Electronic inductive effects

Fluorine in organic compounds is associated with a set of electronic effects inductive and mesomeric, stabilizing and destabilizing, pulling or pushing electrons, which are convincingly... 

A substituent can influence reactivity not only by its electronic effect (inductive and/or resonance), but also, in some cases, by its steric effect an effect due to crowding at some stage of the reaction, and dependent therefore on the size of the substituent. 

See also electronic effect inductive effect polar effect. 

A more complete discussion of acidity and electronic effects can be found is Appendix 2.) A few words about the two types of electronic effects induction and resonance. Inductive effects are a result of polarized a bonds, usually because of electronegative atom substituents. Resonance effects work through n systems, requiring overlap of p orbitals to delocalize electrons. 

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. ... 

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. 

In a M.o. treatment of the electronic effect of the methyl group it was found necessary to take into account both inductive and hypercon-jugative effects. This treatment is commented on in 9.3 below. 

Induced dipole/mduced dipole attraction (Section 2 17) Force of attraction resulting from a mutual and complemen tary polanzation of one molecule by another Also referred to as London forces or dispersion forces Inductive effect (Section 1 15) An electronic effect transmit ted by successive polanzation of the cr bonds within a mol ecule or an ion... 

The 1,3-dipolar cycloadditions offluonnatedallenes provide a rich and varied chemistry Allenes, such as 1,1-difluoroallene and fluoroallene, that have fluorine substitution on only one of their two cumulated double bonds are very reactive toward 1,3-dipoles Such activation derives from the electron attracting inductive and hyperconjugative effects of the allylic fluorine substituent(s) that give nse to a considerable lowering of the energy of the LUMO of the C(2)-C(3) n bond [27]... 

Inductive effect (Section 1.15) An electronic effect transmitted by successive polarization of the cr bonds within a molecule or an ion. 

A further complication arises with Ingold s suggestion" that both the inductive and resonance effects are composed of initial state equilibrium displacements that reveal themselves in equilibrium properties like dipole moments and equilibrium constants and of time-dependent displacements produced during reaction by the approach of an attacking reagent, observed rate effects being resultants of both types of electronic effects. Hammett, however, claims that it is not necessary or possible to make this distinction. 

The ortho effect may consist of several components. The normal electronic effect may receive contributions from inductive and resonance factors, just as with tneta and para substituents. There may also be a proximity or field electronic effect that operates directly between the substituent and the reaction site. In addition there may exist a true steric effect, as a result of the space-filling nature of the substituent (itself ultimately an electronic effect). Finally it is possible that non-covalent interactions, such as hydrogen bonding or charge transfer, may take place. The role of the solvent in both the initial state and the transition state may be different in the presence of ortho substitution. Many attempts have been made to separate these several effects. For example. Farthing and Nam defined an ortho substituent constant in the usual way by = log (K/K ) for the ionization of benzoic acids, postulating that includes both electronic and steric components. They assumed that the electronic portion of the ortho effect is identical to the para effect, writing CTe = o-p, and that the steric component is equal to the difference between the total effect and the electronic effect, or cts = cr — cte- They then used a multiple LFER to correlate data for orrAo-substituted reactants. 

Research on the nature of substituent constants continues, with results that can bewilder the nonspecialist. The dominant approach is a statistical one, and the main goal is to dissect substituent effects into separate electronic causes. This has led to a proliferation of terms, symbols, and conclusions. A central issue is (here we change terminology somewhat from our earlier usage) to determine the balance of field and inductive effects contributing to the observed polar electronic effect. In... 

This apparent characteristic enhancement in the basicity has been used quite frequently for the determination of the position of a double bond with respect to the nitrogen atom in unsaturated amines. The cases such as neostrychnine (134) and dehydroquinuclidine (139) in which the protonation at the 8-carbon atom cannot occur due to the lack of overlap between the electron pair on the nitrogen atom and the tt electrons of the double bond, since this would involve the formation of a double bond at the bridgehead— a violation of Bredt s rule—show a decrease in basicity. For instance the basicities of quinuclidine (140) and dehydroquinuclidine (139) have been shown by Grob et al. (82), to differ by 1.13 pK units in aqueous solution at 25. This decrease in basicity has been attributed to the electron-withdrawing inductive effect of the double bond. 

Recently Stamhuis et al. (33) have determined the base strengths of morpholine, piperidine, and pyrrolidine enamines of isobutyraldehyde in aqueous solutions by kinetic, potentiometric, and spectroscopic methods at 25° and found that these enamines are 200-1000 times weaker bases than the secondary amines from which they are formed and 30-200 times less basic than the corresponding saturated tertiary enamines. The baseweakening effect has been attributed to the electron-withdrawing inductive effect of the double bond and the overlap of the electron pair on the nitrogen atom with the tt electrons of the double bond. It was pointed out that the kinetic protonation in the hydrolysis of these enamines occurs at the nitrogen atom, whereas the protonation under thermodynamic control takes place at the -carbon atom, which is, however, dependent upon the pH of the solution (84,85). The measurement of base strengths of enamines in chloroform solution show that they are 10-30 times weaker bases than the secondary amines from which they are derived (4,86). 

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. 

Meta-directing deactivators, such as —CHO, act through a combination of electron-withdrawing inductive and resonance effects that reinforce each other and are felt most strongly at the ortho and para positions. As a result, the ortho and para intermediates are less stable so reaction with an electrophile occurs at the meta position (Figure 16.16). 

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