Electron acceptance, inductive

For reasons of this sort Taft and Rakshys considered that the chemical shift of a Fluorine atom meta to a particular substituent is a measure of only the inductive effect of that substituent while of course the shift of an ortho or para i F nucleus is a measure of both inductive and conjugative effects. They proposed that the difference between the meta chemical shift and the para chemical shift be taken as a measure of the conjugative effect of a substituent. Their results for some of the systems of interest in the present context are summarised in Table 5. In both cases the second-row substituent appears to have a considerable electron accepting conjugative effect compared to its first-row analogue. This of course can be rationalised by partial double bonding. 

Italian workers have reported extensive studies on the evaluation of a values for thienyl and furyl groups. These are collected in Table XII. The am, Om, and am values reveal the electron-withdrawing inductive effect of the rings, while as expected from the previous discussion of this section, these substituents fall into that small category whose op, oj>, and Op values are all different, revealing an ability to both accept and donate electrons by resonance under the appropriate incentives. 

In spite of some apparent resemblance between the pyridinio (14) and the trimethylammonio groups, the former is more powerful as an inductive withdrawing substituent but it displays a small +M effect (81CCC584). On the whole, its electron-accepting properties are more significant than those of N-protonated pyridyl (15) and pyridyl N-oxide (16) groups (cf. Table III). 

In Summary Substituents that donate electrons by induction and hyperconjugation activate the benzene ring and direct electrophiles ortho and para their inductively electron-accepting counterparts deactivate the benzene ring and direct electrophiles to the meta positions. 

On the other hand, an electron-accepting group tends, through the inductive effect, to increase the double-bond character and hence increases the frequency of the vibration ... 

The weaker electron accepting ability of 1 than TCNQ is based on less aromaticity of the central heterocyclic ring formed upon the reduction process. This is the case for other hetero-TCNQs. The selenophene-TCNQ 2 and the furan-TCNQ 3 have the same first reduction potential (4-0.03 V), which is slightly lower than that of 1. Considering the following order of aromaticity, thiophene > selenophene > furan [31], this result means that the inductive effects of the introduced heteroatoms, especially the oxygen atom of... 

A familiar feature of the electronic theory is the classification of substituents, in terms of the inductive and conjugative or resonance effects, which it provides. Examples from substituents discussed in this book are given in table 7.2. The effects upon orientation and reactivity indicated are only the dominant ones, and one of our tasks is to examine in closer detail how descriptions of substituent effects of this kind meet the facts of nitration. In general, such descriptions find wide acceptance, the more so since they are now known to correspond to parallel descriptions in terms of molecular orbital theory ( 7.2.2, 7.2.3). Only in respect of the interpretation to be placed upon the inductive effect is there still serious disagreement. It will be seen that recent results of nitration studies have produced evidence on this point ( 9.1.1). 

They demonstrated that electron-deficient R groups and electron-rich R substituents at S accelerated the reductive elimination. They proposed 123 (Lj = DPPE, R = Ph, R = Ar) as a transition state, where R acts as an electrophile and thiolate as a nucleophile. The Hammet plot for the reductive elimination showed that the resonance effect of the substituent in R determines the inductive effect of the R group, and the effect in SR showed an acceptable linear relationship with the standard o-values. The relative rate for sulfide elimination as a function of the hybrid valence configuration of the carbon center bonded to palladium followed the trend sp > sp spl... 

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