Thermodynamic control of reaction

Materials chemistry contains all the elements of modem chemistry. These include synthesis, structure, dynamics, and properties. In synthesis, one employs all possible methods and conditions from high-temperature and high-pressure techniques to mild solution methods (chimie douce or soft chemistry).3 Chemical methods generally tend to be more delicate, often yielding novel, metastable products. Kinetic rather than thermodynamic control of reactions favors the formation of such structures. Supramolecular organization provides new ways of designing materials. 

Understand the concept of kinetic vs. thermodynamic control of reactions. 

Polysubstitution raises some complications for three reasons (a) when two polyfluoroalkyl groups are already present these can, in some cases, control the position of further substitution (b) some of the reactions are reversible and (c) substitution at the position most activated to attack sometimes results in crowding and therefore not the most thermodynamically stable system. This can lead to a competition between kinetic and thermodynamic control of reaction products [116, 122-128]. 

A few examples of increased selectivity can be found in the literature [85-90] where the steric course, and the chemo- or regioselectivity of reactions can be altered under the action of microwave irradiation compared with conventional heating. They have been listed and discussed in a recent review by De la Hoz et al. [91] and are discussed in Chapter 5 of this book. The main problem when trying to attribute accurately any MW effect lies in the fact that the kinetic control of reactions is not ensured. Of course, the only serious conclusions need to eliminate the thermodynamic control of reactions which is, unfortunately, highly probable when high temperatures are concerned. 

Finally, Lin (1988) advocates a more detailed elucidation of the differences between the kinetic, as compared with the thermodynamic, control of reactions. This, he suggests, is a way to avoid poor understanding about the nature of the products of a reaction in given conditions. 

Figure 6.28. Reprinted with permission from "A New Perspective on Kinetic and Thermodynamic Control of Reactions," by Snadden, R. B. /. Chem. Educ. 1985, 62, 653-655. Copyright 1985 American Chemical Society. 

The maximum rates of the reactions of most aldehydes and ketones with semi-carbazide occur in the pH range of 4.5-5.0. For the purpose of making derivatives of carbonyl compounds (Sec. 25.7), semicarbazide is best used in an acetate buffer (CH3CO2H/CH3CO2 ) solution, which maintains a pH in the maximum rate range of 4.5-5.0. However, to demonstrate the principle of kinetic and thermodynamic control of reactions, buffers that maintain higher pHs, and thus produce lower rates, are more desirable. Parts A-C of the experimental procedure involve a phosphate buffer system, whereas the bicarbonate system is used in Part D. It is then possible to compare how the difference in rates in the two buffer systems affects the product ratio. Analysis of the products from the various parts of these experiments provides strong clues as to which of the semicarbazones is the product of kinetic control and which is the product of thermodynamic control. 

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