Anilines resonance energies

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... 

Since this change by the factor 1/1.4 X 10 is due entirely to the complete inhibition of the F, G, H resonance by addition of the proton, the quantity RT In 1.4 X 10 = 8.4 kcal/mole represents the F, G, H resonance energy in aniline. This value is probably more accurate than that given by thermochemical data, 6 kcal/mole (Table 6-2), and the agreement between the two is satisfactory. 

The resonance energy of the pseudobase has been approximated by that of the nearest aromatic molecule from which it can be considered to be derived. Thus, benzene is used as the reference molecule for 1,2-dihydro-2-hydroxy-l-methylquinoline, etc. Small differences in the resonance energies of benzene and aniline (1.6 kcal mol- J140 and benzene and styrene etc. have been ignored. 

The high resonance energy of anilines and its absence upon protonation both contribute to the significant difference found for the solid and gaseous amino acids. 

First, the nitrogen atom of aniline is attached to an sp carbon, whereas the nitrogen atom of cyclohexylamine is attached to a less electronegative sp carbon. Second, the nitrogen atom of protonated aniline lacks a lone pair that can be delocalized. When it loses a proton, however, the lone pair that formerly held the proton can be delocalized. Loss of a proton, therefore, is accompanied by an increase in resonance energy. 

Electron delocalization can affect the nature of the product formed in a reaction and the of a compound. A carboxylic acid and a phenol are more acidic than an alcohol such as ethanol, and a protonated aniline is more acidic than a protonated amine because electron withdrawal stabilizes their conjugate bases and the loss of a proton is accompanied by an increase in resonance energy. 

A. Greenberg and T. A. Stevenson, Molecular Structure and Energetics Studies of Organic Molecules (Eds. J. F. Liebman and A. Greenberg), VCH, Deerfield Beach, 1986. See also the discussion in J. F. Liebman and R. M. Pollack, in The Chemistry of Enones, Part I (Eds. S. Patai and Z. Rappoport), Wiley, New York, 1989 wherein the resonance energy of crotonaldehyde was shown to be less than that of piperylene while the rotational barriers are in the reverse order. From Stull, Westmm and Sinke we find the barrier to rotation of the nitro group in nitrobenzene is 25.1 kJmol , to be compared with the rotational barrier of the amino group in aniline of 14.2 kJmol". ... 

Conflicting thermodynamic and kinetic answers are particularly striking in the case of aniline. This compound is thermodynamically stable with a resonance energy similar to benzene, but it is kinetically unstable with a high reactivity due to the possibility of an easy electron-transfer reaction to oxygen. Reactivity is related to the HOMO and LUMO energies. Bird showed that the hardness of a molecule that is half of the HOMO—LUMO gap is related to the REPE. 

For aniline and other arylamines, the resonance stabilization is the result of the interaction of the unshared pair on nitrogen with the ir system of the aromatic ring. The resonance energy of benzene is approximately 151 kj (36 kcal)/mol. For aniline, it is 163 kJ (39 kcal)/mol. Because of fhis resonance interaction, the electron pair on nitrogen is less available for reaction with acid. No such resonance stabilization is possible for alkylamines. Therefore, the electron pair on the nitrogen... 

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. 

VII. Multi-parameter Correlation Equations All authors who have seriously considered the scope and limitations of the linear free-energy relationships have recognized the existence of real deviations. Frequently, the limitations of the Hammett eq. (1) for certain substituents in certain situations were considered to be indicative of a duality of u-constants. Hammett noticed that the reactions of anilines and phenols required a special value for aJt NOt, 1.27, in contrast to the value, 0.778, derived from benzoic acids. An example is the increased acidity of p-nitrophenol over that expected on the basis of the a constant based on benzoic acid. Resonance interaction between the substituent and the side-chain is presumed to be responsible ... 

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... 

There is an extra R.E. of 6 kcal but the basicity of aniline is also very much smaller than that of an aliphatic primary amine, such as cyclohexylamine. This special resonance does not exist any longer in the anilinium ion because the free pair has now become a bonding pair by proton addition. The base constant Kb is related to the (free) energy AF of the proton addition according to the expression AF = —RTlnKb. 

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