Resonance effect, electrophilic

This led to the introduction of the concepts of inductive and resonance effects and to the establishment of the mechanism of electrophilic aromatic substitution. 

There were two schools of thought concerning attempts to extend Hammett s treatment of substituent effects to electrophilic substitutions. It was felt by some that the effects of substituents in electrophilic aromatic substitutions were particularly susceptible to the specific demands of the reagent, and that the variability of the polarizibility effects, or direct resonance interactions, would render impossible any attempted correlation using a two-parameter equation. - o This view was not universally accepted, for Pearson, Baxter and Martin suggested that, by choosing a different model reaction, in which the direct resonance effects of substituents participated, an equation, formally similar to Hammett s equation, might be devised to correlate the rates of electrophilic aromatic and electrophilic side chain reactions. We shall now consider attempts which have been made to do this. 

A tertiary carbonium ion is more stable than a secondary carbonium ion, which is in turn more stable than a primary carbonium ion. Therefore, the alkylation of ben2ene with isobutylene is much easier than is alkylation with ethylene. The reactivity of substituted aromatics for electrophilic substitution is affected by the inductive and resonance effects of a substituent. An electron-donating group, such as the hydroxyl and methyl groups, activates the alkylation and an electron-withdrawing group, such as chloride, deactivates it. 

Resonance effects are the primary influence on orientation and reactivity in electrophilic substitution. The common activating groups in electrophilic aromatic substitution, in approximate order of decreasing effectiveness, are —NR2, —NHR, —NH2, —OH, —OR, —NO, —NHCOR, —OCOR, alkyls, —F, —Cl, —Br, —1, aryls, —CH2COOH, and —CH=CH—COOH. Activating groups are ortho- and para-directing. Mixtures of ortho- and para-isomers are frequently produced the exact proportions are usually a function of steric effects and reaction conditions. 

The substituent effects in aromatic electrophilic substitution are dominated by resonance effects. In other systems, stereoelectronic effects or steric effects might be more important. Whatever the nature of the substituent effects, the Hammond postulate insists diat structural discussion of transition states in terms of reactants, intermediates, or products is valid only when their structures and energies are similar. 

A more quantitative formulation of the varying resonance effects in electrophilic nuclear substitution reactions has been suggested by Tsuno, who has proposed to use Eq. (2), where Aa+ is a resonance exaltation term, and r is a susceptibility constant. 

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). 

One further comparison aromatic aldehydes, such as benzaldehyde, are less reactive in nucleophilic addition reactions than aliphatic aldehydes because the electron-donating resonance effect of the aromatic ring makes the carbonyl group less electrophilic. Comparing electrostatic potential maps of formaldehyde and benzaldehyde, for example, shows that the carbonyl carbon atom is less positive (less blue) in the aromatic aldehyde. 

In so far as the composition of the electrical effect is considered, the values of p given in Table XVll show that for both nucleophilic and electrophilic radicals, the resonance effect seems to predominate, probably in the case of the former and almost certainly in the case of the latter. 

They demonstrated that electron-deficient R groups and electron-rich R substituents at S accelerated the reductive elimination. They proposed 123 (Lj = DPPE, R = Ph, R = Ar) as a transition state, where R acts as an electrophile and thiolate as a nucleophile. The Hammet plot for the reductive elimination showed that the resonance effect of the substituent in R determines the inductive effect of the R group, and the effect in SR showed an acceptable linear relationship with the standard o-values. The relative rate for sulfide elimination as a function of the hybrid valence configuration of the carbon center bonded to palladium followed the trend sp > sp spl... 

It is concluded that the selectivities of electrophilic additions are not directly related to the reactivities but to the transition-state positions. Extensive comparison with similar data on the bromination and hydration of other ethylenic compounds bearing a conjugated group shows that this unexpected reactivity-selectivity behaviour can arise from an imbalance between polar and resonance effects (Ruasse, 1985). Increasing resonance in the ground state would make the transition state earlier and attenuate the kinetic selectivity more strongly than it enhances the reactivity. Hydration and halogenation probably respond differently to this imbalance. 

The effect of a substituent on the aromatic substitution reaction is similar to its effect on electrophilic side chain reactions, but not precisely parallel. Thus the Hammett relationship using the usual sigma or substituent constants gives considerable scatter when applied to aromatic substitution. The scatter is probably due to an increased importance of resonance effects in the nuclear substitution reaction as compared with the side chain reactions. 

Reich and Cram 8 > studied the patterns of electrophilic substitution of the monosubstituted [2.2]paracyclophanes. It was at once clear that the directive influences of the substituents X (see below) could not be correlated with transannular resonance effects in the ground state 84>. The product pattern predicted on the basis of electrostatic ground-state models, such as the canonical structures 65 for electron-releasing and 66... 

In phenols, the reactions that take place on the aromatic ring are electrophilic substitution reactions (Unit 13, Class XI). The -OH group attached to the benzene ring activates it towards electrophilic substitution. Also, it directs the incoming group to ortho and para positions in the ring as these positions become eiectron rich due to the resonance effect caused by -OH group. The resonance structures are shown under acidity of phenols. 

The presence of -OH group In phenols activates the aromatic ring towards electrophilic substitution and directs the Incoming group to ortho and para positions due to resonance effect. Reimer-Tiemann reaction of phenol 5delds sallcylaldehyde. In presence of sodium hydroxide, phenol generates phenoxlde Ion which Is even more reactive than phenol. Thus, In alkaline medium, phenol undergoes Kolbe s reaction. 

Pyridine N-oxides are frequently used in place of pyridines to facilitate electrophilic substitution. In such reactions there is a balance between electron withdrawal, caused by the inductive effect of the oxygen atom, and electron release through resonance from the same atom in the opposite direction. Here, the resonance effect is more important, and electrophiles react at C-2(6) and C-4 (the antithesis of the effect of resonance in pyridine itself). 

Another interpretation for the meta sequence is conceivable. Polarization of the aromatic by an electrophilic reagent (Z+) may induce the latent resonance effects of the m-fluoro substituent ... 

As a result of the overlaping inductive and resonance effects of substituents (acting in the same direction as in the molecules of HFP and PFIB), the central carbon in these olefins has a much higher negative charge than that of the terminal atom. It means that the attack of electrophile E+ on these substrates is aimed at the central carbon in contrast to the attack of nucleophile Nu" which is directed toward terminal carbon carying a positive charge ... 

In the molecule of internal olefin F-butene-2, the inductive and resonance effects of CF3 and F substituents compensate for each other due to symmetry of the molecule. As a result of the much lower polarization of the C=C bond (along with steric shielding of it by a bulky CF3 group), it is responsible for the significantly lower reactivity of this olefin towards electrophilic reagents ... 

Even general AB-type monomers, affording polyamide, polyester, polyether, and so on, undergo chain-growth condensation polymerization if the polymer end group becomes more reactive than the monomer by virtue of the change of substituent effects between the monomer and polymer. Both the resonance effect and inductive effect of the nucleophilic site on the reactivity of the electrophilic site at the para and meta positions of the monomer are applicable, respectively. 

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