Acid-basic catalyzing mechanism

Acid-basic catalyzing mechanism considered the catalysis of enzyme as the proton transferring between enzyme and substrate, i.e. mutual transformation between acid and basic, thus decreased the required active energy of reaction and made 

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Finally, acid-catalyzed mechanisms are generally fast but must overcome the relatively low basicity of the tetrahedral intermediate (with electron-withdrawing substituents necessarily present, the basicity is low). 

This chapter compares the reaction of gas-phase methylation of phenol with methanol in basic and in acid catalysis, with the aim of investigating how the transformations occurring on methanol affect the catalytic performance and the reaction mechanism. It is proposed that with the basic catalyst, Mg/Fe/0, the tme alkylating agent is formaldehyde, obtained by dehydrogenation of methanol. Formaldehyde reacts with phenol to yield salicyl alcohol, which rapidly dehydrogenates to salicyladehyde. The latter was isolated in tests made by feeding directly a formalin/phenol aqueous solution. Salicylaldehyde then transforms to o-cresol, the main product of the basic-catalyzed methylation of phenol, likely by means of an intramolecular H-transfer with formaldehyde. With an acid catalyst, H-mordenite, the main products were anisole and cresols moreover, methanol was transformed to alkylaromatics. 

In general, the reactions in the addition phase of both the base- and acid-catalyzed mechanisms are reversible. The equilibrium constant for addition is usually unfavorable for acyclic ketones. The equilibrium constant for the dehydration phase is usually favorable, because of the conjugated a,/ -unsaturated carbonyl system that is formed. When the reaction conditions are sufficiently vigorous to cause dehydration, the overall reaction will go to completion, even if the equilibrium constant for the addition step is unfavorable. Entry 3 in Scheme 2.1 illustrates a clever way of overcoming the unfavorable equilibrium of the addition step. The basic catalyst is contained in a separate compartment of a Soxhlet extractor. Acetone is repeatedly passed over the basic catalyst by distillation and then returns to the reaction flask. The concentration of the addition product builds up in the reaction flask as the more volatile acetone distills preferentially. Because there is no catalyst in the reaction flask, the adduct remains stable. 

Problem-Solving Strategy Proposing Reaction Mechanisms 1007 Mechanism 21-8 Transesterification 1008 21-7 Hydrolysis of Carboxylic Acid Derivatives 1009 Mechanism 21-9 Saponification of an Ester 1010 Mechanism 21-10 Basic Hydrolysis of an Amide 1012 Mechanism 21-11 Acidic Hydrolysis of an Amide 1012 Mechanism 21-12 Base-Catalyzed Hydrolysis of a Nitrile 1014 21-8 Reduction of Acid Derivatives 1014... 

One of the most notable properties of sultam-modified substrates is that they undergo highly selective reactions in Lewis-acid-catalyzed as well as in thermal processes. There are a number of investigations into the basic selection mechanisms of the sultam auxiliary [5], which were carried out mainly by the groups of W. Oppolzer and D. Curran. In summary, the following model has arisen, which is described here giving the... 

The mechanism classification and the overall transformation classification are orthogonal to each other. For example, substitution reactions can occur by a polar acidic, polar basic, free-radical, pericyclic, or metal-catalyzed mechanism, and a reaction under polar basic conditions can produce an addition, a substitution, an elimination, or a rearrangement. Both classification schemes are important for determining the mechanism of a reaction, because knowing the class of mechanism and the overall transformation rales out certain mechanisms and suggests others. For example, under basic conditions, aromatic substitution reactions take place by one of three mechanisms nucleophilic addition-elimination, elimination-addition, or SrnL If you know the class of the overall transformation and the class of mechanism, your choices are narrowed considerably. 

Retro-aldol and retro-Michael reactions occur under acidic conditions. The mechanisms are the microscopic reverse of the aldol and Michael reactions, as you would expect. One of the most widely used acid-catalyzed retro-aldol reactions is the decarboxylation of jS-ketoacids, malonic acids, and the like. Protonation of a carbonyl group gives a carbocation that undergoes fragmentation to lose CO2 and give the product. Decarboxylation does not proceed under basic conditions because the carboxylate anion is much lower in energy than the enolate product. 

In Chapters 2 through 6 you learned how to draw polar basic, polar acidic, peri-cyclic, free-radical, and transition-metal-mediated and -catalyzed mechanisms. The reactions in the following problems may proceed by any of these mechanisms. Before you solve each problem, then, you need to identify its mechanistic class. See Chapter 1 if you have forgotten how to do so. 

The first step in the acid-catalyzed mechanism (Mechanism 20.5) is protonation of the carbonyl oxygen by the acid catalyst. Protonation of the carbonyl oxygen increases the acidity of the a hydrogen allowing it to be removed by a neutral and weakly basic solvent molecule in the second step. The product is the enol. 

Hydrolysis occurs by an acid-catalyzed mechanism involving a basic, protonated alkoxy substituent. Condensation occurs by a base-catalyzed mechanism involv-... 

The initial product of this reaction is an enol that cannot be isolated because it is rapidly converted into an aldehyde via tautomerization. As we saw in the previous section, tautomerization cannot be prevented, and it is catalyzed by either acid or base. In this case, basic conditions are employed, so the tautomerization process occurs via a base-catalyzed mechanism (Mechanism 10.3). 

Observations of the extent of reesterification of polysiloxanes [e.g., 26,59] indicate that it proceeds much further under acidic conditions than under basic conditions. This led Keefer [48] to the conclusion that the base-catalyzed mechanism involves inversion of configuration, while the acid-catalyzed mechanism does not. It is likely that the first step of the acid-catalyzed reesterification reaction involves the protonation of a silanol group, whereas under base-catalyzed conditions the first step is the deprotonation of an alcohol to form the nucleophile, OR . Therefore, the tendency for reesterification to be more complete under acidic than basic conditions may also result from the greater ease of protonation of silanol groups under the acidic conditions normally employed in sol-gel processing (pH 1-3) than deprotonation of alcohols under the weakly basic conditions normally employed (pH 8-10). 

It is generally agreed that both hydrolysis and condensation occur by adder base-catalyzed bimolecular nucleophilic substitution reactions involving, e.g., Sf Z-Si, S 2 -Si, or S 2 -Si transition states or intermediates. The acid-catalyzed mechanisms are preceded by rapid protonation of the OR or OH substituents bonded to Si, whereas under basic conditions hydroxyl or silanolate anions attack Si directly. Statistical and steric effects are probably most important in influencing the kinetics however. Inductive effects are certainly evident in the hydrolysis of organoalkoxysilanes. 

Under basic condition, first step takes place with dissociation of water to produce hydroxyl anions followed by the attack of these hydroxyl anions on the silicon atom (Fig. 18.8). It has been reported that -0 H displaces -0 Rby attack from the opposite side, which is similar to acid catalysis [37], Potassium hydroxide, HF, and amines are mainly used in acid-catalyzed mechanisms. 

The use of a reagent bearing a basic center or the addition of a base to the reaction mixture was recognized as necessary to prevent the acid-catalyzed elimination of the elements of water from the intermediates. Since the publication of this work, a number of similar intermediates have been isolated from thioamides and a-halogeno carbonyl compounds (608, 609, 619, 739, 754, 801), and as a result of kinetic studies, the exact mechanism of this reaction has been well established (739, 821). 

Possible role of the induced acidity and basicity in catalysis and environmental chemistry is discussed. The suggested mechanism explains the earlier reported promotive effect of some gases in the reactions catalyzed by Bronsted acid sites. Interaction between the weakly adsorbed air pollutants could lead to the enhancement of their uptake by aerosol particles as compared with separate adsoi ption, thus favoring air purification. 

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