Aniline resonance structures

The second way in which the substituent R affects the charge distribution of the molecule is called the resonance effect (or sometimes the tautomeric or electromeric effect). This results when the molecule resonates among several electronic structures. For example, for aniline the structures... 

The first compound of interest is aniline (14) itself. While drawings of its resonance structures permeate textbooks, the research literature acknowledges ambiguities as to its quantitation28. For example, there is a ca 38 kJ mol-1 spread of plausible resonance energies for aniline as defined by the exothermicity of reaction 12... 

Ionic or dipolar valence-bond resonance structures of nitrobenzene (53a,b) and aniline (54a,b) illustrate the difference of 7r-electron density in the ring of these two compounds85. 

Basicity of aniline Aniline, like all other amines, is a basic compound ( Tb = 4.2 X 10 °). Anilinium ion has a = 4.63, whereas methylammo-nium ion has a pK = 10.66. Arylamines, e.g. aniline, are less basic than alkylamines, because the nitrogen lone pair electrons are delocalized by interaction with the aromatic ring tt electron system and are less available for bonding to H+. Arylamines are stabilized relative to alkylamines because of the five resonance structures as shown below. Resonance stabilization is lost on protonation, because only two resonance structures are possible for the arylammonium ion. 

Silicon bound to a phenyl group can also influence the bond system by additional (p- -d) back donation from carbon to silicon. In agreement with this model, p-trimethylsilyl-substituted benzoic acid shows a greater acidity than expected from inductive effects. Furthermore, p-trimethylsilyl phenol exhibits a greater acidity than phenol itself, and p-trimethylsilyl aniline shows a decreased basicity as compared with that of the nonsubstituted compound. This behaviour can be described by the following resonance structures [Eqs. (4) and (5)] ... 

Arylamines are weaker bases than cyclohexylamines because of resonance. Aniline, a typical arylamine, exhibits the resonance structures shown in Figure 8-1. 

The first triarylmethane dyes were synthesized on a stricdy empirical basis in the late 1850s an example is fuchsine, which was prepared from the reaction of vinyl chloride with aniline. Their structural relationship to triphenylmethane was established by Otto and Emil Fischer (5) with the identification of pararosaniline [569-61-9] as 4,4,4,-triaminotriphenylmethane and the structural elucidation of fuchsine. Several different structures have been assigned to the triarylmethane dyes (6—8), but none accounts precisely for the observed spectral characteristics. The triarylmethane dyes are therefore generally considered to be resonance hybrids. However, for convenience, usually only one hybrid is indicated, as shown for crystal violet [548-62-9], Cl Basic Violet 3 (1), for which Amax = 589 nm. 

Draw the important resonance structures for aniline. Use the curved arrow convention to show how the electrons are moved to create each new resonance structure. Discuss the relative contribution of each to the resonance hybrid and the overall resonance stabilization of the compound. 

For example, five resonance structures can be drawn for aniline (C6H5NH2). Because three of them place a negative charge on a carbon atom of the benzene ring, an NH2 group donates electron density to a benzene ring by a resonance effect. 

Use Spartan View to obtain the dipole moments of aniline, nitrobenzene, and 4-nitro-aniline. Is the dipole moment of 4-nitroani ine the sum of the dipole moments of aniline and nitrobenzene Compare electrostatic potential maps and geometries for all three structures, and draw resonance structures for 4-nitroaniline that account for your observations. 

Fig. 8-2 Some resonance structures for aniline, involving charge separation.

This cation is stabilized, similar to the way aniline is, by delocalization of the odd electron on nitrogen. Several resonance structures may be written as a result of this delocalization ... 

Aniline is not as basic as an aliphatic amine due to the presence of the aromatic group. Resonance structures can be drawn for aniline that show the electron pair on... 

We can understand this effect if we write the resonance structures for the arenium ions that would arise from ortho, meta, and para attack on aniline ... 

Two important conclusions can be drawn from the given resonance structures. First, the electron lone pair is not localized on the nitrogen atom as in saturated amines. We recall from the previous chapter that amines are less basic if the nitrogen lone pair is not localized on the N-atom. This was illustrated by an example where we compared the basicities of aniline and pyridine and showed that aniline is a weaker base because its lone pair electrons are delocalized over the entire benzene ring. For the same reason, amides are neutral rather than basic. 

Amides are most nucleophilic on oxygen (the product has more resonance structures than that from W-alkylation). POCI3 is the only electrophile. Anilines are most nucleophilic at the 0- and p-positions, but the latter is more sterically accessible. 

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