Substitution, electrophilic resonance effects

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. 

We will address this issue further in Chapter 10, where the polar effects of the substituents on both the c and n electrons will be considered. For the case of electrophilic aromatic substitution, where the energetics of interaction of an approaching electrophile with the 7t system determines both the rate of reaction and position of substitution, simple resonance arguments are extremely useful. 

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. 

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). 

This is so because the methyl substituent can affect the rate and the position of further substitution. A substituent can either activate or deactivate the aromatic ring towards electrophilic substitution and does so through inductive or resonance effects. A substituent can also direct the next substitution so that it goes mainly ortho/para or mainly meta. 

There are three resonance structures for the intermediate formed in each form of electrophilic substitution, but there are two crucial ones to consider (Fig. N), arising from ortho and para substitution. These resonance structures have the positive charge next to the OH substituent. If oxygen only had an inductive effect, these resonance structures would be highly unstable. However, oxygen can act as a nucleophile and so can use one of its lone pairs of electrons to form a new rbond to the neighbouring electrophilic centre (Fig. O). This results in a fourth resonance structure where... 

In case of halogen substituents, the inductive effect is more important than the resonance effect in deactivating the ring. However, once electrophilic substitution does occur, resonance effects are more important than inductive effects in directing substitution. 

These inductive and resonance effects oppose each other. The carbon-halogen bond (shown at left) is strongly polarized, with the carbon atom at the positive end of the dipole. This polarization draws electron density away from the benzene ring, making it less reactive toward electrophilic substitution. 

The reaction apparently proceeds by the electrophilic attack of an acylium ion or protonated mixed anhydride [ArCO(H)OS02CF3], upon the para-position of an aromatic ether (Fig. 37). Loss of a proton results in the formation of 256. The nonsubstituted aryl group of the diphenyl ether was found to be much less reactive toward electrophilic substitution. This group is deactivated by protonation of the keto group in the strongly acidic environment. Therefore, monomers must be designed so that this type of resonance effect does not inhibit substitution at the second site of substitution [Eq. (53)] [162]. 

The effect of fluorine, chlorine, or bromine as a substituent is unique in that the ring is deactivated, but the entering electrophile is directed to the ortho and para positions. This can be explained by an unusual competition between resonance and inductive effects. In the starting material, halogen-substituted benzenes are deactivated more strongly by the inductive effect than they are activated by the resonance effect. However, in the intermediate carbocation, halogens stabilize the positive charge by resonance more than they destabilize it by the inductive effect. 

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