Resonance effects of substituents

A selective herbicide that kills broad-leaf weeds is made by chlorinating phenoxyacetic acid. Is the substituent an activating or deactivating group  

Is the nitroso group (—N=0) an activating or deactivating group Consider both inductive and 

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There were two schools of thought concerning attempts to extend Hammett s treatment of substituent effects to electrophilic substitutions. It was felt by some that the effects of substituents in electrophilic aromatic substitutions were particularly susceptible to the specific demands of the reagent, and that the variability of the polarizibility effects, or direct resonance interactions, would render impossible any attempted correlation using a two-parameter equation. - o This view was not universally accepted, for Pearson, Baxter and Martin suggested that, by choosing a different model reaction, in which the direct resonance effects of substituents participated, an equation, formally similar to Hammett s equation, might be devised to correlate the rates of electrophilic aromatic and electrophilic side chain reactions. We shall now consider attempts which have been made to do this. 

For illustrative purposes, we shall consider the resonance effect of substituents upon the energies of the pi MO s of the model system ethylene. 

Resonance effect of substituent present on the benzene ring... 

The mechanism involves a nucleophilic attack by R " (or a polar R ) on the hydrogen.2 Evidence is that resonance effects of substituents in R seem to make little difference. When... 

Explain the terms inductive and resonance effect of substituents. What makes a substituent exhibit a negative resonance effect Which types of substituents have a positive resonance effect Can a given substituent exhibit at the same time a negative inductive and a positive resonance effect If yes, give some examples of such substituents. 

A DSP analysis, in which contributions due to inductive and resonance effects of substituents are considered separately, has been applied in order to obtain a deeper level of insight into the electronic factors influencing pyridinium ion acidities. It has been said that a ... single parameter treatment has the merit of simplicity but where this suggests correlations which are unrealistic in terms of chemical experience, then a dual parameter approach is necessary. 53 The correlation has the form of Eq. (8). While only one set of inductive-effect... 

As a result of the overlaping inductive and resonance effects of substituents (acting in the same direction as in the molecules of HFP and PFIB), the central carbon in these olefins has a much higher negative charge than that of the terminal atom. It means that the attack of electrophile E+ on these substrates is aimed at the central carbon in contrast to the attack of nucleophile Nu" which is directed toward terminal carbon carying a positive charge ... 

A. S. Cieplak, Inductive and Resonance Effects of Substituents on 7t-Face Selection, Chem. Rev. 1999,... 

As a consequence, the aim of our own work was to attempt a quantitative analysis of the relationships between thermal stability, ground state strain and resonance effects of substituents and of the influence of strain on structure 9b). The results were expected to be valuable not only as fundamental knowledge but were expected to contribute to fields of applied chemistry such as carbon initiators11), coal pyrolysis12) or thermal stabilities of polymers13). 

As a next step in this analysis we investigated 18) a series of 1,2-diphenyl tetraalkyl-ethanes 27 which generate resonance stabilized tertiary benzyl radicals 28 at elevated temperatures (Fig. 3). Having worked out a method for analysis of the steric effect we hoped to succeed also in quantitatively separating it from the resonance effect of substituents. It is immediately recognized from Fig. 3 and the related correlation Eq. (9 and 10) that thermolysis occurs at much lower temperatures (100 200 °C) and with much lower activation enthalpies than in the aliphatic Cq—Cq series 11. 

To estimate the resonance constants CTr for the same groups use was made of Eq. (6) linking the NH2 chemical shifts in PMR spectra of m- and p-substituted anilines (3) with the resonance effects of substituents, and of a similar dependence [Eq. (15)] for hydroxy group protons in the spectra of substituted phenols (12) (79ZOR1737). In both cases the results were in good agreement. 

As far as electronic effects are concerned, it is revealed that the resonance effects of substituents at the 5-position also have an influence on activity. 

Kvasnicka, V, Sklenak, . and Pospichal, J. (1993b) Neural network classification of inductive and resonance effects of substituents. /. Am. Chem. Soc., 115, 1495-1500. 

The chemical shifts for aromatic carbons are considered to be a sensitive probe in studies on polar and resonance effects of substituents (345). Thus, significant upfield chemical shifts for the ortho and para carbons... 

For a detailed acconnt of theoretical and experimental manifestations of differences in the C-H vs. C-C hyperconjugation, see pages 1280-1282 in Cieplak A. S. (1999). Indnctive and resonance effects of substituents on n -face selection. Chemical Reviews, 99, 1265-1336. 

The effects of substituents can be treated by the linear free energy relationships, some examples of which will be mentioned here. The most general postulate, d) on which these treatments are based, is that the shift of the half-wave potentials, (AE /g) relative to that of the parent compound, is due to the more or less independent contributions of the polar P, steric S and mesomeric M (resonance) effects of substituents ... 

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