Aromatic rings electronic distribution

Because of the presence of nitrogen in the aromatic ring, electrons in pyridine are distributed in such a way that their density is higher in positions 3 and 5 (the P-positions). In these positions, electrophilic substitutions such as halogenation, nitration, and sulfonation take place. On the contrary, positions 2, 4, and 6 (a- and y-positions, respectively) have lower electron density and are therefore centers for nucleophilic displacements such as hydrolysis or Chichibabin reaction. In the case of 3,5-dichlorotrifluoropyridine, hydroxide anion of potassium hydroxide attacks the a- and y-positions because, in addition to the effect of the pyridine nitrogen, fluorine atoms in these position facilitate nucleophilic reaction by decreasing the electron density at the carbon atoms to which they are bonded. In a rate-determining step, hydroxyl becomes attached to the carbon atoms linked to fluorine and converts the aromatic compound into a nonaromatic Meisenheimer complex (see Surprise 67). To restore the aromaticity, fluoride ion is ejected in a fast step, and hydroxy pyridines I and J are obtained as the products [58],... 

Perfluoroalkylation of arenes makes the aromatic ring electron deficient and thus it interacts with electron-rich arenes and Lewis bases. Figure 1.9 shows the TT-electron distribution on the hexafluorobenzene and benzene ring. Most of the TT-electrons in hexafluorobenzene are withdrawn from the aromatic ring by fluorine atoms to make the aromatic ring highly electron deficient, while the —-electrons in benzene are localized inside the ring. Thus both the benzenes interact attractively in a face-to-face manner [1]. 

It is well known that the kinetic effects of several substituents on one or two aromatic rings are not additive. This is exemplified in the bromination of 1,1-diphenylethylenes, stilbenes and a-methylstilbenes. The presence of a substituent, particularly one capable of electron donation by resonance, on the aromatic ring so alters the charge distribution at the transition state that the second substituent in the other ring then interacts with a charge different from that which would prevail if the substituent were alone. This is expressed... 

The relative rates of acetylation in competition experiments in the [m.n]paracyclophane series 38> may be interpreted in terms of trans-annular electronic and steric effects. If the rate of acetylation of [6.6]para-cyclophane [(7), m =n =6] is is taken as one, the relative acetylation rates of the [4.4]-, [4.3]-, and [2.2]paracyclophanes are 1.6, 11, and >48, respectively. As the aromatic rings come closer together, the rate of entry of the first acetyl group into the nucleus increases, while that of the second acetyl group decreases. Both effects clearly indicate that the positive. partial charge can be distributed over both benzene rings in the monoacetylation transition state (64). 

These hybridisation variations are caused by anisotropy within the chemical bonds. This is due to the non-homogeneous electronic distribution around bonded atoms to which can be added the effects of small magnetic fields induced by the movement of electrons (Fig. 9.12). Thus, protons on ethylene are deshielded because they are located in an electron-poor plane. Inversely, protons on acetylene that are located in the C-C bond axis are shielded because they are in an electron-rich environment. Signals related to aromatic protons are strongly shifted towards lower fields because, as well as the anisotropic effect, a local field produced by the movement of the aromatic electrons or the ring current is superimposed on the principal field (Fig. 9.12). 

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