Resonance donors

However, if we were to substitute at the mete-position, the lone pair on the oxygen atom cannot stabilize the positive charge in the intermediate. As we will see later when we quantify these effects, the mete-position is a little less reactive than benzene—while the lone pair of electrons on oxygen makes the OMe group a resonance donor, the electronegativity of oxygen means that it is 

FIGURE 12.32 Lone pairs stabilize a positive center. 

FIGURE 12.33 Mechanism of electrophilic substitution of an aromatic ring bearing a resonance donor. 

Draw a complete mechanism for the tribromination of aniline by molecular bromine. Solution 

Notice that we do not need a halogen carrier for this reaction to work the NHj group is strongly activating, and it is difficult to stop this reaction at the mono- or disubstitution stage. 

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It is instructive to consider Af-substituted azoles in reverse, i.e. the azole ring as the substituent linked to some other group. Hammett and Taft cr-constant values for azoles as substituents are given in Table 11. The values show that all the azoles are rather weak net resonance donors, imidazole being the strongest. They are all rather strong inductive acceptors, with pyrazole considerably weaker in this respect than imidazole or the triazoles. 

With arenes bearing a single resonance donor substituent (NR2, OMe, F), the addition is strongly preferred at the meta position, with small amounts of ortho substitution (0-10%).89,90 Representative examples are shown in Table 2 and equation (30). 

Regioselectivity in m/10-disubstituted arenes is often high and useful. A series of examples is summarized in Figure 1. The resonance donor substituent (OMe) appears to dominate the directing influen-... 

The resonance donor effect of R3M substituents towards Rff (a-n conjugation), shown schematically in 1, increases as M changes in the order C < Si < Ge < Sn < Pb. One can observe a weak a-n conjugation effect even in organic compounds. 

For all benzene substituents which are resonance donors, ap > ap 1. It follows from equation 23 that the differences ap 1 — ap characterizing the strengthening of the a-n conjugation increase with the enhancement of the polarizability of all the bonds within the substituent bonded to the aromatic ring. The quantitative characteristic of the overall substituent polarizability is the sum of the refractions of its bonds, E R > (see Section I). The values of ap+ and ap are approximately equal for organic substituents which are resonance acceptors. If the R3- X M substituents had only a resonance acceptor effect (the d-7T conjugation), the correlation in equation 23 would fail for compounds R3-nX MPh. In fact, equation 24... 

The presence of a strong resonance donor 0-71 conjugation effect in R3MCH2Rjt molecules. 

R Alkyl groups —Ar Aryl groups Weakly activating ortho and para Weak inductive or resonance donors... 

Resonance donor ( + M) and acceptor (-M) substituents may produce substantially larger effects than they do in benzene because they are able to become more coplanar with the heteroaromatic ring. A proper analysis requires a range of substituent types to detect such anomalies. These need, in turn, to be considered carefully because hydrogen bonding may reduce the reactivities of more reactive compounds relative to less reactive ones, so producing attenuated p values. 

Phenyl groups are weak resonance donors, but inductive acceptors. Phenyl groups are therefore expected to reduce the basicity of azoles. 

Methoxy groups are resonance donors but inductive acceptors. The inductive effect would be expected to be dominant for azoles. 

Halogen atoms are inductive acceptors (and weak resonance donors) they are expected to cause a marked decrease in basicity, especially from -positions. 

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