Reactive behavior

Three-Dimensional Modeling of Chemical Structures. The two-dimensional representations of chemical stmctures are necessary to depict chemical species, but have limited utiHty in providing tme understanding of the effects of the three-dimensional molecule on properties and reactive behavior. To better describe chemical behavior, molecular modeling tools that reflect the spatial nature of a given compound are required. 

Instead of the definition in Eq. (7-82), the selectivity is often written as log k,). Another way to consider a selectivity-reactivity relationship is to compare the relative effects of a series of substituents on a pair of reactions. This is what is done when Hammett plots are made for a pair of reactions and their p values are compared. The slope of an LEER is a function of the sensitivity of the process being correlated to structural or solvent changes. Thus, in a family of closely related LFERs, the one with the steepest slope is the most selective, and the one with the smallest slope is the least selective.Moreover, the intercept (or some arbitrarily selected abscissa value, usually log fco for fhe reference substituent) should be a measure of reactivity in each reaction series. Thus, a correlation should exist between the slopes (selectivity) and intercepts (reactivity) of a family of related LFERs. It has been suggested that the slopes and intercepts should be linearly related, but the conditions required for linearity are seldom met, and it is instead common to find only a rough correlation, indicative of normal selectivity-reactivity behavior. The Br nsted slopes, p, for the halogenation of a series of carbonyl compounds catalyzed by carboxylate ions show a smooth but nonlinear correlation with log... 

The quest for improved methods for elucidating and predicting the reactive behavior of molecules and other chemical species is a continuing theme of theoretical chemistry. This has led to the introduction of a variety of indices of reactivity some are rather arbitrary, while others are more or less directly related to real physical properties. They have been designed and are used to provide some quantitative measure of the chemical activities of various sites and/or regions of the molecule. 

Lehmann, H.A. and Grosmann, G., The reactivity behavior of P4 molecules and especially its "slow" reaction to give primary reaction products with oxidation levels between zero and three, Pure Appl. Chem., 52, 905, 1980. 

In order to understand the detailed reaction mechanism such as the regio-selectivity, apart from the global properties, local reactivity parameters are necessary for differentiating the reactive behavior of atoms forming a molecule. The Fukui function [10] if) and local softness [11] t.v) are two of the most commonly used local reactivity parameters. 

Biscarbene 34 was characterized by IR and UV/vis spectroscopy [49], The analysis of the experimental data showed that these are compatible with the presence of two phenylchlorocarbene (6) subunits in 34. This interpretation was further supported by the reactivity behavior of 34, which, like 6, is unreactive toward oxygen under conditions where triplet carbenes react fast. In contrast to its para isomer (22), 34 appears to undergo photochemical ring expansion analogous to that of 6[105]. In addition, the computed [RHF/6-31G(d)] IR spectrum of 34, which is in good agreement with the observed one, is based on the wave function for the singlet (cr /cr ) biscarbene (54 of Fig. 9). 

From the findings presented concerning the reactions of P4 it follows that the formation of Li3P7 and therefore also of P7(SiMe3)3 takes place in several interrelated reaction steps which influence one another, but which cannot yet be detailed. We lack both a detailed picture of the reactive behavior of these compounds and a reliable knowledge of the first steps of the formation reactions. Therefore we sought to understand the problem by investigating simpler systems able to help in its solution. A re " of this approach now follows. 

The reactive behavior of P4(SiMe3)4 (85) with respect to lithium alkyls was also the object of former investigations 47). It is these that firmly established the formation of Li3P7, P(SiMe3)3, and LiP(SiMe3)2. 

Just as it is useful to have a local ionization energy, so would it be desirable, in the context of reactive behavior, to have a local polarizability, a(r). Reflecting the discussion earlier in this section, we have suggested that 7s(r) be viewed as an inverse measure of as(r) we focus upon the surface local ionization energy and surface local polarizability because the outermost electrons are expected to make the greatest contributions to a. The volume dependence that is so important on a macroscopic scale should not be a factor on the local level, which considers only infinitesimal volume elements dr. We have presented evidence in support of the concept expressed by equation 14 ... 

The structure-reactivity behavior found for similar organosodium polymerization initiators of styrene [27] or that for addition reactions with 1,1-diphenylethylene [28] is almost identical with that found for the lithium initiators of Table 3.1. It is interesting to note from Table 3.1 that the reactivity of lithium... 

Other reactions for which a discussion of their structure-reactivity behavior in terms of the PNS has provided valuable insights include nucleophilic addition and substitution reactions on electrophilic alkenes, vinylic compounds, and Fischer carbene complexes reactions involving carbocations and some radical reactions. 

The first two derivatives in the series, monuron and metobromuron, are related to the 4-haloanilines. Their primary photochemistry has been studied by Boulkamh and Richard by means of nanosecond absorption spectroscopy [80]. The transients detected from both compounds in aqueous solution could be assigned to the N-substituted 4-iminocarbene, imino-p-benzoquinone-O-oxide and anilino radical from a complete analogy of their spectral and reactive behavior with that of the species obtained from 4-chloroaniline [55,57]. The quantum yields of carbene formation were determined to be = 0.051 for monuron and

halogen-substituted phenylurea derivatives underwent the same heterolytic dehalogenation process as the 4-haloanilines, which could be understood with reference to the protonability of the amine nitrogen, as in the case of 4-chloro-N,N-dimethylanilinc [55]. 

Figure 1 and Tables 2 and 4 reveal some interesting aspects of structure-reactivity behavior. Clearly, the rate at which the free enamine is protonated decreases in the order... 

Finally, we presented a case for which the large size of reactants and products induces transition state selectivity mainly due to differences in adsorption. The reactivity behavior is shown to follow the Polaniy-Br0nsted relation. 

Reactivity results for the 1273 K chars reacted in 0.1 MPa CO2 and 3.1 kPa steam at 1053 K are shown in Figures 3 and 4, respectively. The same trends evident for the 1273 K chars reacted in 0.1 MPa air are also seen in these reactant gases. Two regions of reactivity behavior are present in both cases. Calcium is seen to be a good char gasification catalyst, while Mg has very little catalytic effect. 

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