Dehydration reactions

Resoles. Like the novolak processes, a typical resole process consists of reaction, dehydration, and finishing. Phenol and formaldehyde solution are added all at once to the reactor at a molar ratio of formaldehyde to phenol of 1.2—3.0 1. Catalyst is added and the pH is checked and adjusted if necessary. The catalyst concentration can range from 1—5% for NaOH, 3—6% for Ba(OH)2, and 6—12% for hexa. A reaction temperature of 80—95°C is used with vacuum-reflux control. The high concentration of water and lower enthalpy compared to novolaks allows better exotherm control. In the reaction phase, the temperature is held at 80—90°C and vacuum-refluxing lasts from 1—3 h as determined in the development phase. SoHd resins and certain hquid resins are dehydrated as quickly as possible to prevent overreacting or gelation. The end point is found by manual determination of a specific hot-plate gel time, which decreases as the polymerization advances. Automation includes on-line viscosity measurement, gc, and gpc. 

Excess reagents are conveniently renoved by either evaporation with a strean of nitrogen or partitioning with an aqueous solution of a weak base Byproduct fonation is generally not a najor problen except for analytes which are labile to acidic or basic conditions at elevated teaperatures [476,478,479,485,486]. Under conditions where the anhydride can lead to undesirable side reactions (dehydration, enolization, etc.), the haloalkylacyl-iaidazole reagent can sometimes be employed. 

As shown in Eq. 6.59, Rapoport has prepared sinefungin, nucleoside antibiotics, via nitro-aldol reaction, dehydration, and reduction with Zn in acetic add.115a [i-Nitrostyrenes are selectivity reduced to the corresponding oximes by indium metal in aqueous methanol under neutral conditions.11515... 

The number of methods available for the synthesis of bicyclic systems containing two fused five-membered rings with one bridgehead nitrogen can be summarized in a few general reactions dehydrative ring closure, oxidative Schiff base cyclization, and base-induced closure (Scheme 3). One-pot reactions involving precursor synthesis followed by cyclization are also known. 

Glycine acts as an acid-base catalyst in this reaction. C8 and Cl 1 are very acidic, and once deprotonated they are very nucleophilic, so they can attack C2 and C3 in an aldol reaction. Dehydration gives a key cyclopentadienone intermediate. (The mechanism of these steps is not written out below.) Cyclopentadienones are antiaromatic, so they are very prone to undergo Diels-Alder reactions. Such a reaction occurs here with norbomadiene. A retro-Diels-Alder reaction followed by a [4 + 1] retrocycloaddition affords the product. 

After the substitution is complete, all that is required is an aldol reaction, dehydration by Elcb, and deprotonation. Workup then gives the product. 

The disappearance of the magnetization above the Curie (or Neel) temperature (see Chap. 6) can be used to identify iron oxides. Thermal reactions - dehydration/dehy-droxylation, oxidation/reduction - with accompanying phase transformations may give additional information about phases present. Therefore, not only the heating cycle, but also the cooling cycle is usually recorded to give the so-called Js(T) curve. 

Tris(phenylthio)antimony, 337 Dechlorination (see Reduction reactions) Deconjugation (see Isomerization) Defluorination (see Reduction reactions) Dehalogenation (see Elimination reactions, Reduction reactions) Dehydration (see also Elimination reactions)... 

Zamaraev and Thomas provide a concise summary of work done with a family of classic catalytic test reactions—dehydration of butyl alcohols—to probe the workings of acidic molecular sieve catalysts. This chapter echoes some of the themes stated by Pines and Manassen, who wrote about alcohol dehydration reactions catalyzed by solid acids in the 1966 volume of Advances in Catalysis. 

On the basis of the success of these initial reports on the proline-catalyzed intramolecular aldol reaction several groups focused on extending this type of synthesis to bicyclic products bearing angular substituents other than methyl and ethyl reported earlier [97-101]. Preparation of bicyclic systems with protected hydroxymethyl substituents, e.g. 99, was reported by Uda et al. (Scheme 6.46, Eq. 1) [113, 114]. As a selected example, the aldol adduct 99 was formed in 70% yield and with 75% ee in the presence of one equivalent of L-proline. Synthesis of a related product with an angular phenylthio substituent, 101, was described by Watt and co-workers (Scheme 6.46, Eq. 2) [115]. After intramolecular proline-catalyzed aldol reaction, dehydration of the ketol intermediate, and subsequent recrystallization... 

Another important bulk chemical that could be derived from glycerol is acrylic acid (Craciun et al., 2005 Shima and Takahashi, 2006 Dubois et al., 2006). Shima and Takahashi (2006) reported a complete process for acrylic acid production involving the steps of glycerol dehydration in a gas phase followed by the application of a gas phase oxidation reaction to a gaseous reaction product formed by the dehydration reaction. Dehydration of glycerol could lead to commercially viable production of acrolein, which is an important and versatile intermediate for the production of acrylic acid esters, superabsorber polymers or detergents (Ott et al., 2006). Sub- and supercritical water have been applied by Ott et al. (2006) as the reaction media for glycerol dehydration, but the conversion and acrolein selectivities that have been achieved so far are not satisfactory for an economical process. 

ZnO exhibits varied catalytic properties, being active for hydrogenation and dehydrogenation reactions, dehydration of alcohols, methanol synthesis, and other reactions. ZnO is a wide-band n-type semiconductor with surface states present in the band gap. It can be doped with cations, and defects can be generated by treatment under oxidizing or reducing conditions (which change the availability of electrons at the surface). 

The precise nature of the carbonyl groups determines what happens next. If R is a leaving group (OR, Cl, etc.), the tetrahedral intermediate collapses to form a ketone and the product is a 1,3-di-ketone. The synthesis of dimedone (later in this chapter) is an example of this process where an alkoxy group is the leaving group. Alternatively, if R is an alkyl or aryl group, loss of R is not an option and the cyclization is an intramolecular aldol reaction, Dehydration produces an a,P-unsaturated ketone, which is a stable final product. 

The addition of a primary or secondary amine to a carbonyl group to form a tetrahedral intermediate is the first step in a number of reactions. Dehydration of the intermediate can give an imine. If, however, the carbonyl group is part of an acyl halide, ester or anhydride, the collapse of the tetrahedral intermediate may lead to an amide. These reactions are exemplified in Scheme 2.34. 

Aromatic amines react with various carbonyl compounds in a useful series of condensation reactions. Dehydration of the carbinolamines to... 

The dehydrohalogenation of alkyl halides, discussed in Chapter 8, is one way to introduce a n bond into a molecule. Another way is to eliminate water from an alcohol in a dehydration reaction. Dehydration, like dehydrohalogenation, is a P elimination reaction in which the elements of OH and H are removed from the a and P carbon atoms, respectively. 

The P-hydroxy carbonyl compounds formed in the aldol reaction dehydrate more readily than other alcohols. In fact, under the basic reaction conditions, the initial aldol product is often not isolated. Instead, it loses the elements of H2O from the a and P carbons to form an a,P-unsaturated carbonyl compound. 

A/-Alkylation of benzylhydroxylamine by a primary alkyl bromide proceeded smoothly at room temperature. HMPA is the solvent of choice and addition of tetraethylammonium iodide facilitates the reaction. Dehydration of the resulting V-alkylhydroxylamines and subsequent acid hydrolysis gave the corresponding primary amines (Scheme 45). ... 

Similar mechanisms can be provided for other reactions, such as parahydrogen conversion, hydrogen exchange reactions, dehydrations, etc. Reactions of the exchange type, where the energy levels in the two species are close, if not identical, can proceed in the van der Waals layer at low temperatures by virtue of the quantum mechanical exchange interaction. However, other classes of reactions are unlikely to proceed exclusively in other than the chemisorbed state. 

Thermal reactions (dehydration, thermal decomposition, solid state reactions) play an important part in the production of inorganic materials from minerals, gels and related raw materials. Solid state NMR can provide useful insights into the progress of these reactions and their mechanistic details, as well as information on phase transformations and other high-temperature phenomena. 

The first step in any kinetic study is to identify all the products and intermediates of the reaction. Dehydration often involves several distinct steps which may be very dependent upon reaction conditions, e,g, copper sulfate pentahydrate may yield trihydrate and/or monohydrate [29,30], Metastable intermediates may be formed, e,g, amorphous magnesium carbonate produced on dehydration undergoes [18] exothermic recrystallization at higher temperatures,... 

The theory of solid state reaction kinetics includes no consideration of surface properties other than the recognition that crystal faces are the most probable location for nucleation in many reactions. Dehydration studies have provided evidence that, in many such processes, all surfaces are modified soon after the onset of chemical change [20,21], This is ascribed to a surface reaction that is limited in extent and can continue only at local sites of special reactivity where the recrystallization required for nucleation is possible. In other decompositions there is evidence that the modified reactivity associated with surfaces may influence the overall reaction [11] and may also preserve the identity of crystals. This, incidentally, masks the occurrence of melting during decomposition [22],... 

Derivation From acetophenone or styrene by Will-gerodt reaction, dehydration of ammonium phenyl acetate. 

Use Condensing or polymerizing agent in organic reactions, dehydrating agent, strong base. 

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