde produced by the dehydration of the carbohydrate). 

In aqueous solutions of sulphuric (< 50%) and perchloric acid (< 45 %) nitrous acid is present predominantly in the molecular form, although some dehydration to dinitrogen trioxide does occur.In solutions contairdng more than 60 % and 65 % of perchloric and sulphuric acid respectively, the stoichiometric concentration of nitrous acid is present entirely as the nitrosonium ion (see the discussion of dinitrogen trioxide 4.1). Evidence for the formation of this ion comes from the occurrence of an absorption band in the Raman spectrum almost identical with the relevant absorption observed in crystalline nitrosonium perchlorate. Under conditions in which molecular nitrous... 

Aldol additions and ester condensations have always been and still are the most popular reactions for the formation of carbon-carbon bonds (A.T. Nielsen, 1968). The earbonyl group acts as an a -synthon, the enoi or enolate as a d -synthon. Both reactions will be treated together here, and arguments, which are given for aldol additions, are also valid for ester condensations. Many famous name reactions belong to this category ). The products of aldol additions may be either /J-hydroxy carbonyl compounds or, after dehydration, or, -unsaturated carbonyl compounds. 

In his cephalosporin synthesis methyl levulinate was condensed with cysteine in acidic medium to give a bicyclic thiazolidine. One may rationalize the regioselective formation of this bicycle with the assumption that in the acidic reaction mixture the tMoI group is the only nucleophile present, which can add to the ketone. Intramolecular amide formation from the methyl ester and acid-catalyzed dehydration would then lead to the thiazolidine and y-lactam rings. The stereochemistry at the carboxylic acid a-... 

Another difficulty in this reaction lies in the preparation of pure chloroacetaldehyde. The low yield observed is due to simultaneous formation of by-products (polyhalogenation). So vinylchloride was used as a starting material for this synthesis (449). A simpler method is to react chlorine with vinylchloride in aqueous solution and then to dehydrate the semihydrated chloroacetaldehyde by distillation through a column of calcium chloride heated to 70 to 90 C (451). 

Zaitsev s rule as applied to the acid catalyzed dehydration of alcohols is now more often expressed in a different way elimination reactions of alcohols yield the most highly substituted alkene as the major product Because as was discussed in Section 5 6 the most highly substituted alkene is also normally the most stable one Zaitsev s rule is sometimes expressed as a preference for predominant formation of the most stable alkene that could arise by elimination... 

Alkene synthesis via alcohol dehydration is complicated by carbocation rearrangements A less stable carbocation can rearrange to a more sta ble one by an alkyl group migration or by a hydride shift opening the possibility for alkene formation from two different carbocations... 

In Problem 5 17 (Section 5 13) we saw that acid catalyzed dehydration of 2 2 dimethyl cyclohexanol afforded 1 2 dimethylcyclohexene To explain this product we must wnte a mecha nism for the reaction in which a methyl shift transforms a secondary carbocation to a tertiary one Another product of the dehydration of 2 2 dimethylcyclohexanol is isopropyhdenecyclopentane Wnte a mechanism to rationalize its formation... 

We now have a new problem Where does the necessary alkene come from Alkenes are prepared from alcohols by acid catalyzed dehydration (Section 5 9) or from alkyl halides by dehydrohalogenation (Section 5 14) Because our designated starting material is tert butyl alcohol we can combine its dehydration with bromohydrm formation to give the correct sequence of steps... 

As a method for the preparation of alkenes a weakness in the acid catalyzed dehydration of alcohols IS that the initially formed alkene (or mixture of alkenes) sometimes isomenzes under the conditions of its formation Write a stepwise mechanism showing how 2 methyl 1 butene might isomenze to 2 methyl 2 butene in the presence of sulfuric acid... 

Dehydration of 2 2 3 4 4 pentamethyl 3 pentanol gave two alkenes A and B Ozonolysis of the lower boiling alkene A gave formaldehyde (H2C=0) and 2 2 4 4 tetramethyl 3 pentanone Ozonolysis of B gave formaldehyde and 3 3 4 4 tetramethyl 2 pentanone Identify A and B and suggest an explanation for the formation of B in the dehydration reaction... 

Primary amines undergo nucleo philic addition to the carbonyl group of aldehydes and ketones to form carbinol amines These carbinolamines dehydrate under the conditions of their formation to give N substituted imines Secondary amines yield enamines... 

Aldol condensation (Sections 18 9-18 10) When an aldol ad dition IS carried out so that the 3 hydroxy aldehyde or ke tone dehydrates under the conditions of its formation the product IS described as ansing by an aldol condensation... 

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