Nature and Position of Substituents

Molecular Flexibility, Polyfunctionality, and Spacing/Distance Between Reactive Croups 

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The nature and position of substituents on the arylidene ring markedly affect the ratio of addition products to cleavage products. As expected, electron-releasing substituents enhance the cleavage reaction. 

The oxazolo[3,4-a]azepinones 4, in which 5 7 ring fusion imparts considerable planarity and hence antiaromatic character on the ring system, undergo spontaneous dimerization.153 The mode of dimerization appears to depend on the nature and position of substituents. The unsubstituted system and the 9-chloro derivative 4 (R1 = Cl R2 = H) produce the exo.anti-dimers, e.g. 5, upon spray-vacuum pyrolysis at 300 C, whereas the 7-/ert-butyl, 7-bromo, 7-methyl, and 7,9-dichloro (4, R1 = R2 = Cl) compounds yield the exo,syn-dimcrs, e.g. 6. 

The other structural parameters, such as the ester group content, the presence, nature, and position of substituents on monomer units, and the number and nature of comonomers, are of less importance and do not modify this classification of hnear polyesters. 

The nature and position of substituents relative to the ring oxygen atom have an important effect on the acid-catalyzed isomerization of oxepins. It has been observed that, in addition to the hydrogen isotopes ( H, 2H, 3H), chloro, bromo, methyl (72E1129) and alkoxycarbonyl (79JA2470) substituents also show the migration-retention sequence found in the NIH shift. 

Introduction of substituent(s) adjacent to the electrophilic group, or of bulky substituent(s) on the molecule of attachment, will minimize its carcinogenic/ mutagenic potential. This is because the nature and position of substituents may influence electronic or steric effects on the bioactivation and reactivity/stability of many ultimate electrophilic metabolites. 

Annular tautomerism between one or more NH-forms and various CH forms of 1,2-diazepines is possible with the favored species dependent upon the nature and position of substituents (69ACS3125, 70T739). Interestingly, unlike monocyclic azepines and tautomeric 1,2-diazepines, 5H- 1,2-diazepines (47) exist preferentially in their bicyclic diazanorcaradiene form (48) (72JA2770). 

The UV spectra of many coumarins have been studied in different solvents with a view to correlating the effects of the nature and position of substituents (74MI22201). The spectral parameters do indeed show a dependence on these properties and several general rules have been proposed to enable the spectral features of a substituted coumarin to be predicted. Not unexpectedly, such predictions are not exact and a study of some disubstituted derivatives of coumarin has shown that the rules are not always obeyed (78T1221). The compounds which do not adhere to the rules possess a 4-methoxy group and the spectra are characterized by two double absorption bands in the ranges 267-296 and 303-322 nm. This behaviour has been attributed to the additional cross-conjugation provided by the methoxy substituent. 

Since compounds 10 and 30 are rather complicated, the authors could not attempt to determine the normal frequencies of the molecule, but had to restrict the analysis of spectra to the determination of characteristic frequencies. Thus, six frequencies have been found that can be used to identify 2-benzopyrylium cations. Band I appears between 1650-1610 cm-, and the pyrylium ring is responsible for this vibration [8a band according to Wilson s notation (34MI1)]. The position of this band and its intensity are dependent on the nature and position of substituents in the cation, and these changes are similar to data of monocyclic pyrylium salts [82AHC(Suppl)]. 

There has been considerable effort directed towards obtaining a fundamental understanding of the factors that govern the reactivities of carbon-centered radicals in bimolecular reactions, particularly with respect to their addition to alkenes [84]. From early liquid and gas phase studies, reactivity in such addition reactions was concluded to derive from a complex interplay of polar, steric, and bond-strength terms [85], which is much influenced by the nature and position of substituents on both the radical and the alkene. 

As with the benzocondensated series, the production of 3,1-benzoxazepines via the irradiation of quinoline-N-oxides is strongly affected by the solvent, and also by the nature and position of substituents on the pyridine moiety of the quinoline (aprotic solvents and Ph, CN, OMe, or CF3 groups attached to C(2) position favor formation of... 

The 13C NMR (79MI41801) and IR spectra (63CB2029) of numerous oxazolidines have been reported. The conformation of oxazolidines depends on the nature and position of substituents (74MI41800) ah initio calculations on the parent molecule indicate that the envelope conformation (56) with the nitrogen atom at the tip has the lowest energy (79JST(5l)l33). 

All of the azines are weak bases, in which the natures and positions of substituents alter the ease and orientation of N-protonation. This review does not aim to provide a comprehensive survey of basic characteristics within the series, but merely highlights the reactivities of particular ring nitrogen atoms with proton acids, since these reactivities are frequently related to trends observed in N-alkylation, N-acylation, and N-oxidation. 

Describe adenine and guanine in terms that indicate the nature and positions of substituent chemical groups on the purine ring. 

A distinctive feature of the H NMR spectra of homotropenylium ion derivatives is that the magnitude of the chemical shift difference (A ) between the S-exo and S-endo proton resonances is found to be dependent on the nature and position of substituents on the seven-membered ring . In all cases, a donor substituent on one of the basal ring carbons attenuates the chemical shift difference. For example, the 1- and 2-hydroxy substituted homotropenylium ions, 16 and 17, have values only some 50-55% of that found for the parent cation Systematic variation in the donor properties of the oxygen substituent using the Lewis acid scale developed by Childs and colleagues led to a linear change in A. ... 

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