Acid acting catalysts

Furthermore this chapter deals chiefly with polymerizations which are catalyzed by acid-acting catalysts. A comprehensive discussion of not only the thermal but even the photochemical and free radical-initiated polymerizations is outside its scope. The free radical-initiated reactions include those which are induced by metal alkylies, peroxides, oxygen and certain other substances. They depend on free radical initiation of a chain reaction whether or not these free radicals should be considered to be catalysts has been questioned because the radicals enter into the reaction chain and are part of the reaction product. 

In general, the catalysts may be classified as acids and metal halides. As will be explained below, both types of catalysts are acid-acting catalysts in the modern sense of the term. Some metals (e.g., sodium, copper, and iron) are catalysts for the polymerization of alkenes, especially ethylene. They are active probably because they can combine with one of the pi electrons of the alkene and form a free radical which can then initiate a chain reaction (p. 25). 

The alkylation of paraSins with olefins to yield higher molecular weight branched-chain paraffins may be carried out thermally or catalyt-ically. The catalysts for the reaction fall into two principal classes, both of which may be referred to as acid-acting catalysts (1) anhydrous halides of the Friedel-Crafts type and (2) acids. Representatives of the first type are aluminum chloride, aluminum bromide, zirconium chloride, and boron fluoride gaseous hydrogen halides serve as promoters for these catalysts. The chief acid catalysts are concentrated sulfuric acid and liquid hydrogen fluoride. Catalytic alkylations are carried out under sufficient pressure to keep at least part of the reactants in the liquid phase. 

Formaldehyde polymerizes even without added catalyst, but it is possible that traces of formic acid act as adventitious catalysts in this system. 

The products betu a strong formal resemblance to styrene and may be polymerised. For commercial purposes the monomers are not separated but are polymerised in situ in the crude naphtha, sulphuric acid acting as an ionic catalyst to give polymers with a degree of polymerisation of 20-25. 

Ostwald first came to catalysis through his work on the acceleration of homogeneous reactions by acids. This work was popular at the time although ultimately it would be shown to be incorrect because he believed that the acid, acting as a catalyst, did not enter into the chemical change which it influenced but rather acted by its mere presence (contact catalysis). 

The description of Eqs. (58) and (59) in terms of the mixture fraction and reaction-progress variables is described in detail by Fox (2003). Here we will consider a variation of Eq. (59) wherein the acid acts as a catalyst in the second reaction (Baldyga et al., 1998) ... 

Homocitrate acting as acid/base catalyst for protonating bound substrates or metal centers to produce hydrides... 

However, the driving force in this reaction is not large, so one usually ends up with an equilibrium mixture of water, salicylic acid, ASA and acetic acid. A better approach to produce ASA (aspirin) is to react acetic anhydride with salicylic acid in the presence of phosphoric or sulfuric acid acting as a catalyst ... 

In the case of the esterification of the diacid, the reaction is self-catalyzed as the terephthalic acid acts as its own acid catalyst. The reverse reaction, the formation of TPA and EG from BHET is catalytic with regard to the usual metal oxides used to make PET, but is enhanced by either the presence of hydroxyl groups or protons. In the case of transesterification of dimethyl terephthalate with ethylene glycol, the reaction is catalytic, with a metal oxide needed to bring the reaction rate to commercial potential. The catalysts used to produce BHET are the same as those needed to depolymerize both the polymer to BHET and BHET to its simpler esters. Typically, titanium, manganese and zinc oxides are used for catalysts. 

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. 

An acid-rhenium catalyst mixture acts on ( )-4-(4-hydroxyphenyl)butan-2-one oxime (44) to produce a high yield of the spiro compound (45), which then rearranges to the substituted quinoline (46)." ° The Beckmann rearrangement product (47)... 

The initiating reaction between aldoses and amines, or amino acids, appears to involve a reversible formation of an N-substituted aldosyl-amine (75) see Scheme 14. Without an acidic catalyst, hexoses form the aldosylamine condensation-product in 80-90% yield. An acidic catalyst raises the reaction rate and yet, too much acid rapidly promotes the formation of 1-amino-l-deoxy-2-ketoses. Amino acids act in an autocat-alytic manner, and the condensation proceeds even in the absence of additional acid. A considerable number of glycosylamines have been prepared by heating the saccharides and an amine in anhydrous ethanol in the presence of an acidic catalyst. N.m.r. spectroscopy has been used to show that primary amines condense with D-ribose to give D-ribopyrano-sylamines. ... 

Abstract The last few years have seen a considerable increase in our understanding of catalysis by naturally occurring RNA molecules, called ribozymes. The biological functions of RNA molecules depend upon their adoption of appropriate three-dimensional structures. The structure of RNA has a very important electrostatic component, which results from the presence of charged phosphodiester bonds. Metal ions are usually required to stabilize the folded structures and/or catalysis. Some ribozymes utilize metal ions as catalysts while others use the metal ions to maintain appropriate three-dimensional structures. In the latter case, the correct folding of the RNA structures can perturb the pKa values of the nucleo-tide(s) within a catalytic pocket such that they act as general acid/base catalysts. The various types of ribozyme exploit different cleavage mechanisms, which depend upon the architecture of the individual ribozyme. 

A novel finding related to the mechanism of catalysis by the genomic HDV ribozyme is that the pKa of C75 is perturbed to neutrality in the ri-bozyme-substrate complex and, more importantly, that C75 acts as a general acid catalyst in combination with a metal hydroxide which acts as a general base catalyst (Fig. 9A) [105]. The discovery of this phenomenon provided the first direct proof that a nucleobase can act as an acid/base catalyst in RNA. As a result, as shown by the solid curve in Fig. 9B, the curve that represents the dependence on the pH of the self-cleavage of the precursor genomic HDV ribozyme has a slope of unity at pH values that are below 7 (the activity increases linearly as the pH increases, with a slope of +1). Then, at higher pH values, the observed rate constant is not affected by the pH. 

There are several different routes to carboxamides. In most of these reactions, a carboxylic acid is converted to a more reactive intermediate, e.g. the acid chloride, which is then allowed to react with an amine. For practical reasons, it is preferable to form the reactive intermediate in situ. Arylboronic acids with electron-withdrawing groups such as (3,4,5-trifluorophenyl)boronic acid act as highly efficient catalysts in the amidation between carboxylic acids and amines. (3-Nitrophenyl)boronic acid and [3,5-bis(trifluoromethyl)phenyl]boronic acid are also effective eimidation catalysts and commercially available. 

Water is inexpensive, nontoxic and nonflammable. Replacing organic solvents with water may reduce volatile organic compound (VOC) emissions and CO2 production from solvent incineration. Supercritical water is less polar than ambient water and will dissolve many organic compounds that would not otherwise be soluble (Katritzky et al., 1996). At the same time, it can act as an acid, base, or acid-base catalyst (Katritzky et al., 1996). This can eliminate the wastes generated from neutralization steps. 

Oxides of transition metals can act as acid-base or redox catalysts. Oxides of non-transition metals (AI2O3, SiOj) are, however, good acid-base catalysts. There is a large family of aluminosilicate zeolitic acids (e.g. H -ZSM-5, H-mordenite). Micropor-ous aluminium phosphates (ALPOs) can be modified to yield acidic SAPOs (Si replaces... 

The catalyst is sometimes diluted with charcoal in the ratio of 1 1 to 2 1 of catalyst to charcoal (21). The charcoal acts as an adsorbent for the phosphoric acid released under operating conditions and distributes the acid over a larger portion of the bed. The phosphoric acid acts as the actual catalytic agent. 

By characterizing various zeolite catalysts under the same reaction conditions, the authors found weaker MAS NMR signals of alkoxy species for the less active zeolites HY and HZSM-5 than for the more active zeolite H-beta (250). This observation suggests that the alkoxy species observed under steady-state conditions act as reactive surface species in the MTBE synthesis from isobutylene and methanol on acidic zeolite catalysts. 

The oxonium ylide mechanism requires a bifunctional acid-base catalyst. The validity of the oxonium ylide mechanism on zeolites was questioned459,461,464 because zeolites do not necessarily possess sufficiently strong basic sites to abstract a proton from the trimethyloxonium ion to form an ylide. It should, however, be pointed out, as emphasized by Olah,447,465 that over solid acid-base catalysts (including zeolites) the initial coordination of an electron-deficient (i.e., Lewis acidic) site of the catalysts allows formation of a catalyst-coordinated dimethyl ether complex. It then can act as an oxonium ion forming the catalyst-coordinated oxonium ylide complex (10) with the participation of surface bound CH30 ions ... 

Overall second-order kinetics have been observed for catalysis by bases, alcohols or acids, and in the base-catalysed reactions the formation of ether groups is relatively insignificant46147). The base catalysis can be further activated by an acid co-catalyst, HA. For example any resident hydroxyl groups can act as internal co-catalysts. Tanaka and Kakiuchi48) proposed the following scheme for the reaction catalysed by base (B) with acid co-catalyst ... 

In enzymes, the most common nucleophilic groups that are functional in catalysis are the serine hydroxyl—which occurs in the serine proteases, cholinesterases, esterases, lipases, and alkaline phosphatases—and the cysteine thiol—which occurs in the thiol proteases (papain, ficin, and bromelain), in glyceraldehyde 3-phosphate dehydrogenase, etc. The imidazole of histidine usually functions as an acid-base catalyst and enhances the nucleophilicity of hydroxyl and thiol groups, but it sometimes acts as a nucleophile with the phos-phoryl group in phosphate transfer (Table 2.5). 

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