Hydration reactions base-catalyzed

A MECHANISM FOR THE REACTION ] Base-Catalyzed Hemiacetal Formation 736 [ A MECHANISM FOR THE REACTION ] Hydrate Formation 737... 

The mechanism of this reaction is outlined m Figure 17 8 It is analogous to the mech anism of base catalyzed hydration m that the nucleophile (cyanide ion) attacks the car bonyl carbon m the first step of the reaction followed by proton transfer to the carbonyl oxygen in the second step... 

With pteridine (1) the covalent hydration is a complex matter since the general acid-base catalyzed reaction provides a good example of a kinetically controlled addition to the... 

The mechanistic pattern established by study of hydration and alcohol addition reactions of ketones and aldehydes is followed in a number of other reactions of carbonyl compounds. Reactions at carbonyl centers usually involve a series of addition and elimination steps proceeding through tetrahedral intermediates. These steps can be either acid-catalyzed or base-catalyzed. The rate and products of the reaction are determined by the reactivity of these tetrahedral intermediates. 

E. Rapid-Reaction Technique Because this technique and the apparatus involved are considered in detail in the following review, only a qualitative discussion is given here. This is the most valuable method for the confirmation of covalent hydration because it can usually give conclusive results even when the percentage of the hydrated species is as low as 2%. It makes use of the facts that aU known examples of the formation or disappearance of the hydrated species followed first-order kinetics and that the rates are both acid- and base-catalyzed. It also depends on the usual state of affairs that the ratio of the hydrated to the anhydrous species, although pH independent (see Section II, A), is different in the three species, i.e. in the cation, neutral species, and anion. In principle, a solution of one... 

The reversible hydration of nitrogen-containing heterocyclic bases and their hydroxy and mercapto derivatives is acid-base catalyzed and, at constant pH, the reactions obey first-order rate equations. 2 26.27-33... 

The nucleophilic addition of water to an aldehyde or ketone is slow under neutral conditions but is catalyzed by both base and acid. The base-catalyzed hydration reaction takes place as shown in Figure 19.4. The nucleophile is the... 

Both these methods require equilibrium constants for the microscopic rate determining step, and a detailed mechanism for the reaction. The approaches can be illustrated by base and acid-catalyzed carbonyl hydration. For the base-catalyzed process, the most general mechanism is written as general base catalysis by hydroxide in the case of a relatively unreactive carbonyl compound, the proton transfer is probably complete at the transition state so that the reaction is in effect a simple addition of hydroxide. By MMT this is treated as a two-dimensional reaction proton transfer and C-0 bond formation, and requires two intrinsic barriers, for proton transfer and for C-0 bond formation. By NBT this is a three-dimensional reaction proton transfer, C-0 bond formation, and geometry change at carbon, and all three are taken as having no barrier. 

Fig. 10.27. Mechanistic and stereochemical aspects of the hydration of trans-(R,K)-anethole epoxide in the absence (Reactions a and b) and presence of epoxide hydrolase (Reaction c) [178][179]. The ratio of Reaction a to bis ca. 4 1 in acid- and base-catalyzed hydration to yield the threo- and erythro-diols, respectively. The enzymatic Reaction c also yields preferentially the erythro-diol.

Aldol condensation of acetone is a well-known base-catalyzed reaction, and barium hydroxide is one of the catalysts for this reaction mentioned in textbooks. A family of barium hydroxide samples hydrated to various degress determined by the calcination temperature (473, 573, 873, and 973 K) of the starting commercial Ba(OH)2 8H2O were reported to be active as basic catalysts for acetone aldol condensation (282,286). The reaction was carried out in a batch reactor equipped with a Soxhlet extractor, where the catalyst was placed. The results show that Ba(OH)2 8H2O is less active than any of the other activated Ba(HO)2 samples, and the Ba(OH)2 calcined at 473 K was the most active and selective catalyst for formation of diacetone alcohol, achieving nearly 58% acetone conversion after 8h at 367 K in a batch reactor. When the reaction temperature was increased to 385 K, 78% acetone conversion with 92% selectivity to diacetone alcohol was obtained after 8h. The yield of diacetone alcohol was similar to that described in the literature in applications with commercial barium hydroxide, but this catalyst required longer reaction times (72-120 h) (287). No deactivation of the catalyst was observed in the process, and it could be used at least 9 times without loss of activity. 

Addition of hydrogen sulfide and thiols is qualitatively similar to reaction with alcohols in that there are two stages, formation of hemithioacetal (or hemithio-ketal) followed by acid-catalyzed elimination of the hydroxy group and substitution of a second —SR (Equations 8.47 and 8.48). The transformation has been studied less extensively than hydration and acetal formation, and relatively little information on mechanism is available. The initial addition appears to be specific base-catalyzed, an observation that implies that RS is the species that adds. The situation is thus similar to cyanide addition. General acid catalysis has, however, been found at pH 1 to 2 for addition of weakly acidic alkyl thiols, and the reaction rate as a function of pH has a minimum and rises both on the... 

Base-catalyzed hydration of conjugated carbonyls, followed by retro-aldol fragmentation has been a common strategy for studying the reaction cascade (1-4). The kinetically important step in the base-catalyzed hydration of an alpha/beta unsaturated carbonyl is similar to a nucleophilic substitution reaction at carbon 3. The reaction cascade proceeds rapidly from the conjugated carbonyl through its hydration and subsequent fragmentation. 

The rate-determining step is 3, the transfer of the OH proton to the conjugate base A . For the reverse process, the hydration of acetaldehyde, reaction 4 is rate-determining. For the base-catalyzed reaction the proposed mechanism is very similar ... 

Carbonyl often exists as gem-diol (hydrate) in a aqueous solution (R— CO—R + H2O. R—C(0H)2—R ). The nucleophile (hydroxide ion, HO—) in a base-catalyzed reaction is much more powerful than a water molecule, the nucleophile in neutral media. Aldehydes react faster than ketones because equilibrium constants for hydration are more favorable for aldehydes than they are for ketones. Aldehydes [CO(R )(H)] also react readily with amines (primary amines form an imine or Schiff bases R—N=C(R )(H) -1- H2O while secondary amines form an enamine (must have an aldehyde with hydrogen on alpha carbon). 

Several examples in previous sections fall under the heading of acid-base catalysis. Nitration of aromatics with catalyst acid HB and its conjugate base B is one of these (see reaction 4.6 in Section 4.1). Also, acid-catalyzed hydrolysis and hydration reactions such as 8.3 and 8.7 can be viewed as belonging to this category because the original catalyst actually is H30+ rather than H+ and is reconstituted in a step that involves its conjugate base, H20, as co-reactant. 

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