Uncatalyzed reactions, kinetics

A. K. Galwey, Reactions in the Sohd State, in Bamford and Tipper, eds.. Comprehensive Chemical Kinetics, vol. 22, Elsevier, 1980. Galwey, A. K., Chemistry of Solids, Chapman and Hall, 1967. Sohn, H. Y, and W. E. Wadsworth, eds.. Rate Frocesses of Extractive Metallurgy, Plenum Press, 1979. Szekely, J., J. W. Evans, and H. Y. Sohn, Gas-Solid Reactions, Academic Press, 1976. Uhmann, ed., Enzyklopaedie der technischen Chemie, Uncatalyzed Reactions with Solids, vol.. 3, 4th ed., Verlag Chemie, 1973, pp. 395-464. 

Bromination is catalyzed by Lewis acids, and a study of the kinetics of bromination of benzene and toluene in the presence of aluminum chloride has been reported. Toluene is found to be about 35 times more reactive than benzene under these conditions. The catalyzed reaction thus shows a good deal less substrate selectivity than the uncatalyzed reaction, as would be expected on the basis of the greater reactivity of the aluminum chloride-bromine complex. 

The catalytic activity of doped nickel oxide on the solid state decomposition of CsN3 decreased [714] in the sequence NiO(l% Li) > NiO > NiO(l% Cr) > uncatalyzed reaction. While these results are in qualitative accordance with the assumption that the additive provided electron traps, further observations, showing that ZnO (an rc-type semi-conductor) inhibited the reaction and that CdO (also an rc-type semi-conductor) catalyzed the reaction, were not consistent with this explanation. It was noted, however, that both NiO and CdO could be reduced by the product caesium metal, whereas ZnO is not, and that the reaction with NiO yielded caesium oxide, which is identified as the active catalyst. Detailed kinetic data for these rate processes are not available but the pattern of behaviour described clearly demonstrates that the interface reactions were more complicated than had been anticipated. 

The overall change in free energy for the catalytic reaction equals that of the uncatalyzed reaction. Hence, the catalyst does not affect the equilibrium constant for the overall reaction of A -i- B to P. Thus, if a reaction is thermodynamically unfavorable, a catalyst cannot change this situation. A catalyst changes the kinetics but not the thermodynamics. 

Pure parathion is a pale yellow, practically odorless oil, which crystallizes in long white needles melting at 6.0° C. (17). It is soluble in organic solvents, except kerosenes of low aromatic content, and is only slightly soluble in water (15 to 20 p.p.m. at 20° to 25° C.). Peck (35) measured its rate of hydrolysis to diethyl thiophosphate and nitro-phenate ions in alkaline solutions. He found that the reaction kinetics are first order with respect to the ester and to hydroxyl ion. In normal sulfuric acid the rate of hydrolysis was the same as in distilled water. Peck concluded that hydrolysis takes place by two mechanisms—a reaction catalyzed by hydroxyl ions and an independent uncatalyzed reaction with water. He calculated that at a pH below 10 the time for 50% hydrolysis at 25° C. is 120 days in the presence of saturated lime water the time is 8 hours. The over-all velocity constant at 25° C. is k = 0.047 [OH-] + 4 X 10-6 min.-1... 

As a model esterification reaction, the formation of ethyl lactate has been studied and its complete kinetic and thermodynamic analysis has been performed. The formation rate of ethyl lactate has been examined as a function of temperature and catalyst loading. In early experiments, it was determined that lactic acid itself catalyzes esterification, so that there is significant conversion even without ion exchange resin present. The Arrhenius plot for both resin-catalyzed and uncatalyzed reactions indicates that the uncatalyzed... 

The kinetics of enzyme-catalyzed reactions (i. e the dependence of the reaction rate on the reaction conditions) is mainly determined by the properties of the catalyst, it is therefore more complex than the kinetics of an uncatalyzed reaction (see p.22). Here we discuss these issues using the example of a simple first-order reaction (see p.22)... 

The derivation (Sec. 2-2b) of the kinetics of catalyzed polyesterification assumes that the catalyzed reaction is much faster than the uncatalyzed reaction, that is, k 3> k. This assumption is usually valid and therefore one can ignore the contribution by the uncatalyzed polyesterification to the total polymerization rate. For example, k is close to two orders of magnitude larger than k for a typical polyesterification. For the atypical situation where k is not negligible relative to k , the kinetic expression for [M] or Xn as a function of reaction time must be derived [Hamann et al., 1968] starting with a statement of the polymerization rate as the sum of the rates of the catalyzed and uncatalyzed polymerizations ... 

The triphase hydrolysis of 1-bromoadamantane with catalysts 50 (n = 1-16) and 51 was studied kinetically 170). The enthalpies of activation (AH ) for the catalyzed reactions were 6-12 kcal/mol lower than for the uncatalyzed reaction. (The free energies of activation were 1-2 kcal/mol lower). This considerable variation in AH was attributed to a much different microenvironment in catalysts 50 and 51 from that in the absence of the catalyst. Thus Regen 170> pictures both the organic phase and the aqueous phase in the polymer matrix, as if homogeneous. Studies using spin-labeled compounds also indicated that catalyst 50 affects the polarity and the motional freedom of the microenvironment171). 

The parameters kcat and Km also allow us to evaluate the kinetic efficiency of enzymes, but either parameter alone is insufficient for this task. Two enzymes catalyzing different reactions may have the same kcat (turnover number), yet the rates of the uncatalyzed reactions may be different and thus the rate enhancements brought about by the enzymes may differ greatly. Experimentally, the Km for an enzyme tends to be similar to the cellular concentration of its substrate. An enzyme that acts on a substrate present at a very low concentration in the cell usually has a lower Km than an enzyme that acts on a substrate that is more abundant. 

The enzymatic activity of the L-19 IVS ribozyme results from a cycle of transesterification reactions mechanistically similar to self-splicing. Each ribozyme molecule can process about 100 substrate molecules per hour and is not altered in the reaction therefore the intron acts as a catalyst. It follows Michaelis-Menten kinetics, is specific for RNA oligonucleotide substrates, and can be competitively inhibited. The kcat/Km (specificity constant) is 10s m- 1 s lower than that of many enzymes, but the ribozyme accelerates hydrolysis by a factor of 1010 relative to the uncatalyzed reaction. It makes use of substrate orientation, covalent catalysis, and metalion catalysis—strategies used by protein enzymes. 

Addition ofbenzenethiol to 71 in the presence of a catalytic amount of tetrabutylammonium fluoride67 afforded only the trans isomer 7-(3,6-dideoxy - 2 - S - phenyl - 2 - thio -/ - l- erythro - hexopyranosyl- 4 -ulose)theo-phylline (105), In the absence of catalyst, 71 reacted with benzenethiol to give 7-(3,6-dideoxy-2-S-phenyl-2-thio-/ -L-threo-hexopyranosyl-4-ulose)theophylline (106). These results suggest that the uncatalyzed reaction gave the kinetic product, 106, whereas the catalyzed reaction led42 to the thermodynamic product 105. 

The [H + ] dependence of kobs has been interpreted in terms of an acid-catalyzed and an uncatalyzed reaction path, as shown in Scheme 6. The acid-catalyzed path has been proposed to involve protonation of one hydroxo bridge to give a labile aqua-bridged intermediate. The aqua-bridged complexes have in no case been identified, but spectroscopic results indicate that such species are very strong acids with Ka3 1. The kinetic parameters calculated from the rate expression, Eq. (55), are... 

The obtained function shows that the reactions follow first-order kinetics in monosaccharide. From formula 2 it is clear that the observed reaction rate is a combination of the rates of conversion of the catalyzed and the uncatalyzed reactions. Therefore the function obtained from the uncatalyzed reaction was subtracted from the overall function leaving the net catalyzed reaction. The reaction rate is obtained by taking the first derivative at t-0 of the function describing the net catalyzed reaction. 

Another subject that captured the attention of researchers in the 1970s was the identification of reaction conditions under which catalyzed and uncatalyzed reactions exhibit multiple steady states and/or oscillatory behavior. Theoretical investigations demonstrated that such behavior could arise from the nonlinear character of the reaction kinetics or from an interplay between the kinetics of a reaction and mass transport processes. A rich body of literature has now emerged detailing the space of reaction conditions and parameters within which multiple steady states and oscillations can be expected [15]. 

A catalyst speeds up a reaction by providing new pathways (elementary steps) with more favorable reaction kinetics than those that exist in the uncatalyzed reaction. Catalysts do so by interacting with the reactant in a reaction pathway that significantly lowers the activation energy, Ea, in comparison to the uncatalyzed reaction. 

With alkenes having internal C=C bonds, hydroalumination is disfavored by both kinetic and thermodynamic factors, and the uncatalyzed reaction is generally unfeasible. The hydroalumination of internal alkenes can be catalyzed by the addition of titanium(IV) alkoxides but the same catalysts also promote the isomerization of the secondary aluminum alkyls generated into their primary isomers (equation 16). ... 

The differences in the relative rate of the uncatalyzed reactions are due to the fact that the reactions involving alcohols, water, and carbanilide follow different kinetics and therefore are influenced in a different manner by reactant concentrations. 

As an example of an amine of unusually high catal3dic activity, Farkas et al. studied the 1,4-diazabicyclooctane catalyzed reaction of phenyl isocyanate with 2-ethylhexanol 39). This amine is a di-tertiary base, N (C2H4)3N, with the N atoms at the bridge heads. The reaction was found to follow the second-order kinetics, and the rate of reaction was proportional to the diazabicyclooctane concentration. The temperature dependence of the uncatalyzed and the catalyzed reaction between the 23° and 47° corresponds to an energy activation of 11.1 and 5.5 kcal./mole for the uncatalyzed reactions, respectively. 

At higher temperatures, chains become short (approaching 1) and limiting rates are reached in either catalyzed or uncatalyzed reactions. Under these conditions, kinetic equations for the simple hydroperoxide cycle indicate that the limiting rate is not a function of catalyst concentration. Many real systems approach this behavior [12]. 

The reactions used in kinetic methods fall into two categories catalyzed or uncatalyzed. As noted earlier, catalyzed reactions are the most widely used because of their superior sensitivity and selectivity. Uncatalyzed reactions are used to advantage when high-speed, automated measurements are required or when the sensitivity of the detection method is great. ... 

As noted earlier, kinetic methods based on uncatalyzed reactions are not nearly as widely used as those in which a catalyst is involved. We have already described two of these methods (pages 896 and 898). 

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