Hydrolysis kinetics steric effects

Kinetic studies show that hydrolysis of 1-organyl- and 1-alkoxysilatranes in neutral aqueous solutions is a first-order reaction catalyzed by the formed tris(2-hydroxyalkyl)amine13 294. As a rule, electron release and steric effects of the substituent X hinder the reaction. However, the hydrolytic stability of 1-methylsilatrane is just below that of 1-chloromethylsilatrane294. Successive introduction of methyl groups into the 3, 7 and 10 sites of the silatrane skeleton13,294 and substitution with ethyl group on C-459 retard sharply the hydrolysis rate. It was proposed294 that nucleophilic attack at silicon by water proceeds via formation of the four-centered intermediate 57 (equation 56). 

Until relatively recently no kinetic studies on the nitrosation of alcohols had been reported, presumably since the reactions are very rapid and require special techniques. Some kinetic measurements on the reverse reaction, the hydrolysis of alkyl nitrites have been reported here conventional kinetic methods were used. Early workers examined the reactions of the series methyl, ethyl, i-propyl and t-butyl nitrites in an acetic acid-acetate buffer and found a small increase in rate constant along the series (Skrabal et a ., 1939). Later Allen measured the rate constants for the hydrolysis of a number of alkyl nitrites in aqueous dioxan solvent for both acid- and base-catalysed reactions (Allen, 1954). The rate constants for the O-nitrosation of alcohols were determined indirectly by measurement of the overall equilibrium constant for the process, by noting the change in the rate constant for the nitrosation of phenol in the presence of added alcohols. These, combined with the known data for the reverse hydrolysis reaction, enabled the rate constants for the forward reaction to be obtained (Schmid and Riedl, 1967). The reactivity sequence MeOH > EtOH > i-PrOH > t-BuOH was deduced, and attributed to a steric effect. 

Hammett (and related sigma) relationships have been applied to aquatic reactions of several classes of aromatic contaminants. For example, alkaline hydrolysis of triaryl phosphate esters fits a Hammett relationship (Table 3) is t he sum of the substituent constants for the aromatic groups and k0 is the hydrolysis rate constant for triphenyl phosphate (0.27 M 1 s-1 t1/2 = 30 days at pH 8). Triaryl esters thus hydrolyze much more rapidly than trialkyl or dialkyl-monoaryl esters under alkaline conditions. Rates of photooxidation of deprotonated substituted phenols by singlet oxygen have been found to be correlated with Hammett a constants (Scully and Hoigne, 1987). The electronic cllects of substituents on pKa values of substituted 2-nitrophenols also fit a I lammett relationship this, of course, is not a kinetic LFER. Two compounds (4-phenyl-2-NP and 3-methyl-2-NP) did not fit the relationship and were not included in the regression. Steric effects may account for the discrepancy for the latter compound. Nitrophenols are used as intermediates in synthesis of dyes and pesticides and also used directly as herbicides and insecticides. 

However, NMR studies, coupled with statistical modeling, contradict these arguments. Pouxviel and co-workers (12, 13) studied acid-catalyzed reactions of TEOS with a variety of W values. Simulated kinetic curves for temporal evolution of various silicon species observed by 29Si NMR spectroscopy were consistent with relative hydrolysis rate constants for sequential hydrolysis of the four -OEt substituents of 1 5.3 20 36. These trends were confirmed by more recent H NMR spectroscopy (14), which yielded values for the four successive hydrolysis steps for TEOS of 0.0143, 0.064, 0.29, and 1.3 min-1, respectively. Clearly these results indicate that inductive effects cannot be solely used to explain the relative hydrolysis rates. Steric bulk of the alkoxy substituent relative to hydroxy may have a dominant effect on hydrolysis rates. 

Reactives of Side-Chains of Monocyclic Thiophens. - The rate constants for the esterification of some 3-, 4-, and 5-substituted thiophen-2-carboxylic acids and of some 2- and 4-substituted thiophen-3-carboxylic acids with diazodiphenylmethane in methanol solution have been measured, and linear correlations gave information about the transmission of substituent effects. The rates of alkaline hydrolysis of ethyl thiophen-2-carboxylate in ethanol-water and DMSO-water media have been measured and compared with those of other heterocyclic esters. The kinetics of iodination of 2-acetylthiophen in methanol-water, using different carboxylate buffers, have been studied.Basicity constants have been measured for j3-(2-thienyl)-acrylamides and compared with those of the corresponding benzene and furan derivatives. The acidity constants of ( )-a-phenyl-j3-(2-thienyl)-acrylic acids and analogous furan-, selenophen-, and pyridine-substituted compounds have been measured, and have been rationalized by an equation involving separate contributions of polar, conjugative, and steric effects of the heterocycles. ... 

The hydrolysis kinetics of aliphatic esters (R, and R2 = alkyl groups) are also sensitive to electronic effects. The hydrolysis data for a series of aliphatic esters are summarized in Table 2.4. The addition of chloride substituents to the Rj group drastically increases the neutral and base hydrolysis rate constants. These data also demonstrate that as the steric bulk of R2 increases, there is a significant decrease in kb (compare ethyl acetate to t-butyl acetate). 

Of course, not all substitutions of interest occur on aromatic systems. Robert Taft s studies in the 1950s on the kinetics of the formation and hydrolysis of carboxylic esters laid the groundwork for the separation of the polar (electronic) effect of a substituent and its steric effect. He formulated equation... 

Lambe et al. (1978) studied the enhanced steric stabilization of polystyrene latices by poly(vinyl alcohol). This is included in this sub-section on copolymers because the samples studied were not fully hydrolysed. This means that the parent poly(vinyl acetate) from which they were derived was only partially (88%) hydrolysed (this is often accomplished by alcoholysis). The resultant polymer is not, however, a completely random copolymer because adjacent group effects influence the hydrolysis kinetics in such a way that some degree of blockiness is introduced. On average, these blocks consist of 2 ester groups to every 18 alcohol groups but blocks of average size 5-6 acetate groups are common. The chemical structure of the polymers should therefore formally represented by a structure intermediate between poly(vinyl acetate-6-vinyl dcohol) and poly(vinyl acetate-co-vinyl alcohol) rather than poly(vinyl alcohol) as such. The random (or statistical) copolymer can be prepared by partial reacetylation of fully hydrolysed poly(vinyl alcohol). 

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. 

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