Inductive effects structure

In unsaturated molecules electronic effects can be transmitted by mesomerism as well as by inductive effects. As with the latter, the mesomeric properties of a group are described by reference to hydrogen. Groups which release electrons to the unsaturated residue of the molecule are said to exert a +Af effect, whereas groups which attract electrons are said to exert a —Af effect. In aromatic structures the important feature of an M-substituent is that it influences the 0- and p-positions selectively. 

The greater positive character hence the increased acidity of the O—H proton of 2 2 2 tnfluoroethanol can be seen m the electrostatic potential maps displayed m Figure 1 8 Structural effects such as this that are transmitted through bonds are called indue tive effects A substituent induces a polarization m the bonds between it and some remote site A similar inductive effect is evident when comparing acetic acid and its trifluoro derivative Trifluoroacetic acid is more than 4 units stronger than acetic acid... 

Electron Delocalization in the Conjugate Base With a of —1 4 nitnc acid is almost completely ionized m water If we look at the Lewis structure of nitric acid m light of what we have said about inductive effects we can see why The N atom m nitric acid IS not only electronegative m its own right but bears a formal charge of +1 which enhances its ability to attract electrons away from the —OH group... 

Mesomeric shifts of the types shown in structures (82) and (83) increase the electron density on the nitrogen atom and facilitate reaction with electrophilic reagents. However, the heteroatom Z also has an adverse inductive effect the pK, of NH2OH is 6.0 and that of N2H4 is 8.0, both considerably lower than that of NH3 which is 9.5. 

Hiickel models of molecular electronic structure enjoyed many years of popularity, particularly the r-electron variants. Authors sought to extract the last possible amount of information from these models, perhaps because nothing more refined was technically feasible at the time. Thus, for example, the inductive effect was studied. The inductive effect is a key concept in organic chemistry a group R should show a - -1 or a —I effect (according to the nature of the group R) when it is substituted into a benzene ring. 

The foregoing resonance structures describe the influence of the substituents on the 7r-electron distribution in the thiophene ring. Besides this effect the inductive effect of the substituents on the a-electi on system must be considered when discussing physical and chemical properties of thiophenes. 

The fact that the equilibrium for aminopyridine 1-oxides is displaced further in favor of the amino form than is the equilibrium for aminopyridines is in accord with the mesomerism of these compounds. The stabilization of the amino forms (e.g., 241) by structures of type 240 is more effective than the corresponding stabilization in the pyridine series since the negative charge is associated with the oxygen atom. The stabilization of the imino form (e.g., 242) by structures of type 243 is less than in the pyridine series because of the adverse inductive effect of the oxygen atom. ... 

Secondary steric effects of nitro groups are more easily detected by comparing the reactivities with those of aza derivatives. For example, in structure 20 the rate depression on passing from methyl to -butyl is only 2.5-fold and can be attributed to an inductive effect, whereas in structure 21 a similar change involves the factor 16, which can be attributed in part to steric inhibition of resonance (S.I.R.) of thep-N02 group (reaction with piperidine). 

We can imagine the transition state for alkene protonation to be a structure in which one of the alkene carbon atoms has almost completely rehybridized from sp2 to sp- and in which the remaining alkene carbon bears much of the positive charge (Figure 6.16). This transition state is stabilized by hyperconjuga-lion and inductive effects in the same way as the product carbocation. The more alkyl groups that are present, the greater the extent of stabilization and the faster the transition state forms. 

In the first molecule the resonance produces an increased concentration of electrons on the o and p carbon atoms, and in the second it produces a decreased concentration. As with the inductive effect, the meta positions remain nearly unaffected, since structures of the type... 

The error in Hiickel s treatment lies not in the quantum mechanical calculations themselves, which are correct as far as they go, but in the oversimplification of the problem and in the incorrect interpretation of the results. Consequently it has seemed desirable to us to make the necessary extensions and corrections in order to see if the theory can lead to a consistent picture. In the following discussion we have found it necessary to consider all of the different factors mentioned heretofore the resonance effect, the inductive effect, and the effect of polarization by the attacking group. The inclusion of these several effects in the theory has led to the introduction of a number of more or less arbitrary parameters, and has thus tended to remove significance from the agreement with experiment which is achieved. We feel, however, that the effects included are all justified empirically and must be considered in any satisfactory theory, and that the values used for the arbitrary parameters are reasonable. The results communicated in this paper show that the quantum mechanical theory of the structure of aromatic molecules can account for the phenomenon of directed substitution in a reasonable way. 

Inductive effects fall off rapidly with distance, so there is a large difference between the following structures ... 

In the structure on the left, the charge is somewhat stabilized by the inductive effects of the neighboring chlorine atoms. In contrast, the structure on the right is destabilized by the presence of methyl groups. Therefore, the structure on the left is more stable. 

Since the (n-Bu)3Sn radical (R ) is nucleophilic (17), a partial negative charge must be produced at the methine carbon whose chlorine is being abstracted. The rate of this abstraction should clearly be enhanced by electron-withdrawing groups on R due to their stabilization of this charge by inductive effects. As observed, the removal of Cl from EW (or W diad) is expected to be more facile than from EVE (or VE diad) as a result of a -halogen effect in the former structure. 

The actual structure is somewhere in between, i.e. (2lab) a hybrid of which (21a) and (21b) are the canonical forms. There will also be an inductive effect, as shown in (2lab) but this will be much smaller than the mesomeric effect as a electrons are much less polarisable, and hence less readily shifted, than tt electrons. 

There are a number of structural features in R—H that promote the removal of H by bases through making it more acidic, and also features that serve to stabilise the resultant carbanion, Re in some cases both effects are promoted by the same feature. The main features that serve to stabilise carbanions are (cf. factors that serve to stabilise carboca-tions, p. 104) (a) increase in s character at the carbanion carbon, (b) electron-withdrawing inductive effects, (c) conjugation of the carbanion lone pair with a polarised multiple bond, and (d) aromat-isation. 

As with the inductive effect, resonance effects on ground state properties have already been included in the procedure, PEPE, for calculating partial atomic charges. This has been achieved by generating and weighting the various resonance structures of a molecule. The significance and quality of the results has been shown by correlations and calculations of physical data 47>48-52>. 

The inductive effect is also illustrated by the strengths of acids such as HN03 and HN02. The structures for the molecules are as follows ... 

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