Reactivities of organic compounds

Robert Filler, The Pentafluorophenyl Croup Effects on Reactivity of Organic Compounds... 

Ever since its original formulation, the Hammett equation has been one of the most widely discussed and applied relations between structure and reactivity of organic compounds, and it has been... 

A proposed mechanism must be consistent with all the facts about the reaction and with the reactivity of organic compounds. 

The reactivity of organic compounds toward eh varies systematically (Hart and Anbar, 1970), the variation often attributable to electron density of a specific functional group. Other generalizations are the following ... 

Siskin M, Katritzky AR (1991) Reactivity of organic compounds in hot water geochemical and technological implications. Science 254 231-237 Schubert K, Brandner J, Fichtner M, Linder G, Schygulla U, Wenka A (2001) Micro structure devices for applications in thermal and chemical process engineering. Microscale Thermophys Eng 5 17-39 Thayer AM (2005) Harnessing microreactions. Chem Eng News Coverstory 83 43-52. 

The introduction of a substituent in an organic compound may affect its reactivity in a given reaction. A number of quantitative relationships have been suggested in connection with the effect of substituents on the rate constant of the reaction. Such structure-reactivity co-relations are helpful in predicting the reactivity of organic compounds in various reactions and also in verifying the reaction mechanism. One such useful relationship was proposed by Hemmett, which relates the equilibrium and rate constants for the reaction of meta and para substituted benzene derivatives. 

Some general remarks on the reactivity of organic compounds... 

As the first commercial NMR instruments became available, a significant part of the empirical knowledge related to the structure and reactivity of organic compounds was under close scrutiny. Model compounds that could be used to test certain concepts or effects were subject to spectroscopic techniques and a framework for interpreting spectra based on structural properties began to develop. 

We conclude this chapter by introducing a simple tool with which we will be able to put the reactivities of organic compounds into an environmental context the well-mixed reactor or one-box model. 

The substitution method and the interconversion reactions discussed for proof of structure possibly may give you erroneous ideas about the reactions and reactivity of organic compounds. We certainly do not wish to imply that it is a simple, straightforward process to make all of the possible substitution products of a compound such as... 

This section is concerned with the quantitative correlation of reaction rates and equilibria of organic reactions with the structure of the reactants. We will restrict the discussion to benzene derivatives. The focus is on a remarkably simple treatment developed by L. P. Hammett in 1935, which has been tremendously influential. Hammett s correlation covers chemical reactivity, spectroscopy and other physical properties, and even the biological activity of drugs. Virtually all quantitative treatments of reactivity of organic compounds in solution start with the kinds of correlations that are discussed in this section. 

It is well documented that the structure of organic pollutants and speciation of inorganic pollutants determine the effectiveness of treatment by different technologies. For example, AOPs are used to destroy organic pollutants based on the molecular structures of the pollutants. In terms of reactivity of organic compounds, the following order may be expected ... 

Because ELUMO is a measure of the ability of a compound to accept electrons (i.e., act as an electrophilic species or undergo reduction), the above correlations show that the kinetic rates and activation energy increase as ELUM0 increases. Therefore, the ability of the compound to behave as an electrophilic species increases as ELUMO increases. The increased reactivity of organic compounds may have a direct influence on its degradation rate constants as follows ... 

We have described the use of liquid ammonia and liquid ammonia solutions of potassium amide in studies of hydrogen isotope exchange reactions in organic compounds. This method has been used to obtain new data on the reactivity of organic compounds, the effect of the reagents upon it, and the acidity of hydrocarbons. 

Some preliminary examples of hydrolysis reactions illustrate the very wide range of reactivity of organic compounds. For example, triesters of phosphoric acid hydrolyze in nearneutral solution at ambient temperatures with half-lives ranging from several days to several years (Wolfe, 1980), whereas the halogenated alkanes pentachloroethane, carbon tetrachloride, and hexachloroethane have "environmental" (pH = 7 25°C) half-lives of about 2 hr, 50 yr, and 1000 millennia, respectively (Mabey and Mill, 1978 Jeffers et al., 1989). On the other hand, pure hydrocarbons from methane through the PAHs are not hydrolyzed under any circumstances that are environmentally relevant. 

Reactivity of organic compounds toward HO is estimated readily in most cases because of the large databases of values measured for reactions in water and air and the correlation equations relating rates of oxidation of aliphatic compounds in air and water, as well as the availability of Hammett SARs for reactions of aromatic compounds. In addition, 90% of HO rate constants in water and air are within 50% of 5 x 109 M 1 s Thus, the limiting indirect photoreaction rate constant for almost any compound in freshwater with more than 1 mg/L of NO3 will be 1.5 x 10 7 s 1 at 40°-50° latitude in summer, equivalent to a 50-day half life. In low-nitrate waters, the limiting half life will be extended correspondingly. 

Ground-state stabilization by hyperconjugation can modify the reactivity of organic compounds significantly, and must be taken into account when estimating the stability of certain types of compound. Di- or polysubstituted methanes can have high ground-state stability if the substituents are simultaneously a-acceptors and jt-donors, as is observed for many heteroatoms. If, on the other hand, the substitu-... 

R. W. Taft (1922-1996) was professor of chemistry at Pennsylvania State College and then for thirty years at the University of California, Irvine. He did distinguished work in several fields of physical organic chemistry, e.g. structure-reactivity relationships, gas-phase reactivity of organic compounds, and the correlation analysis of solvent effects.299,300... 

Narayan, S., H. Muldoon, M.G. Finn, V.V. Fokin, H.C. Kolb and K.B. Sharpless, On Water Unique Reactivity of Organic Compounds in Aqueous Suspension, Angewandte Chemie International Edition, 44, 3275-3279 (2005). 

Siskin, M. and A.R. Katritzky, Reactivity of Organic Compounds in Superheated Water General Background, Chemical Reviews, 101, 825-836 (2001). 

L. M. Epshtein Hydrogen Bonds and the Reactivity of Organic Compounds in Proton Transfer and Nucleophilic Substitution Reactions, Usp. Khim. 48, 1600 (1979) Russ. Chem. Rev. 48, 854... 

As saturated hydrocarbons, the alkanes have the lowest chemical reactivity of organic compounds. However, under certain conditions these compounds can undergo substitution reactions, especially with the halogens. An example of such a reaction is that between an alkane, such as methane, and a halogen, such as chlorine. In this substitution reaction, a chlorine atom replaces a hydrogen atom on the methane molecule. 

Reactivity indices are linked to the activation energy of the process, defined as the potential energy difference between the initial state and the transition complex, both considered in an isolated state. In other words, no allowance is made for the magnitude of the entropy term and for the contribution from solvation and ion pairing, which may significantly alter the energy profile of the substitution reaction (Fig. 1). A feasible situation can occur when the structure of the transition state is not satisfactorily identifiable with the structure of the a intermediate or with that of the isolated substrate. When both approximations fail, application of reactivity indices may lead to a poor appraisal of the relative reactivity of organic compounds. 

Now that you have learned how to identify the mechanistic class of a particular reaction, you are ready to learn how to write the mechanism itself. This chapter is devoted to polar mechanisms that occur under basic conditions. Archetypical mechanisms will be presented and discussed so that you become familiar with the patterns of reactivity of organic compounds under basic conditions. 

In the following sections of this chapter we will present a few of the kinds of reactions that organic compounds undergo. A topic of such vast scope as reactivity of organic compounds can be made manageable only if we divide our study of this field into subtopics. Nearly all organic transformations involve at least one of three fundamental classes of reactions. The following three sections will address substitution, addition, and elimination reactions. We will also look at some reaction sequences that combine reaction steps from more than one of the fundamental classes. 

---