Other Factors Affecting Chemical Reactivity

Of the many factors controlling chemical reactivity some are obviously involved in the derivation of Equation 3.4, but some are not. Strain in the a framework, whether gained or lost, is not included, except insofar as it affects the energies of those orbitals which are involved. Factors which affect the entropy of activation are not included, nor are solvent effects. 

We cannot, then, expect this approach to understanding chemical reactivity to explain everything. Most attempts to check the validity of frontier orbital theory computationally indicate that the sum of all the interactions of the filled with the unfilled orbitals swamp the contribution from the frontier orbitals alone. Even though the frontier orbitals make a weighted contribution to the third term of the Salem-Klopman equation, they do not account quantitatively for the many features of chemical reactions for which they seem to provide such an uncannily compelling explanation. Organic chemists, with a theory that they can handle easily, have fallen on frontier orbital theory with relief, and comfort themselves with the suspicion that something deep in the patterns of molecular orbitals must be reflected in the frontier orbitals in some disproportionate way. 

One problem with this simple picture is that it seems inherently unlikely that pairs of electrons should act in concert—a pair of electrons in a single orbital spend as much time as far away from each other as possible, so why should a pair of electrons act together to move from one bond into another It seems more reasonable for them to move one at a time. The transfer of one electron from one molecule to another is well known—it is the basis for one-electron oxidation and one-electron reduction, with many examples in electrochemistry, in sodium-in-ammonia reductions, and in inorganic redox reactions—but is it a common pathway in ionic organic chemistry, or something that only happens in favourable circumstances  

We cannot, then, expect this approach to understanding chemical reactivity to explain everything. We should bear in mind its limitations, particularly when dealing with reactions in which steric effects are likely to be important, and in which solvent effects are involved. Solvent effects are well known, for example, to be a large part of the explanation of ambident reactivity and other manifestations of the principal of HSAB.246 Some mention of all these factors will be made again in the course of this book. Arguments based on the interaction of frontier orbitals are powerful, as we shall see, but they must not be taken so far that we forget these important limitations. 

---

This chapter deals with chemical and physical properties other than ones for which the nature of the hydration products must be considered, which are treated in Chapters 5 to 8. In general, properties of the whole clinker or cement are alone considered, those of the constituent phases having been dealt with in Chapter 1, but factors affecting the reactivities of these phases are included as a link with the following chapters on hydration. 

All these methods demonstrate that the 2-positions of pyridine, pyrimidine, and other azines are the most electron deficient in the ground state. However, considerably greater chemical reactivity toward nucleophiles at the 4-position is often observed in syntheses and is supported by kinetic studies. Electron deficiency in the ground state is related to the ability to stabilize the pair of electrons donated by the nucleophile in the transition state. However, it is not so directly related that it can explain the relative reactivity at different ring-positions. Certain factors which appear to affect positional selectivity are discussed in Section II, B. 

In this article, the authors have attempted to supply a reference to the majority of pertinent papers on gas-carbon reactions. Reasons for the large amount of apparently conflicting data on orders and activation energies for the reactions are advanced. A detailed quantitative discussion of the role which inherent chemical reactivity of the carbon and mass transport of the reactants and products can play in affecting the kinetics of gas-carbon reactions is presented. The possibilities of using bulk-density and surface-area profile data on reacted carbons for better understanding of reaction mechanisms is discussed. Finally, some factors, other than mass transport, affecting gas-carbon reactions are reviewed. 

For all these reasons, most of the acoustical energy involved in generating the cavities and in their collapse is ultimately spent in decomposing water into H2 and 02. This is the main factor affecting sonochemical efficiency (i.e., the ratio between the rate of the reaction of interest and the applied power density, W/L). In order to improve the efficiency of a sonochemical process, chemical or physical modifications can be introduced into the system, which may reduce this loss (see Sec. IV.G). The efficiency can also be affected by the presence of other chemicals in the solution, which may react with the radicals, thus reducing the number of reactive species available to the target molecules. A preprocess might be conceived to separate some of these unwanted chemicals from the solution prior to sonochemical treatment. 

For metals, the nature of the active metal surface determines its reactivity, as do both surface cleanness and metal particle size (4,9-11). Finely divided metals, with their correspondingly larger surface areas, show markedly greater reactivity towards alkyl halides than do corresponding bulk metals (20). Sono-chemical treatment of metal surfaces removes impurities and renders such surfaces correspondingly more reactive (21). Various other factors about metal surfaces that affect their adsorption of organic molecules are discussed in a book by Albert and Yates (22). 

The reactions of a series of arsonium ylides with p-nitrobenzaldehyde have been shown to be first order for each reagent and there is a general tendency for the more basic ylides to be the more reactive" ". The correlation is not, however, complete, since factors other than basicity, e.g. steric, and interactions between ylidic substituents and the arsenic atom, also affect the reactivity " , but as a generalization it is largely valid and also must be a significant factor in the greater reactivity of arsonium compared to phosphonium and sulphonium ylides. A fair correlation has also been noted between the chemical shift of the signal from the methine proton in a series of ylides and their rates of reaction with p-nitrobenzaldehyde" ". ... 

Other factors that affect the solubility of a gas are its size, and reactivity with the solvent. Heavier, larger gases experience greater van der Waals forces and tend to be more soluble. Gases that chemically react with a solvent have greater solubility. 

Chemical reaction depends on the presence of reactive substrates and on the probability of their encounters. Thus, the possibilities of reactions can be numerous. The literature describes reactions of OH groups on the surface of kaolin with isocyanates, vulcanization of nitrile rubber by ZnO, reactions of carboxyl groups on the filler surface with amines and epoxy groups, reactions of carboxyl groups with diols," and many others.The presence of a reactant on the surface of a material particle increases the probability of chemical reaction. Other factors include statistical probabilities, surface barriers which affect contact, dilution factors, molecular mobility, and viscosity changes in the system. These are discussed in other sections of this book. 

Many chemical component-s present in such aerosols are relatively stable they can be measured long after (days, week.s, or more) the aero.sol has been collected on a filter or impactor plate, for example. Short-lived reactive and/or volatile species such as peroxides and aldehydes are not usually determined. This may make it difficult to evaluate the health and ecological effects of aerosols because chemically reactive chemical species tend to be the most active biochemically. The chemical components present in the particles collected on a filter or impactor plate may react with each other when they are in close proximity. Particle deposits in filters or on surfaces may also react with molecular components of the gases flowing over them. Chemical reactions between the gas and aerosol may not affect mea.surement.s of metallic elements but may modify chemical speciation (compound form) on the collector surface. All of these factors must be taken into account in selecting sampling and measurement methods for aerosol chemical properties. 

Besides the shape, another important factor that affects the electronic properties and chemical reactivity of PAHs is the nature of the periphery. According to Clar s classification, the graphitic molecules with armchair and cove peripheries shown in Fig. 3.14 (A and B) are all-benzenoid PAHs. In addition to these linear topologies, Stein and Brown considered two other peripheral structures, i.e. acene-like (C) and quinoidaT (D) structures, which lie in a higher energy state and thus show higher chemical reactivity [62]. 

---