E Reactive intermediates

The oxidation of pesticide compounds usually generates products with aqueous mobilities that are either similar to or greater than that of the parent compound. The oxidation of aldicarb, for example, produces aldicarb sulfoxide and aldicarb sulfone, both of which have lower A/qc values than aldicarb (Moye and Miles, 1988). Similarly, because most phototransformations involve either the hydrolysis or oxidation of the parent compound, they yield products that are generally more polar (Mill and Mabey, 1985), and thus more water soluble than the parent compound. Reduction reactions, by contrast, may result in products that are less water soluble than their parent compound. Examples include the reduction of aldicarb sulfoxide to aldicarb (Miles and Delfino, 1985 Lightfoot et al, 1987) and the reduction of phorate sulfoxide to phorate (Coats, 1991). The reactivity of transformation products may be either higher or lower than that of their parent compounds. However, those in the former category (i.e., reactive intermediates) are, of course, much less likely to be detected in the hydrologic system than more stable products. 

Thermal reactions of 1,4,2-dioxa-, 1,4,2-oxathia- and 1,4,2-dithia-azoles are summarized in Scheme 1. The reactive intermediates generated in these thermolyses can often be trapped, e.g. the nitrile sulfide dipole with DMAD. 

The behaviour of pyrazoles towards nitrosation is similar to their behaviour described above towards diazo coupling, i.e. aminopyrazoles and pyrazolones readily react with nitrosation agents, like alkyl nitrites (81FES1019), to afford stable nitroso derivatives. Some simple nitrosopyrazoles have been isolated, for example the blue-green 3,5-dimethyl-4-nitrosopyrazole, and many others have been proposed as reactive intermediates in the direct conversion of pyrazoles into diazonium or diazo derivatives (Scheme 25) (B-76MI40402). 

The biotransformation of a given chemical compound in experimental animals and in humans may differ. Furthermore, high doses of chemical compounds are used in studies with experimental animals, and this may cause alterations in biotransformation of the tested chemicals that do not occur at the lower doses relevant to the human exposure situation. For example, a metabolic pathway dominating at low doses may become saturated, and a salvage metabolic pathway, e.g., one that produces reactive intermediates of the compound, may become involved in the biotransformation of the chemical. Since this intermediate could never be produced at the exposure levels encountered in humans, the overall result... 

In the course of this development, knowledge about low valent (in the sense of formal low oxidation states) reactive intermediates has significantly increased [26-30]. On the basis of numerous direct observations of silylenes (silanediyles), e.g., by matrix isolation techniques, the physical data and reactivities of these intermediates are now precisely known [31], The number of kinetic studies and theoretical articles on reactive intermediates of silicon is still continuously growing... 

Structures which cannot be obtained by experiment, e.g. highly reactive intermediates and activated complexes, can be investigated. 

The model process Eq. (15) has been studied by means of the MINDO/3 method to clarify the energetic conditions during the formation of cyclic reactive intermediates in cationic propagation of alkoxy-substituted monomers. The enthalpies of formation in the gas phase AH°g of both the alternative structures e and /were supplemented by the solvation energies Eso]v for transition into solvent CH2C12 with the assistance of the continuum model of Huron and Claverie which leads to heats of formation in solution AH° s. Table 13 contains the calculated results. 

Acid and base can, of course, induce many reactions of organocobalt(III) complexes (see Section B below, and Section VI,B, C, D), presumably through the formation of reactive intermediates by the gain or loss of a proton, e.g.. 

Heating of bis(trimethylsilylmethyl)sulfoxide 1166 generates HMDSO 7 and, via 1167, the reactive intermediate thioformaldehyde-S-methyhde 1168, which can be trapped in situ, e.g. by N-methylmaleimide, to give 81% of the l,3-dipolar cycloaddition product 1169 [14] (Scheme 8.3). Further analogous 1,3-dipolar cycloadditions with acetylenes are discussed elsewhere [15]. 

The absence of overlapping of bands of various matrix-isolated compounds and the possibility of freezing highly reactive intermediates make this method very convenient for the direct study of reaction mechanisms. Additionally, direct IR spectroscopy of intermediates allows estimation of important structural parameters, e.g. valence force fields, which show the character of bonds in these species. 

NO2 NO -I- O. Oxygen atoms are known to be highly reactive, so it is reasonable to predict that this intermediate reacts rapidly with NO2 molecules. Compared with the fast second step of this mechanism, the step that forms the oxygen atoms is e.xpected to be slow and rate-determining. Similarly, the first step of Mechanism It, NO2 NO3 + NO, produces NO3. This species is known to be unstable, so it will decompose in the second step of Mechanism II almost as soon as it forms. Again, the second step of the mechanism is expected to be fast, so the step that forms the reactive intermediate is slow and rate-determining. Later in this chapter, we discuss experiments that make it possible to distinguish between Mechanisms I and II. 

---