Strong Acid Catalysis

Stronger bases or acids promote the cyclization of the 1 1 adduct, 51, to pyrazines and, as was shown for the case of o-phenylenediamine, 1 equivalent or more of a strong acid both catalyzes and directs the course of the condensation by promoting the loss of ammonium ion. Strong acid catalysis is described in more detail in the following section (72JOC4136). 

Preparation of 53 by the reaction of 2,3-dichloro-5,6-dicyanopyrazine (Section V,D,2) with ammonia in dimethylformamide has been reported (78MI1). 

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Ketones with labile hydrogen atoms undergo enol acetylation on reaction with ketene. Strong acid catalysis is required. If acetone is used, isoptopenyl acetate [108-22-5] (10) is formed (82—85). Isopropenyl acetate is the starting material for the production of 2,4-pentanedione (acetylacetone) [123-54-6] (11). 

Alkaline Catalysts, Resoles. Resole-type phenoHc resins are produced with a molar ratio of formaldehyde to phenol of 1.2 1 to 3.0 1. For substituted phenols, the ratio is usually 1.2 1 to 1.8 1. Common alkaline catalysts are NaOH, Ca(OH)2, and Ba(OH)2. Whereas novolak resins and strong acid catalysis result in a limited number of stmctures and properties, resoles cover a much wider spectmm. Resoles may be soHds or Hquids, water-soluble or -insoluble, alkaline or neutral, slowly curing or highly reactive. In the first step, the phenolate anion is formed by delocali2ation of the negative charge to the ortho and para positions. 

The reaction between acrylonitrile and formaldehyde (as paraformaldehyde or tri-oxane), under strong acid catalysis (usually sulphuric) and most often in presence of catalytic quantities of acetic anyhydride, to produce triacrylohexahydrotriazine, is inclined to violent exotherm after an induction period. The runaway can be uncontrollable on sub-molar scale. It may be due to acrylate polymerisation or to increasing reactivity of the formaldehyde equivalent due to progressive de-oligomerisation. Procedures claimed to prevent the risk have been described in the literature but do not seem reliable. 

THE EARLY PRODUCT AND STRONG-ACID CATALYSIS DEVELOPMENT... 

Explain why compound 2 hydrolyzes at a rate that is independent of pH in the range pH 1.5 to 0.1 M NaOH, despite the fact that as a general rule acetals and monothioacetals show strong acid catalysis. 

The usefulness of the product in moist crosslinking is based on its good hydrolysis stability, which permits a strongly acid catalysis e.g. with CONDENSOL FN. The chlorine resistance of finishes obtained by moist crosslinking is good, provided crosslinking is complete. 

Treatment of a variety of substituted 2-aminobenzonitriles 15 with formic acid under strong acid catalysis provides quinazolin-4(l/f)-ones 16 in good yield.For tautomerism of 16, see p 9. 

Conversely, the presumed heterolytic C-N bond separation should involve a prior protonation step since this reaction takes place only under strong acid catalysis when it is run at room temperature. In the absence of acid, however, the same process may be thermally induced. There are two potential sites for proton attachment The aziridine nitrogen and the carbonyl oxygen. Both—as protonated species—are suitable to initiate fragmentation of the three-membered ring by way of intermediates VII and VIII, respectively (see Scheme 14.2). While evidence supporting the existence of VIII is available from proton nmr spectral analysis of a V-acrylaziridine in superacid media, other researchers ... 

In the following reaction, run under strong acid catalysis, two enol acetates could form. Predict which would be the major product. 

Similarly, many carbon-heteroatom bonds are also cleaved under strong acid catalysis involving pentacoordinate carbon intermediates [Eq. (6.17)]. 

The widespread use of their ion-exchange properties in the water-softening/detergent industry and their strong acid catalysis properties in petroleum-refining have made zeolites the workhorse materials in both applications. While this has attracted many researchers to the zeolite field, it has also had the effect of typecasting these remarkable materials into a limited number of chemical roles. The work reported below is a brief review of our own work here at Du Pont which attempts to dispel this stereotype by demonstrating that there are many remarkable similarities between zeolite structures and those of protein portions of natural... 

Early hydrocarboxylation-hydroesterification literature deals largely with Ni and Co as activating metals, but during the last three decades the noble group VIII metals, especially Pd, Pt, Rh and Ir, have been studied. Similarly, the use of pyridine promoted Co catalysts has been optimized. This section will not include references to metals of lesser or more specialized activity, such as Fe, Ru and Cu(I), nor strong acid catalysis, nor oxidative carbonylation of alkanes. 

Mesoporous aluminosilicates are of special interest as catalysts for the alkylation of ketoses, which are prone to degradation when treated with strong acid catalysts. Although disaccharides are generally subject to alcoholysis under strong acid catalysis, the use of MCM-41 as acid catalyst enables the selective preparation of alkylated disaccharides in good yield. 

Fully alkylated amino resins require strong acid catalysis for fast and/or low-temperature cross-linking. Their catalysis mechanism is different from that of partially alkylated resins which respond to weak acid catalysts or general acid catalysis. A fully alkylated melamine resin catalyzed by a strong acid catalyst is a faster curing (cross-linking) agent than a partially butylated amino resin. 

Features Gives rapid low temp, crosslinking, without need for strong acid catalysis... 

Though aromatic, pyridine is very resistant to electrophilic aromatic substitution and undergoes reaction only under drastic conditions. For example, nitration or bromina-tion requires high temperatures and strong acid catalysis. 

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