Direct inductive effect

Three aspects of the inductive effect have to be considered the inductive effect, the inducto-electromeric or rr-inductive effect, and the direct field effect. The first of these is the one most frequently... 

Excluding the phenomenon of hyperconjugation, the only other means by which electronic effects can be transmitted within saturated molecules, or exerted by inductive substituents in aromatic molecules, is by direct electrostatic interaction, the direct field effect. In early discussions of substitution this was usually neglected for qualitative purposes since it would operate in the same direction (though it would be expected to diminish in the order ortho > meta > para) as the cr-inductive effect and assessment of the relative importance of each is difficult however, the field effect was recognised as having quantitative significance. ... 

Ingold introduces the terms substrate field effect and reagent field effect to describe those aspects of the direct field effect numbered (z) and (3) in 9.1.2. His description of the substituent effect of the trimethylammonio group is thus given substantially in terms of the substrate field effect and the TT-inductive effect, i.e. it is an isolated molecule description. The reagent field effect is seen to be significant in nitration and to produce qualitatively the same 226... 

Closely related to the inductive effect and operating in the same direction is the field effect In the field effect the electronegativity of a substituent is communicated not by successive polarization of bonds but via the medium usually the solvent A substituent m a molecule polarizes surrounding solvent molecules and this polarization is transmit ted through other solvent molecules to the remote site... 

One underlying physical basis for the failure of Hammett reaction series is that substituent interactions are some mixture of resonance, field, and inductive effects. When direct resonance interaction is possible, the extent of the resonance increases, and the substituent constants appropriate to the normal mix of resonance and field effects then fail. There have been many attempts to develop sets of a values that take into account extra resonance interactions. 

According to Roberts et al. the direction of addition of ammonia to 3-substituted benzynes might be predicted by considering the amide ion to add so as to provide the most favorable location of the negative charge with respect to the inductive effect of the orienting substituent. Thus, ammonia adds to 3-methoxybenzyne (39) producing chiefly n-aminoanisole (40). 

One further point inductive effects and resonance effects don t necessarily act in the same direction. Halogen, hydroxyl, alkoxyl, and amino substituents, for instance, have electron -withdrawing inductive effects because of the electronegativity of the -X, -O, or —N atom bonded to the aromatic ring but have resonance effects because of the lone-pair electrons on those same —X, -O, or —N atoms. When the two effects act in opposite directions, the stronger of the two dominates. 

Inductive and resonance effects account for the directing effects of substituents as well as for their activating or deactivating effects. Take alkyl groups, for instance, which have an electron-donating inductive effect and are ortho and para directors. The results of toluene nitration are shown in Figure 16.13. 

Finally, since besides the inductive effect of the sulfoxide and the sulfone functional groups, hydrogen bonding, field effects and steric effects to solvation may or may not work in the same direction, the pKx values can be useful in assigning configurations of suitable pairs of stereoisomeric sulfoxide and sulfone carboxylic acids291. 

In nitrobenzene the nitro group has a large electron affinity, and accordingly draws electrons away from the ring. The resonance effect works in the same direction, and, as a result, all positions have a deficiency of electrons. The meta positions are least affected, and the substitution takes place there with difficulty. In aniline, the inductive effect and the resonance effect oppose each other, but the latter wins out, and very easy o-p substitution takes place. 

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. 

Both sets of results may also be discussed in terms of inductive differences between hydrogen and deuterium (see Halevi, 1963). Brown et al. (1966) jDoint out that both the inductive and steric explanations qualitatively predict isotope effects in the same direction, but that an inductive effect would be expected to operate from the 3 and 4 positions nearly as effectively as from the 2 position . Furthermore, there is no observable isotope effect on the heat of reaction of 2,6-(dimethyl-de)-pyridine with the relatively small molecule diborane A AH = —20 18 cal mol ), but a significant effect is obtained with the larger molecule boron trifluoride AAH = 230 + 150 cal mol ). 

In an effort to better understand the differences observed upon substitution in carvone possible changes in valence electron density produced by inductive effects, and so on, were investigated [38, 52]. A particularly pertinent way to probe for this in the case of core ionizations is by examining shifts in the core electron-binding energies (CEBEs). These respond directly to increase or decrease in valence electron density at the relevant site. The CEBEs were therefore calculated for the C=0 C 1 orbital, and also the asymmetric carbon atom, using Chong s AEa s method [75-77] with a relativistic correction [78]. 

Mg/Me (Me=Al, Fe) mixed oxides prepared from hydrotalcite precursors were compared in the gas-phase m-cresol methylation in order to find out a relationship between catalytic activity and physico-chemical properties. It was found that the regio-selectivity in the methylation is considerably affected by the surface acid-basic properties of the catalysts. The co-existence of Lewis acid sites and basic sites leads to an enhancement of the selectivity to the product of ortho-C-alkylation with respect to the sole presence of basic sites. This derives from the combination of two effects, (i) The H+-abstraction properties of the basic site lead to the generation of the phenolate anion, (ii) The coordinative properties of Lewis acid sites, through their interaction with the aromatic ring, make the mesomeric effect less efficient, with predominance of the inductive effect of the -O species in directing the regio-selectivity of the C-methylation into the ortho position. 

The effect of introducing electron-withdrawing substituents into simple aliphatic acids is more marked. Thus halogen, with an inductive effect acting in the opposite direction to alkyl, might be expected to increase the strength of an acid so substituted, and this is indeed observed as pKa values show ... 

The particularly marked effect with o-NOj may be due to the very short distance over which the powerful inductive effect is operating, but some direct interaction between the adjacent N02 and C02H groups cannot be ruled out. 

The extra base-weakening effect, when the substituent is in the o-position, is due in part to the short distance over which its inductive effect is operating, and also to direct interaction, both steric and by hydrogen bonding, with the NH2 group (cf. the case of o-substituted benzoic acids, p. 63). o-Nitroaniline is such a weak base that its salts... 

With substituents such as OH and OMe that have unshared electron pairs, an electron-donating, i.e. base-strengthening, mesomeric effect can be exerted from the o- and p-, but not from the m-position, with the result that the p-substituted aniline is a stronger base than the corresponding w-compound. The m-compound is a weaker base than aniline itself, due to the electron-withdrawing inductive effect exerted by the oxygen atom in each case. As so often, the effect of the o-substituent remains somewhat anomalous, due to direct interaction with the NH2 group by both steric and polar effects. The substituted anilines are found to have related pAa values as follows ... 

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