Directing effects resonance

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. 

Radiation techniques, application to the study of organic radicals, 12, 223 Radical addition reactions, gas-phase, directive effects in, 16, 51 Radicals, cation in solution, formation, properties and reactions of, 13, 155 Radicals, organic application of radiation techniques, 12,223 Radicals, organic cation, in solution kinetics and mechanisms of reaction of, 20, 55 Radicals, organic free, identification by electron spin resonance, 1,284 Radicals, short-lived organic, electron spin resonance studies of, 5, 53 Rates and mechanisms of solvolytic reactions, medium effects on, 14, 1 Reaction kinetics, polarography and, 5, 1... 

Two of three nitrofluorobenzene isomers react with methoxide, but the third is unreactive. Obtain energies of methoxide anion (at left), ortho, meta and para-nitrofluorobenzene, and the corresponding ortho, meta and para-methoxide anion adducts (so-called Meisenheimer complexes). Calculate the energy of methoxide addition to each of the three substrates. Which substrate is probably unreactive What is the apparent directing effect of a nitro group Does a nitro group have the same effect on nucleophilic aromatic substitution that it has on electrophilic aromatic substitution (see Chapter 13, Problem 4) Examine the structures and electrostatic potential maps of the Meisenheimer complexes. Use resonance arguments to rationalize what you observe. 

Figure 12a) also the Na2C08 specimens are not blue after irradiation. Further, carbonate free NaOH prepared from distilled sodium metal gave the same results as AnalaR NaOH which contains about 2.5% NaCO as an impurity. Finally, the new resonance signals found in the alkali hydroxide systems are not caused by a direct effect of radiation on the solute, since solid NaOH itself on 7-irradiation at 770 K. neither turns blue, nor shows the characteristic ESR spectra discussed above (16,45). 

Despite the success in parameterizing acid/base and many other properties for a range of different compounds, it is obvious that the simple electrostatic model used by Taft and extended by others [27, 28] have fundamental weaknesses - both with regards to the domain of validity and at a more fundamental level. The model is more intuitive than physical, in the sense that the inductive effect, polarisation effect, resonance effect, mesomeric effect and steric effect have no proper quantum mechanical definition, and can therefore not be derived directly from the system s wave function [29,30]. 

Understand the activating and directing effects of substituents on aromatic rings, and use inductive and resonance arguments to predict orientation and reactivity. 

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