Inductive and Field Effects - Localized Chemical Bonding - Introduction - March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 7th Edition (2013)

March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure, 7th Edition (2013)

Part I. Introduction

Chapter 1. Localized Chemical Bonding

1.I. Inductive and Field Effects

The C–C bond in ethane has no polarity because it connects two equivalent atoms, with identical electronegativities. The presence of a more electronegative atom attached to one of the carbon atoms will lead to bond polarization, however, in what is known as an induced dipole. The C–C bond in chloroethane, for example, is polarized by the presence of the electronegative chlorine atom. This polarization is actually the sum

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of two effects. In the first of these, the C-1 atom is deprived of some of its electron density by the greater electronegativity of Cl, and this effect is partially compensated by drawing the C–C electrons closer to itself. The result is a polarization of this bond and a slightly positive charge on the C-2 atom: an induced dipole. This polarization of one bond caused by the polarization of an adjacent bond is known as an inductive effect. The effect is greatest for adjacent bonds but may also be felt farther away; thus the polarization of the C–C bond causes a (slight) polarization of the three methyl C–H bonds. As a practical matter, the effect is negligible if the polarizing group is more than three bonds away.

The other effect operates not through bonds, but directly through space or solvent molecules, and is called a field effect.43 It is often very difficult to separate the two kinds of effect, but a number of cases have been reported. This is generally accomplished by taking advantage of the fact that the field effect depends on the geometry of the molecule, but the inductive effect depends only on the nature of the bonds. For example, in isomers, 1 and 244 the inductive effect of the chlorine atoms on the position of the electrons in the COOH group (and hence on the

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acidity, see Chap. 8) should be the same since the same bonds intervene. The field effect is different, however, because the chlorine atoms are closer in space to the COOH in 1 than they are in 2. Thus, a comparison of the acidity of 1 and 2 should reveal whether a field effect is truly operating. The evidence obtained from such experiments is overwhelming that field effects are much more important than inductive effects.45 In most cases, the two types of effect are considered together; in this book, they will not be separated but will use the name field effect to refer to their combined action.46 Note that the field effect for 1 may be viewed as internal hydrogen bonding (see Sec. 3.A).

Functional groups can be classified as electron withdrawing (−I) or electron donating (+I) groups relative to hydrogen. This means, for example, that NO2, a −I group, will draw electrons to itself more than a hydrogen atom would if it occupied the same position in the molecule.

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Thus, in α-nitrotoluene, the electrons in the N–C bond are farther away from the carbon atom than the electrons in the H–C bond of toluene. Similarly, the electrons of the C–Ph bond are farther away from the ring in α-nitrotoluene than they are in toluene. Field effects are always comparison effects. For example, compare the −I or +I effect of one group with another (usually hydrogen). Therefore, it may be said that, compared with hydrogen, the NO2 group is electron withdrawing and the O group is electron donating or electron releasing. However, there is no actual donation or withdrawal of electrons, but rather electron distortion or electron redistribution. While withdrawing and releasing terms are convenient to use, the terms merely represent a difference in the position of electrons due to the difference in electronegativity between H and NO2 or between H and O.

Table 1.3 lists a number of the most common −I and +I groups.47 It can be seen that compared with hydrogen, most groups are electron withdrawing. The only electron-donating groups are those with a formal negative charge (but not even all these), atoms of low electronegativity (Si,48 Mg, etc., and perhaps alkyl groups). Alkyl groups49 were formerly regarded as electron donating, but many examples of behavior have been found that can be interpreted only by the conclusion that alkyl groups are electron withdrawing compared with hydrogen.50 In accord with this is the value of 2.472 for the group electronegativity of CH3 (Table 1.2) compared with 2.176 for H. When an alkyl group is attached to an unsaturated or trivalent carbon (or other atom), its behavior is best explained by assuming it is +I (see, e.g., Sec. 5.A.ii, 5.B.i, 8.E, 11.B.i), but when it is connected to a saturated atom, the results are not as clear, and alkyl groups seem to be +I in some cases and −I in others51 (see also, Sec. 8.F). When connected to a positive carbon, alkyl groups are clearly electron releasing.

Table 1.3 Field Effects of Various Groups Relative to Hydrogena

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aThe groups are listed approximately in order of decreasing strength for both −I and +I groups. [Reprinted with permission from Ceppi, E.; Eckhardt, W.; Grob, C.A. Tetrahedron Lett. 1973, 3627. Copyright © 1973, with permission from Elsevier Science.]

It is clear that the field-effect order of alkyl groups attached to unsaturated systems is tertiary > secondary > primary > CH3, but this order is not always maintained when the groups are attached to saturated systems. Deuterium is electron donating with respect to hydrogen.52 Other things being equal, atoms with sp bonding generally have a greater electron-withdrawing power than those with sp2 bonding, which in turn have more electron-withdrawing power than those with sp3 bonding.53 This accounts for the fact that aryl, vinylic, and alkynyl groups are −I. Field effects always decrease with increasing distance, and in most cases (except when a very powerful +I or −I group is involved), cause very little difference in a bond four bonds away or more. There is evidence that field effects can be affected by the solvent.54

For discussions of field effects on acid and base strength and on reactivity, see Chapters 8 and 9, respectively.