Extra-resonances

One underlying physical basis for the failure of Hammett reaction series is that substituent interactions are some mixture of resonance, field, and inductive effects. When direct resonance interaction is possible, the extent of the resonance increases, and the substituent constants appropriate to the normal mix of resonance and field effects then fail. There have been many attempts to develop sets of a values that take into account extra resonance interactions. 

In this equation, the substituent parameters and reflect the incremental resonance interaction with electron-demanding and electron-releasing reaction centers, respectively. The variables and r are established for a reaction series by regression analysis and are measures of the extent of the extra resonance contribution. The larger the value of r, the greater is the extra resonance contribution. Because both donor and acceptor capacity will not contribute in a single reaction process, either or r would be expected to be zero. 

Another example of enhanced sensitivity to substituent effects in the gas phase can be seen in a comparison of the gas-phase basicity for a series of substituted acetophenones and methyl benzoates. It was foimd that scnsitivtiy of the free energy to substituent changes was about four times that in solution, as measured by the comparison of A( for each substituent. The gas-phase data for both series were correlated by the Yukawa-Tsuno equation. For both series, the p value was about 12. However, the parameter r" ", which reflects the contribution of extra resonance effects, was greater in the acetophenone series than in the methyl benzoate series. This can be attributed to the substantial resonance stabilization provided by the methoxy group in the esters, which diminishes the extent of conjugation with the substituents. 

For symmetrical and unsymmetrical diphenyl-urea we expect some extra resonance energy in addition to that for two benzene rings and urea because of conjugation of these groups the values found are 0.55 v.e. and 0.50 v.e., respectively. 

The data given in Table VII for the quinones show large extra resonance energies of 0.57 v.e. in quinone, 1.42 v.e. in anthraquinone, and 1.4 v.e. in phenanthraquinone, in addition to the resonance energy of two benzene rings in the last two compounds. These large values we at-... 

Compound Formula Structure E E Reso- nance energy Extra resonance energy... 

The quantum-mechanical treatment previously applied to benzene, naphthalene, and the hydrocarbon free radicals is used in the calculation of extra resonance energy of conjugation in systems of double bonds, the dihydro-naphthalenes and dihydroanthracenes, phenylethylene, stilbene, isostilbene, triphenylethylene, tetraphenylethyl-... 

The curve corresponding to this equation is also shown in Fig. 1. It is seen that it lies below the curve representing resonance between a single bond and a ry (or ir ) double bond by the amount 0.026 A. at 0.5, and by 0.045 A. at the point for a pure double bond, n — 1.0, these differences representing the effect of the extra resonance stabilization when two ir electron pairs are involved. Values corresponding to the curve are given in Table I. 

Extra resonance energy, not including that within the benzene ring. 

In this equation nf is the bond order it is equal to the bond number n for n = 1, 2, and 3, but has a different interpretation for fractional values. Whereas we have taken the bond number for the carbon-carbon bonds in benzene to be 1, so that the valence of carbon remains equal to the sum of the bond numbers of the bonds formed by the atom, the bond order is taken to be somewhat larger, reflecting the extra resonance energy of the molecule. The value of n calculated... 

The extra factor 22 for resonance in the ortho and para positions corresponds to an extra resonance energy of 1.8 kcal/mole ( = RT In 22) for the ion relative to the unionized molecule. This is not unreasonable there is only one structure of this type involved for each ortho or para nitro group, and the unfavorable distribution of charge makes it of little significance for the unionized molecule.87... 

The quantum-mechanical treatment of this problem70 indicates that the single bonds in a conjugated system have about 20 percent double-bond character and that the extra resonance energy resulting from the conjugation of two double bonds is about 5 to 8 kcal/mole. The calculations also show that a double bond and a benzene nucleus are approximately equivalent in conjugating power. 

Some bonds, classically represented as single bonds, are found to be abnormally short. Using the above interpretation, such bonds are considered to have partial double-bond charaeter and extra resonance pictures are thus drawn which are said to contribute to the structure of the molecule. Almost always, such shortening occurs in a single bond adjacent to a double or triple bond if the bond is between two multiple bonds, the shortening is even more marked ... 

With stilbene and azobenzene there are two isomers. In these cases a plane structure is realized for the //mf-isomers, in contrast to the cis forms where this is impossible on spatial grounds. The stability of the trans forms is a consequence of the extra resonance (extra R.E. /z -stilbene 7.0 kcal energy content of frmy-azobenzene 10 kcal lower than the cis form). This resonance also appears from the distances, stilbene, trans C—C 1.445 A, C=C 1.33 A azobenzene, trans C—N 1.415 A, cis C—N 1.46 A (sum of atomic distances 1.54 A and 1.47 A, respectively) in this latter compound the planes of the rings make an angle of 50°. 

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