Flashing bond values

Table 10.5 lists the physical properties of the aromatic-type pyridine and the three isomeric methyl derivatives (picolines) as well as the 2-ethyl and 4-ethyl pyridine derivatives. Complete hydrogenation of the pyridine ring yields the cyclic secondary amine, piperidine, which is also listed in Table 10.5. The pyridine derivatives are medium boiling liquids with flash points in the range of 37-134°F. The substitution of a nitrogen atom for a carbon atom in the benzene ring gives the pyridine derivative with much higher Hansen polarity and hydrogen bonding values (5 = 8.8 and 5 , = 5.9) than the benzene molecule (6 = 0 and 6 = 2.0).

According to results from laser flash photolysis, the p-(methoxyphenyl) sulfanyl radical adds exclusively to the central atom in of 2,4-dimethylpenta-2,3-diene (If) with a rate constant of 1.1 x 10s M-1 s-1 (23 1 °C) (Scheme 11.6) [45], A correlation between the measured rate constants for addition of para-substituted arylsulfanyl radicals to allene If was feasible using Brown and Okomoto s o+ constant [46], The p+ value of 1.83, which was obtained from this analysis, was interpreted in terms of a polar transition state for C-S bond formation with the sulfanyl radical being the electrophilic part [45]. This observation is in agreement with an increase in relative rate constant for phenylsulfanyl radical addition to 1-substituted allene in the series of methoxyallene lg, via dimethylallene Id, to phenylsulfanylallene lh, to ester-substituted 1,2-diene li (Table 11.2). 

Experimental Analysis Despite the large number of works devoted to the investigation of the radical R" reactivity, the determination of the experimental values of the radical addition rate constants to a monomer double bond remains difficult. Only a small part of these values is based on a direct detection of the produced radical (flash photolysis, ESR). In fact, radicals are very often difficult to observe because of their absorption band below or close to 300 nm. Moreover, the classical overlap of the absorption spectra in the UV (due to the presence of different transient and ground-state species inherent to the production route when using laser flash photolysis) often prevents the direct observation. Indirect methods were tentatively used but they involve a complex set of reactions and are rather difficult to carry out. The same holds true for laser-induced photocalorimetry [265]. 

In this category appear all the alkenes having at least one (3(C-C) bond. The occurrence of the primary (3(C-C) bond rupture in acyclic olefins was first shown to be the main process by Callear and Lee. Using a flash photolytic system ( > 160 nm), they generated various allylic radicals and were thus able to measure the electronic spectrum of these radicals in the 210-250 nm region (61). The quantum yields of the rupture of this bond is 0.8 (47,49,62). For example, the 147.0 and 163.3 nm photolysis of n-l-hexene leads to the primary (3(C-C) bond rupture with a high quantum value (63) ... 

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