Substituent effects rings

A nitro group behaves the same way m both reactions it attracts electrons Reaction is retarded when electrons flow from the aromatic ring to the attacking species (electrophilic aromatic substitution) Reaction is facilitated when electrons flow from the attacking species to the aromatic ring (nucleophilic aromatic substitution) By being aware of the connection between reactivity and substituent effects you will sharpen your appreciation of how chemical reactions occur... 

Many studies have been made of substituent effects in saturated heterocyclic six-membered rings. For a detailed discussion the review of Eliel and Pietrusziewicz should be consulted [Pg.15]

Potassium t-butoxide in t-butyl alcohol requires powerful electron-attracting substituents at C-4 to effect ring opening of pyrazoles but sodamide does not (Scheme 26) (B-76MI40402). As the key to the transformation is the generation of the anion, similar results were obtained by heating some pyrazole-3-carboxylic acids with quinoline. 

Representative chemical shifts from the large amount of available data on isothiazoles are included in Table 4. The chemical shifts of the ring hydrogens depend on electron density, ring currents and substituent anisotropies, and substituent effects can usually be predicted, at least qualitatively, by comparison with other aromatic systems. The resonance of H(5) is usually at a lower field than that of H(3) but in some cases this order is reversed. As is discussed later (Section 4.17.3.4) the chemical shift of H(5) is more sensitive to substitution in the 4-position than is that of H(3), and it is also worth noting that the resonance of H(5) is shifted downfield (typically 0.5 p.p.m.) when DMSO is used as solvent, a reflection of the ability of this hydrogen atom to interact with proton acceptors. This matter is discussed again in Section 4.17.3.7. 

Substituent effects (electronegativity, configuration) influence these coupling constants in four-, five- and seven-membered ring systems, sometimes reversing the cis-tmns relationship so that other NMR methods of structure elucidation, e.g. NOE difference spectra (see Section 2.3.5), are needed to provide conclusive results. However, the coupling constants of vicinal protons in cyclohexane and its heterocyclic analogues (pyranoses, piperidines) and also in alkenes (Table 2.10) are particularly informative. 

A choice can be made between these two with the help of published C substituent effects for the substituents (-NH2, -NH3, -COOR see Section 2.5.4) on the benzene ring in A and B ... 

Adding these substituent effects gives the following calculated shift values (as compared with the observed values in parentheses) for C-1 to C-4 of the pura-disubstituted benzene ring in A and B ... 

Substituent effects calculated for structure B lead to values which are not perfect but which agree more closely than for A with the measured C shifts of the benzene ring carbon atoms. The dia-stereotopism of the NC//2 protons in the H NMR spectrum also points to B as the Newman projection C along the C/fj-ammonium-N bond shows ... 

Nevertheless, many free-radical processes respond to introduction of polar substituents, just as do heterolytic processes that involve polar or ionic intermediates. The substituent effects on toluene bromination, for example, are correlated by the Hammett equation, which gives a p value of — 1.4, indicating that the benzene ring acts as an electron donor in the transition state. Other radicals, for example the t-butyl radical, show a positive p for hydrogen abstraction reactions involving toluene. ... 

IV. Transmission of Substituent Effects through Heterocyclic Systems A. Six-Membered Ring Systems... 

Reaction Series Involving the Transmission of Substituent Effects Through Heterocyclic Rings... 

Reactions involving monocyclic six-membered heteroaromatic rings have not been studied sufficiently extensively to allow a quantitative treatment of substituent effects. However, comparison with aza-naphthalene reactivities indicates that aza- and polyaza-benzene systems must also be highly selective. 

Matsui et al. and Goi have extensively studied substituent effects in the aromatic ring of anilino groups attached to nuclear... 

Substituent Effects for the Methoxy-Dechlorination of Some Fused-Ring Systems ... 

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