Kinetic control of product distribution

Thermodynamic Versus Kinetic Control of Product Distributions. 228... 

Bandstra JZ, R Miehr, RL Johnson, PG Tratnyek (2005) Reduction of 2,4,6-trinitrotoluene by iron metal kinetic controls on product distributions in batch experiments. Environ Sci Technol 39 230-238. 

We shall consider reactions catalysed by two different types of pro-catalyst the first (type A) employs Pd-allyl cations ([Pd(a]lyl)(PCy3)]+/Et3SiH or [Pd(allyl)(MeCN)2] + ), and the second (type B) employs Pd-alkyl or chloro complexes ([(phen)Pd(Me)(MeCN)]+, where phen = phenanthroline, and [(RCN)2PdCl2]). These two types of catalysts give very different products in the cyclo-isomerisation of typical 1,6-dienes such as the diallyl-malonates (10), Scheme 12.6. Since there is known to be a clear order of thermodynamic stability 11 < 12 <13, with a difference of ca. 3-4 kcal mol 1 between successive pairs, any isomerisation of products under the reaction conditions will tend towards production of 12 and 13 from 11 and 13 from 12. Clearly, when 11 is the major product (as with pro-catalysts of type A), it must be the kinetic product (see Chapter 2 for a discussion of kinetic and thermodynamic control of product distributions). However, when 12 is generated selectively, as it is with pro-catalysts of type B, there is the possibility that this is either generated by rapid (and selective) isomerisation of 11 or generated directly from 10. 

Miehr R, Bandstra JZ, Po R, Tratnyek PG. Remediation of 2, 4,6-trinitrotoluene (TNT) by iron metal kinetic controls on product distributions in batch and column experiments. 225th National Meeting, New Orleans, LA, American Chemical Society 2003 43(1). 

Similarly, reaction of chlorobenzene with isopropyl chloride imder Friedel—Crafts conditions produced primarily the ortho and para alkylation products after a short reaction period. Allowing the reaction mixture to stand for a week before workup, however, produced primarily the meta product. In both of these cases, the product isolated after a short time is said to be the product of kinetic control of the product distribution, since the product isolated is the one that is formed faster. The product isolated after a longer time is said to be the product of thermodynamic control of product distribution, since it is the more stable product. ... 

In later chapters, we will encounter several other reactions where competition occurs between the formation of the kinetic product and the thermodynamic product. However, most reactions are conducted under conditions where only one of these factors controls the product distribution. In addition, the kinetic product and the thermodynamic product are often the same, so no competition occurs in these reactions either. 

Over the range of conditions, 1-butene decomposes more rapidly than either of the 2-butene isomers. Double-bond shift and geometrical isomerization accompany the decomposition of the n-butenes however, skeletal isomerization does not occur, as isobutene is not found among the products of the pyrolysis. Isomerization reactions apparently are kinetically controlled, as equilibrium distributions are not generally observed. Trans cis ratios in the products do not correspond to equilibrium at either the maximum or the average reactor temperatures, and in some cases the ratio falls below equilibrium values based on American Petroleum Institute (API) data (14). However, none of these data exceed the equilibrium values based on more recent thermodynamic data (15). 

First, it is important to note that most aromatic electrophilic substitution reactions are under kinetic, and not thermodynamic, control. This is because most of the reactions are irreversible, and the remainder are usually stopped before equilibrium is reached. In a kinetically controlled reaction, the distribution of products (or product spread), i.e. the ratio of the various products formed, is determined not by the thermodynamic stabilities of the products, but by the activation energy barrier that controls the rate determining step. In a two-step reaction, it is a reasonable assumption that the transition state of the rate determining step is close in energy to that of the intermediate, which in this case is the Wheland intermediate and so by invoking the Hammond postulate, one may assume that they have similar geometries. 

Figure 1 Free-energy profiles for organic reactions under kinetic and thermodynamic control. In reactions under kinetic control, it is the stability of the transition states (for example, AG g) that determines the product distribution. For reactions under thermodynamic control, the product distribution is determined by the stability of the products (for example, AGac)-...

To determine the extent of bloating or expansion in an industrial rotary kiln, one must carry out laboratory tests using bench scale furnaces for the evolution kinetics and further correlation tests in a pilot rotary kiln for appropriate temperature profiles. The temporal events determined are, in turn, used to plan quarry operations for product quality control. The same data may also be useful in developing a mechanistic mathematical model that can predict temperature distribution and density changes in the raw material as they journey through the kiln (Boateng et al., 1997). Such tools have proven to be useful for the control of product quality as new mines are explored or even as different strata of the existing mine are explored for feedstock. Some of these time-temperature histories are discussed herein. 

Low cost High productivity Proper kinetic Control of polymer particle morphology Efficient comonomer incorporation Comonomer distribution control Influence on melt flow properties... 

Another way to assess thiophene s reactivity is to compare the intermediate ions formed by addition of N02. Examine the structures, charge distributions and electrostatic potential maps of thiophene+nitronium at C2 and thiophene+nitronium at C3. Draw all of the resonance contributors needed to describe these structures. Which, if either, better delocalizes the positive charge Compare the energies of the two intermediates. Which product should form preferentially if the reaction is under kinetic control Are these results consistent with FMO theory ... 

For the kinetically controlled formation of 1,3-disubstituted tetrahydro-P-carbolines, placing both substituents in equatorial positions to reduce 1,3-diaxial interactions resulted in the cw-selectivity usually observed in these reactions." Condensation reactions carried out at or below room temperature in the presence of an acid catalyst gave the kinetic product distribution with the cw-diastereomer being the major product observed, as illustrated by the condensation of L-tryptophan methyl ester 41 with benzaldehyde. At higher reaction temperatures, the condensation reaction was reversible and a thermodynamic product distribution was observed. Cis and trans diastereomers were often obtained in nearly equal amounts suggesting that they have similar energies."... 

Non-stirred, aerated vessels are used in the process for traditional products such as wine, beer and cheese production. Most of the newly found bioprocesses require microbial growth in an aerated and agitated system. The percentage distribution of aerated and stirred vessels for bioreactor applications is shown in Table 6.1. The performances of various bioreactor systems are compared in Table 6.2. Since these processes are kinetically controlled, transport phenomena are of minor importance. 

Since the transition state for alcohol oxidation and ketone reduction must be identical, the product distribution (under kinetic control) for reducing 2-butanone and 2-pentanone is also predictable. Thus, one would expect to isolate (R)-2-butanol if the temperature of the reaction was above 26 °C. On the contrary, if the temperature is less than 26 °C, (S)-2-butanol should result in fact, the reduction of... 

Even after a very short reaction time (001 sec) it is doubtful whether the isomer distribution (in the small amount of product that has as yet been formed) is purely kinetically controlled—the proportion of m-isomer is already relatively large—and after 10 sec it clearly is not m-benzyltoluene, the thermodynamically most stable isomer, predominating and the control now clearly being equilibrium or thermodynamic (p. 43). 

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