Base-catalyzed hydrolysis, mechanism

The condensation of silanols in solution or with surfaces has not been as extensively studied and therefore is less well understood. The limitation until recently has been the lack of suitable analytical methods necessary to monitor in real time the many condensation products that form when di- or trifunctional silanols are used as substrates. With the advent of high-field wSi-NMR techniques, this limitation has been overcome and recent studies have provided insights into the effects of silanol structure, catalysts, solvent, pH, and temperature on the reaction rates and mechanisms. Analysis of the available data has indicated that the base catalyzed condensation of silanols proceeds by a rapid deprotonation of the silanol, followed by slow attack of the resulting silanolate on another silanol molecule. By analogy with the base catalyzed hydrolysis mechanism, this probably occurs by an SN2 -Si or SN2 -Si type mechanism with a pentavalent intermediate. The acid catalyzed condensation of silanols most likely proceeds by rapid protonation of the silanol followed by slow attack on a neutral molecule by an SN2-Si type mechanism. 

In contrast to acetals, which are base-stable, hemiacetals undergo base-catalyzed hydrolysis. In the alkaline pH range, the mechanism shifts toward a base-catalyzed elimination. 

Some elucidation of the mechanism of elastomer reinforcement is being obtained by precipitating chemically-generated fillers into network structures rather than blending badly agglomerated filler particles into elastomers prior to their cross-linking. This has been done for a variety of fillers, for example, silica by hydrolysis of organosilicates, titania from titanates, alumina from aluminates, etc. [85-87], A typical, and important, reaction is the acid- or base-catalyzed hydrolysis of tetraethylorthosilicate ... 

Bob s research interests and knowledge across chemistry were great. Throughout his career he retained an interest in biomimetic chemistry, specifically the study of metal ion-promoted reactions and reactions of molecules activated by metal ion coordination. His early interests in carbohydrate chemistry inspired him to study metal ion catalysis of both peptide formation and hydrolysis as well as studies in inorganic reaction mechanisms. He was particularly interested in the mechanisms of base-catalyzed hydrolysis within metal complexes and the development of the so-called dissociative conjugate-base (DCB) mechanism for base-catalyzed substitution reactions at inert d6 metal ions such as Co(III). 

Fig. 7.2. a) The most common mechanism of base-catalyzed ester hydrolysis, namely specific base catalysis (HCT catalysis) with tetrahedral intermediate and acyl cleavage. Not shown here are an W mechanism with alkyl cleavage observed with some tertiary alkyl esters, and an 5n2 mechanism with alkyl cleavage sometimes observed with primary alkyl esters, particularly methyl esters, b) Schematic mechanism of general base catalysis in ester hydrolysis. Intermolecular catalysis (bl) and intramolecular catalysis (b2). c) The base-catalyzed hydrolysis of esters is but a particular case of nucleophilic attack. Intermolecular (cl) and intramolecular (c2). d) Spontaneous (uncatalyzed) hydrolysis. This becomes possible when the R moiety is... 

An esterifying substituent of value in prodrug design is the (2-oxo-l,3-dioxol-4-yl)methyl group (8.62 in Fig. 8.4). The interest in this moiety was first documented with prodrugs of antibiotics, as discussed below. To the best of our knowledge, no detailed or systematic studies on the mechanism of activation of (oxodioxolyl)methyl derivatives have been published. The available evidence supports a clean reaction of base-catalyzed hydrolysis as shown in Fig. 8.4, although the actual mechanism may be more complex and/or condition-dependent [74][75]. Briefly, hydrolysis liberates the pro-... 

Fig. 8.5. Mechanism postulated for competitive, specific base catalyzed hydrolysis and acyl migration of catechol monoesters, as seen with 4-pivaloyl-L-dopa (8.81) [114a]. Deprotonation (Reactions a and b) accelerates intramolecular nucleophilic attack (Reactions c and d) to form a tetrahedral transition state. The latter is postulated to be the intermediate common to hydrolysis (Reaction e) and acyl migration. 

The reactivity of organic nitrites toward hydrolysis differs markedly from that of organic nitrates. The reaction mechanisms are summarized in Fig. 9.2. The base-catalyzed hydrolysis of organic nitrites (Fig. 9.2,a) is quite slow, in... 

Fig. 9.2. Simplified reaction mechanisms in the hydrolytic decomposition of organic nitrites. Pathway a Base-catalyzed hydrolysis with liberation of nitrite. Pathway b Reversible nitro-syl exchange between organic nitrites and alcohols. Pathway c General acid catalysis with concerted mechanism in the acid hydrolysis of organic nitrites.

The hydrolysis of diphenhydramine and analogues (11.24, Fig. 11.2) has been studied extensively [46 - 48], These compounds are essentially inert toward base-catalyzed hydrolysis, but do undergo proton-catalyzed hydrolysis, the mechanism of which is shown in Fig. 11.2. The reaction begins with protonation of the ether O-atom and continues with the irreversible heterolytic cleavage of the C-0 bond to produce the benzhydryl cation. This reaction is greatly facilitated by the weakening effect of the benzhydryl moiety on the adjacent C-0 bond. The benzhydryl cation itself is stabilized by resonance, which also explains why the reaction is facilitated. The last step is the for-... 

There are an extremely large number of reactions of 2-oxetanones with nucleophilic reagents, and space will allow inclusion of only representative examples. /3-Lactones show the interesting Bal.2 mechanism for base-catalyzed hydrolysis and the Aal2 mechanism for acid-catalyzed hydrolysis, according to data on kinetics and optical rotation studies of optically active lactones. The mechanistic interpretations are complicated, however, by the possibilities for subsequent elimination and addition reactions to occur, so that both of the two sites for nucleophilic attack on the 0-lactone skeleton, C-2 and C-4, may become involved. In fact 0-lactones are unusually insensitive to base, as well as acid, catalysis, the slow reaction with neutral water predominating between pH 1 and 9 (74JCS(P2)377). 

Taft, following Ingold,39 assumed that for the hydrolysis of carboxylic esters, steric and resonance effects will be the same whether the hydrolysis is catalyzed by acid or base (see the discussion of ester-hydrolysis mechanisms, reaction 0-10). Rate differences would therefore be caused only by the field effects of R and R in RCOOR. This is presumably a good system to use for this purpose because the transition state for acid-catalyzed hydrolysis (7) has a greater positive charge (and is hence destabilized by - / and stabilized by + / substituents) than the starting ester, while the transition state for base-catalyzed hydrolysis (8)... 

Base-Catalyzed Hydrolysis. Let us now look at the reaction of a carboxylic ester with OH", that is, the base-catalyzed hydrolysis. The reaction scheme for the most common reaction mechanism is given in Fig. 13.11. As indicated in reaction step 2, in contrast to the acid-catalyzed reaction (Fig. 13.10), the breakdown of the tetrahedral intermediate, I, may be kinetically important. Thus we write for the overall reaction rate ... 

Figure 13.13 Reaction scheme for the base-catalyzed hydrolysis of carbamates when the mechanism involves a tetrahedral intermediate.

Abstract—A review of the literature is presented for the hydrolysis of alkoxysilane esters and for the condensation of silanols in solution or with surfaces. Studies using mono-, di-, and trifunctional silane esters and silanols with different alkyl substituents are used to discuss the steric and electronic effects of alkyl substitution on the reaction rates and kinetics. The influences of acids, bases, pH, solvent, and temperature on the reaction kinetics are examined. Using these rate data. Taft equations and Brensied plots are constructed and then used to discuss the mechanisms for acid and base-catalyzed hydrolysis of silane esters and condensation of silanols. Practical implications for using organofunctional silane esters and silanols in industrial applications are presented. 

The relatively large value of 1.67 for s seems to fit an SN2 -Si or SN2 -Si mechanism better rather than an SN2-Si type mechanism. In addition, SN2-type mechanisms generally give a small negative p value. For example, solvolysis of primary alkyl p-toluenesulfonaies in ethanol has a p value of —0.746 [53]. The large positive p for base catalyzed hydrolysis is inconsistent with a simple SN2-type mechanism. 

The Sn2-S1 mechanism of neutral hydrolysis and the parallel operation of the SN2-Si and SnI-SI mechanisms in base-catalyzed hydrolysis of 1-aryloxysilatranes in aqueous solutions and in media of different polarity were proposed295-297. The contribution of the SnI-SI pathway in the latter process was presumed to vary from 0 to 60%, depending on the conditions297. 

Figure 78 Hydrolysis reactions of Mitomycin C involving acid-catalyzed (SN1 mechanisms) aziridine ring opening to cis and trans-mitosQnQ and base-catalyzed, hydrolysis of the amino moiety.

The base catalyzed hydrolysis (figure 1.6) is a two-step process, with formation of a pentacoordinate intermediate. Acid catalyzed hydrolysis proceeds by an SN2-type mechanism. The leaving alkoxy group is rapidly protonated and a water molecule performs a nucleophilic attack at the central silicon atom. 

Mechanism of the base-catalyzed hydrolysis (saponification) of an ester. 

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... 

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