Resonance stabilization of aniline

The gas-phase exothermicity is due to the enhanced resonance stabilization of aniline compared to benzene for the nonzwitterionic amino acids, as found in the gas phase. On the other hand, the aniline resonance stabilization is lost in the zwitterionic amino acids of the solid phase and thus the reaction is essentially thermoneutral. This is, of course, related to the weak basicity of aniline compared to related nonaromatic bases such as cyclohexylamine, as exhibited by the ca 50 kJ mol-1 exothermicity of the formal reaction 29. 

Resonance stabilization of aniline, as compared with cyclohexylamine l... 

Direct connection of pendant heteroatom to polystyrene aryl is a synthetically more difficult, but often still feasible (37), alternative. However, though bonds from phenyl to many common heteroatoms are relatively strong, resonance stabilization of partial positive charge developed on an arylated atom activates it to leave other substituents alkyl anilium salts (12) and anilines (38), as well as phenolic esters (39), are relatively easy to cleave. Aryl linkages,... 

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. 

Arylamines have a larger positive AG° for protonation and are therefore less basic than alkylamines, primarily because of resonance stabilization of their ground state. Electrostatic potential maps show that lone-pair electron density is delocalized in aniline compared to cyclohexylamine but that the corresponding ammonium ions localize charge in the same way. 

Because of the opposite nature of the n interactions between the ring and either the nitro or the amino substituent, let us assess the stabilization energies of nitrobenzene and aniline. In equation 13, the resonance stabilizing energy of aniline was defined as the exothermicity of a reaction involving arbitrary reference states. By analogy to equation 13, we may write equation 57 for nitrobenzene and the same arbitrary reference states, R = /-Pr or t-Bu. 

The only nitrenium ions that have been directly characterized by electrochemical generation are those that are kineticaUy stabilized by resonance and steric hindrance. The main limitation to the electrochemical method appears to be competing polymerization processes (Fig. 13.7). It is known that electrolysis of aniline and its... 

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. 

Which is the stronger base, aniline or ammonia The same reasoning applies. Aniline is stabilized by resonance. The basic pair of electrons is delocalized into the ring and is less available to form a covalent bond, so it is a weaker base. (Of course, if the conjugate acid of aniline is stronger than ammonium ion, then aniline must be a weaker... 

Ab initio Hartree-Fock calculations of the stabilities of enols and carbonyl compounds (Table 5) have been performed in recent years by Hehre and Lathan (1972), Bouma et al. (1977, 1980 see also Bouma and Radom, 1978a,b) and Noack (1979). The order or magnitude of the differences in energies (AE) is the same as that estimated for acetone in the gas phase (AG = 13.9 + 2 kcal mol-1 at 25 °C) by Pollack and Hehre (1977) from an ion cyclotron resonance spectroscopy study of the proton and deuteron transfers from CD3C(OH)CD to aniline. This gave relative values for the O—H and... 

Many of the ashless anti-knocks are amines or phenols [26] and are related to liquid-phase oxidation inhibitors. They probably work by reacting with active radicals (particularly OH) to produce radicals which are inert. For instance, N-methyl aniline (NMA) CeHsNHCHs probably produces stabilized CeHsNCHs radicals which, because of their resonance stabilization, are unable to react to regenerate active radicals again and may undergo only radical recombination reactions. The rate of radical removal by this process is likely to be limited in the most favourable case by how fast the additive can react with OH to produce stabilized radicals. Although exact rates are not known, this is probably already a fast process for NMA, and unlikely to be very much faster for any other substance. Indeed, the most effective ashless anti-knock found by MacKinven [26] in an extensive study of 970 substances was a tetra-aryl hydrazine, with a molar effectiveness 2.9 times that of NMA. 

Contribution from the three structures V, VI, and VII stabilizes the amine in a way that is not possible for the ammonium ion resonance thus lowers the energy content of aniline more than it lowers the energy content of the anilinium ion. The net effect is to shift the equilibrium in the direction of less ionization, that is, to make smaller (Fig. 23.1). (See, however, the discussion in Sec. 18.11.)... 

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