Inductive/field effects

Y group such as NMe3 will exert a powerful, electron-withdrawing, inductive/field effect on both the (3-carbon atoms, and thus on the H atoms attached to them,... 

The Hammett cr-values contain contributions from both inductive/field effects and the resonance effect. The cr-constant can be separated quantitatively into a resonance component R, which operates mainly in the para position, and an inductive component F, which is assumed to be equal in the meta and para positions. Hansch, Leo and Taft11 have calculated the F and R values of Me3Si to be 0.01 and —0.08, respectively, as quoted in Table 1. These values seem somewhat at odds with experimentally determined values12-18 for the inductive and resonance parameters, which give mean values of —0.08 and 0.06, respectively. These values confirm the generally accepted view that MesSi is electron-supplying by inductive effects and electron-withdrawing by resonance effects. 

The ability of thienothiophenes and selenoloselenophenes to transmit substituent effects has been determined by H and l3C NMR, and also calculated by the CNDO/2 method [82CS(20)208 83CS(22)22]. As expected, resonance effects are better transmitted between the 2- and 5-positions in 8.185 than in 8.186, since only in the former is direct conjugation between these positions possible. Both resonance and inductive/field effects were greater in the sulfur- relative to the selenium-containing compounds, but in general, replacement of sulfur by selenium caused only minor perturbations of data, which follows from the fact that the main mode of transmission of substituent effects in five-membered heterocycles must be via the carbon chain (cf. Chapter 6, Section 9.C). 

The concept of fluorine negative hyperconjugation (FNHC), which in its simplest form can be written as indicated in Figure 4.4, has had a controversial history although there can be little doubt about the firm theoretical requirement for such an effect [18-20]. Problems have arisen because there are few rate-constant measurements that require FNHC, in addition to inductive field effects, to account for the observations [21], although structural evidence for 4.5A and 4.5B (Figure 4.5) now illustrates the effect well [22, 23]. 

The inductive-field effect (/) is visualized as arising from the differential in electrostatic charge-dipole stabilization or destabilization. In the point-charge dipole approximation, this energy is given by... 

An ab initio theoretical treatment of inductive-field effects has been carried out for -substituted ethylamines, where hyperconjugative and polarizability effects on proton affinities were minimized and intramolecular... 

The rule is bard to use because we do not have a good value for the p/fa of the product of the addition of hydroxide. Steric hindrance from R raises the pA abH of un alkoxide anion (poorer intermolecular solvation), whereas the inductive/field effect from the nitrogen should lower the p/ifabH- If these effects balance out, the tetrahedral intermediate product of nucleophilic attack would have a pATabH of about 13 (HOCH2OH is pKa 13.3). 

The Hammett a-values contain a contribution from both inductive/field effects and the resonance effect. The quantitative separation of -constants into resonance and inductive effects was first suggested by Taft34,35 who proposed the following relationship ... 

As discussed in the previous section, diazocarbonyl compounds are another common photochemical precursor to ketenes. A number of substituted ketenes have been prepared in this way and studied by TRIR spectroscopy. [123]. The diagnostic ketene asymmetric stretching frequency, typically observed between 2100 and 2200 cm has been correlated with the inductive (field) effect of... 

It is useful to make a qualitative prediction of the effect of a substituent on one chemical reaction, but we would like to be able to extend this understanding to predict how the same substituent wiU affect some other reaction. Moreover, it would be helpful to have a procedure for using our knowledge of the effect that different substituents have on a particular reaction to help us imderstand the mechanism of that reaction. It is more difficult to treat both steric and electronic effects simultaneously, so we will first consider systems in which the effects of a substituent are primarily induction/field effects or resonance effects but not steric effects. The approach that we will take illustrates the use of a linear free energy relationship (LFER) to relate values of AG or AG from one reaction to another. ... 

In the previous examples it was possible to discuss the effect of a substituent only in terms of the induction/field effect or primarily in terms of the resonance effect. Both effects are important for many substituents, and the location of the substituent on the ring influences how the effects influence the reaction site. The positive value for the methoxy group indicates that it... 

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