Physical Properties as Related to Chemical Reactions

Diffusion coefficients are typically higher in SCFs than in liquids. This is partly because the substances used as the solvent, such as carbon dioxide, have typically lighter and smaller molecules than organic liquid solvents and partly because the density of an SCF is typically less than a liquid. Consequently, reactions controlled by diffusion may be faster than in a liquid, giving the advantage of smaller process plant size. However, in the region of the critical point, diffusion coefficients can show an anomalous lowering, which can effect reaction rates. The behavior of diffusion coefficients is therefore discussed in Section 1.3.1 and its effect on reactions in Section 1.3.2. 

For reactions controlled by activation, solvation effects on the reagents and the transition state can affect the equilibrium coefficient for the formation of the transition state and therefore the reaction rate. Equilibria can also be affected by solvation effects on reagents and products. In a supercritical fluid, solvation effects can be controlled by density, and therefore at constant 

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Clifford AA (1999) Physical Properties of SCFs as Related to Chemical Reactions. In Jessop PG, Leitner W (eds) Chemical Synthesis in Supercritical Fluids. Wiley-VCH,... 

Enantiomers have identical chemical properties in relation to their reactions with achiral reagents. Their physical properties are identical (e.g. solubility, partition coefficients, boiling points, etc.) So why the interest in enantiomer composition This arises from the fact that in a chiral environment enantiomers behave as different compounds. The natural world is constructed of chiral systems that employ structure recognition mechanisms as a regulatory function [1,4,8], The single enantiomers of racemic drugs exhibit differences in their bioavailability, distribution, metabolism, and excretion. It is often the case that one enantiomer is the more active isomer for a given... 

A complete discussion of entropy is not required for an understanding of the properties of free energy as related to biochemical reactions. For more information about the properties and significance of entropy, refer to physical chemistry texts or Chemical Thermodynamics by I. M. Klotz, Prentice-Hall, New York (1950). 

When a molecule takes part in a reaction, it is properties at the molecular level which determine its chemical behaviour. Such intrinsic properties cannot be measured directly, however. What can be measured are macroscopic molecular properties which are likely to be manifestations of the intrinsic properties. It is therefore reasonable to assume that we can use macroscopic properties as probes on intrinsic properties. Through physical chemical models it is sometimes possible to relate macroscopic properties to intrinsic properties. For instance 13C NMR shifts can be used to estimate electron densities on different carbon atoms in a molecule. It is reasonable to expect that macroscopic observable properties which depend on the same intrinsic property will be more or less correlated to each other. It is also likely that observed properties which depend on different intrinsic properties will not be strongly correlated. A few examples illustrate this In a homologous series of compounds, the melting points and the boiling points are correlated. They depend on the strengths of intermolecular forces. To some extent such forces are due to van der Waals interactions, and hence, it is reasonable to assume a correlation also to the molar mass. Another example is furnished by the rather fuzzy concept nucleophilicity . What is usually meant by this term is the ability to donate electron density to an electron-deficient site. A number of measurable properties are related to this intrinsic property, e.g. refractive index, basicity as measured by pK, ionization potential, HOMO-LUMO energies, n — n ... 

We shall not be much concerned with differences in physical properties, as these differences are generally too small to be of much chemical consequence except for very light elements. Deu-terium oxide and ordinary water, for example, differ sufficiently in their solvent properties to affect noticeably rates and equilibria of reactions in aqueous solution. Even the small differences between isotopes of heavy elements are not devoid of all practical consequence, as witness the separation of uranium-235 from uranium-238 by diffusion processes. Indeed, some quite remarkable feats have been performed in the design of apparatus for separation of isotopes. This work, however, is more closely related to engineering than to chemistry and will not be considered further. 

It was during the first half of the 17th century that scientists began to study chemical reactions experimentally. Jan Baptista van Helmont laid the foundations of the law of conservation of mass. Van Helmont showed that in a number of reactions an aerial fluid was liberated which he defined as a gas. A new class of substances with their own physical properties was shown to exist. A kinetic-molecular theory of gases began to develop. Notable in this field were the experiments of Robert Boyle whose studies, later known as Boyle s law, provided an equation describing the inverse relation between pressure and volume of gas (see the ideal gas law in Chapter 3). 

Purity is typically only part of the equation determining the usefulness of an electronic material - most of the rest is performance. Performance can have many aspects including electronic properties of the material such as conductivity, free carrier mobility, etc., and physical and chemical properties such as mechanical strength, stability against diffusional mixing or reaction with adjacent materials, and many more. Electronic and optical properties are related to the way in which electrons interact with the atomic structure of the material. Chemical properties depend upon the atomic bonding and the possible reactions that can take place between one material and others that it touches. 

Adsorbers, distillation colunuis, and packed lowers are more complicated vessels and as a result, the potential exists for more serious hazards. These vessels are subject to tlie same potential haz. uds discussed previously in relation to leaks, corrosion, and stress. However, llicse separation columns contain a wide variety of internals or separation devices. Adsorbers or strippers usually contain packing, packing supports, liquid distributors, hold-down plates, and weirs. Depending on tlie physical and chemical properties of the fluids being passed tlirough tlie tower, potential liazards may result if incompatible materials are used for llie internals. Reactivity with llie metals used may cause undesirable reactions, which may lead to elevated temperatures and pressures and, ullinialely, to vessel rupture. Distillation columns may contain internals such as sieve trays, bubble caps, and valve plates, wliicli are also in conlacl with tlie... 

Chapter 12 discusses the use of the various monomers obtained from a petroleum origin for producing commercial polymers. Not only does it cover the chemical reactions involved in the synthesis of these polymers, but it also presents their chemical, physical and mechanical properties. These properties are well related to the applicability of a polymer as a plastic, an elastomer, or as a fiber. 

In processing, it is frequently necessary to separate a mixture into its components and, in a physical process, differences in a particular property are exploited as the basis for the separation process. Thus, fractional distillation depends on differences in volatility. gas absorption on differences in solubility of the gases in a selective absorbent and, similarly, liquid-liquid extraction is based on on the selectivity of an immiscible liquid solvent for one of the constituents. The rate at which the process takes place is dependent both on the driving force (concentration difference) and on the mass transfer resistance. In most of these applications, mass transfer takes place across a phase boundary where the concentrations on either side of the interface are related by the phase equilibrium relationship. Where a chemical reaction takes place during the course of the mass transfer process, the overall transfer rate depends on both the chemical kinetics of the reaction and on the mass transfer resistance, and it is important to understand the relative significance of these two factors in any practical application. 

The pentacoordinate molecules of trigonal bipyramidal form, like PF5, are a very nice example for the study of the formal properties of stereoisomerizations. They are characterized by an appreciable nonrigidity and they permit the description of kinetics among a reasonable number of isomers, at least in particular cases (see below). Therefore the physical and chemical properties of these molecules have been thoroughly investigated in relation to stereoisomerization. Recent reviews may be found in the literature on some aspects of this problem. Mislow has described the role of Berry pseudorotation on nucleophilic addition-elimination reactions and Muetterties has reviewed the stereochemical consequences of non-rigidity, especially for five- and six-atom families as far as their nmr spectra are concerned. 

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