Rate-limiting reactions

An example of a commercial semibatch polymerization process is the early Union Carbide process for Dynel, one of the first flame-retardant modacryhc fibers (23,24). Dynel, a staple fiber that was wet spun from acetone, was introduced in 1951. The polymer is made up of 40% acrylonitrile and 60% vinyl chloride. The reactivity ratios for this monomer pair are 3.7 and 0.074 for acrylonitrile and vinyl chloride in solution at 60°C. Thus acrylonitrile is much more reactive than vinyl chloride in this copolymerization. In addition, vinyl chloride is a strong chain-transfer agent. To make the Dynel composition of 60% vinyl chloride, the monomer composition must be maintained at 82% vinyl chloride. Since acrylonitrile is consumed much more rapidly than vinyl chloride, if no control is exercised over the monomer composition, the acrylonitrile content of the monomer decreases to approximately 1% after only 25% conversion. The low acrylonitrile content of the monomer required for this process introduces yet another problem. That is, with an acrylonitrile weight fraction of only 0.18 in the unreacted monomer mixture, the low concentration of acrylonitrile becomes a rate-limiting reaction step. Therefore, the overall rate of chain growth is low and under normal conditions, with chain transfer and radical recombination, the molecular weight of the polymer is very low. 

Controlling an Enzyme That Catalyzes a Rate-Limiting Reaction Regulates an Entire Metabolic Pathway... 

Active control of metabolite flux involves changes in the concentration, catalytic activity, or both of an enzyme that catalyzes a committed, rate-limiting reaction. 

ALASl. This repression-derepression mechanism is depicted diagrammatically in Figure 32-9. Thus, the rate of synthesis of ALASl increases greatly in the absence of heme and is diminished in its ptesence. The turnover rate of ALASl in rat liver is normally rapid (half-life about 1 hour), a common feature of an enzyme catalyzing a rate-limiting reaction. Heme also affects translation of the enzyme and its transfer from the cytosol to the mitochondrion. 

If we assume that all the PB produced in the HDN of DHQ under our present reaction conditions was formed through the reaction of OPA, and that the PCH/PB ratio is the same for the reaction path through OPA in the HDN of DHQ and in the HDN of pure OPA, then the HC/PB ratios om the HDN of OPA can be apphed to the HDN of DHQ under the same reaction conditions the relative contributions of the two reaction pathways can be estimated. The results show that about 40% of the HDN reaction of DHQ takes place through the reaction path DHQ THQl OPA->HC in the absence of H2S but less than 10% in the presence of H2S. The very low concentration of PCHA (compared with that in the presence of H2S) also indicates that the rate limiting reaction steps might have changed in the absence of H2S. 

A classical non-linear model of chemical kinetics is defined by the Michaelis-Menten equation for rate-limited reactions, which has already been mentioned in Section 39.1.1 ... 

Stopped flow mixing of organic and aqueous phases is an excellent way to produce dispersion within a few milliseconds. The specific interfacial area of the dispersion can become as high as 700 cm and the interfacial reaction in the dispersed system can be measured by a photodiode array spectrophotometer. A drawback of this method is the limitation of a measurable time, although it depends on the viscosity. After 200 ms, the dispersion system starts to separate, even in a rather viscous solvent like a dodecane. Therefore, rather fast interfacial reactions such as diffusion-rate-limiting reactions are preferable systems to be measured. 

A plot of kohs against DNA concentration for reaction of tetraphenyl-porphyrin-l-tryptophan-cobalt(II) with calf thymus DNA is linear k 2 x 104 M 1 s 1 at room temperature. The suggested mechanism is of rapid reversible addition of water to 5 -coordinate [Co(trp)(tpp)] followed by rate-limiting reaction of [Co(trp)(tpp)(H20)] with the DNA (285). 

The rate-limiting process in Equation (8.20) involves the two species (peroxide and octane) colliding within the car cylinder and combining chemically. Because two species react in the rate-limiting reaction step, we say that the reaction step represents a bi molecular reaction. In alternative phraseology, we say the molecularity of the reaction is two . 

Numerous experimental studies have investigated the atomistic details of HE decomposition by examining the net products after thermal (low-pressure) decomposition (see, for example, Ref. 54). For RDX and HMX, the rate limiting reaction is most likely NO2 dissociation and a plethora of final products in the decomposition process have been isolated. Several theoretical studies have also... 

Purines are synthesized de novo beginning with PRPP as shown in Figure 1-18-4. The most important enzyme is PRPP amidotransferase, which catalyzes the first and rate-limiting reaction of the pathway. It is inhibited by the three purine nucleotide end products AMP, GMP, and IMP. 

BIMOLECULAR RATE PROCESSES. Many chemical processes involve the rate-limiting reaction of two components ... 

As pointed out earlier, CVD is a steady-state, but rarely equilibrium, process. It can thus be rate-limited by either mass transport (steps 2, 4, and 7) or chemical kinetics (steps 1 and 5 also steps 3 and 6, which can be described with kinetic-like expressions). What we seek from this model is an expression for the deposition rate, or growth rate of the thin film, on the substrate. The ideal deposition expression would be derived via analysis of all possible sequential and competing reactions in the reaction mechanism. This is typically not possible, however, due to the lack of activation or adsorption energies and preexponential factors. The most practical approach is to obtain deposition rate data as a function of deposition conditions such as temperature, concentration, and flow rate and fit these to suspected rate-limiting reactions. 

Percutaneous absorption is a time-dependent process, with passage through the stratum comeum as the rate-limiting reaction. Therefore, duration of exposnre to a xenobiotic is critical. It follows that the quick removal of spills is of the utmost importance. 

The frozen in photoconductivity, as was concluded by Melnick, will arise from effects of the surface barrier layer or, of course, would arise similarly from any other rate-limiting process in the adsorption of oxygen. For our model in this discussion we shall use electron transfer over the surface barrier as the rate-limiting reaction. In this case, the rate at which adsorption occurs is proportional to exp ( —Ei/kT), where E2 is the barrier height. Thus if we measure the decay in photoconductivity (the chemisorption of oxygen) at room temperature, and then suddenly quench the sample to 130°K, it is obvious that the rate of decay in photoconductivity will decrease considerably. The change in the rate will be dependent on Ei and the temperature to which the sample is quenched. 

B. catalyzes a rate-limiting reaction in porphyrin biosynthesis. 

A group at Monsanto has also studied the catalysis of the water-gas shift reaction by rhodium carbonyl iodide (103b). The main difference between their work and our own is the choice of reaction conditions. Their study was conducted at 185°C under 200-400 psig carbon monoxide. Despite this drastic difference in reaction conditions, the studies are surprisingly consistent. In particular, the Monsanto group also finds evidence for two rate-limiting reactions. They did not find this by temperature variation, but instead, consistent with our own work, find that at low acid and iodide... 

Formation of 7-hydroxycholesterol. The committed and rate-limiting reaction for bile acid synthesis is catalyzed by the endoplasmic reticulum enzyme 7c -hydroxylase. 

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