Hydrolysis kinetics inductive effects

Recently Stamhuis et al. (33) have determined the base strengths of morpholine, piperidine, and pyrrolidine enamines of isobutyraldehyde in aqueous solutions by kinetic, potentiometric, and spectroscopic methods at 25° and found that these enamines are 200-1000 times weaker bases than the secondary amines from which they are formed and 30-200 times less basic than the corresponding saturated tertiary enamines. The baseweakening effect has been attributed to the electron-withdrawing inductive effect of the double bond and the overlap of the electron pair on the nitrogen atom with the tt electrons of the double bond. It was pointed out that the kinetic protonation in the hydrolysis of these enamines occurs at the nitrogen atom, whereas the protonation under thermodynamic control takes place at the -carbon atom, which is, however, dependent upon the pH of the solution (84,85). The measurement of base strengths of enamines in chloroform solution show that they are 10-30 times weaker bases than the secondary amines from which they are derived (4,86). 

Studies of kinetic isotopic effects by Chong et al. [24] provided evidence for the formation of an oxocarbonium ion in the NA reaction. The prior results implicated that the formation of the oxocaibonium ion at C-2 is a key step in NA hydrolysis, but it has not yet been proposed any mechanism for its induction and stabilisation that may be fiilly compatible with all the structural, biochemical, and kinetic data. 

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. 

The previous sections have documented that the hydrolysis reaction of tetraalkoxy- and organoalkoxysilanes is influenced by sterlc and inductive effects and appears to be specific-acid-(H30 ) and specific-base-(OH ) catalyzed. The reaction order with respect to water and silicate is observed to be two and one, resulting in third- and second-order overall kinetics, respectively. Based on these factors, it is generally argued that hydrolysis proceeds by bimolecular nucleophilic displacement reactions (S y2-Si reactions) involving pentacoordinate intermediates or transition states... 

Silicon alkoxide condensation, 139-152 acid catalyzed, 148-150 base catalyzed, 145-148 catalyst effect, 140-142 effect of reverse reaction, 150-152 H20 Si effect, 197, 209 kinetics, 152-160 mechanism, 145-152 pH effect, 140-142, 197, 209 pressure effect, 147, 148 rate constant, 154, 160 solvent effect, 143-145 steric and inductive effects, 142, 143 Silicon alkoxide hydrolysis, 108, 109, 116-139,197 acid catalyzed, 131-134 base catalyzed, 134-136 effect of catalyst, 116-119 effect of fluorine, 118-119 effect of HjO.-Si, 123-127 kinetics, 118, 121-127, 152-160 mechanism, 116, 130-136 pressure effect, 134 rate constant, 154-155 solvent effect, 127-130 steric and inductive effects, 119-123 Silicon carbide-reinforced alumina, 865 Silicon carbide, 287-289, 736-737 Silicon cross-polarization NMR, 167-170, 220, 221... 

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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