Resonance structures contribution

Equivalent resonance structures contribute equally to the hybrid. 

Conjugate addition occurs because there are two sites on the electrophile where a nucleophile can attack. The structure of the resonance hybrid and the two resonance structures contributing to the hybrid cire shown in Figure 11-22. The presence of this resonance is apparent in the infrcired spectrum because the carbonyl stretch shifts to a longer wavenumber. 

In using the resonance method, we assume that all the resonance structures contributing to a given resonance hybrid have exactly the same spatial arrangements of the nuclei but different pairing schemes for the electrons. Therefore 11, 12, and 13 are not to be confused with bicyclo[2.2.0]-2,5-hexadiene, 15, because 15 is a known (albeit not very stable) molecule with different atom positions and therefore vastly different bond angles and bond lengths from benzene ... 

In other words, a more stable resonance structure contributes more to the resonance hybrid and is said to be more important. Structures of equal stability contribute equally. 

The lower its energy, the more a resonance structure contributes to the overall structure of the hybrid. 

The resonance stabilization in these systems is thought to arise from the conjugation between the C-C and C-X double bonds, whereby the dipolar resonance structures contribute significantly to the resonance hybrid. 

The resonance structures contribute to the planar arrangement by giving the CON bond partial double-bond character. 

C How Do We Decide When One Resonance Structure Contributes More to the Hybrid Than Another ... 

On the other hand, when electrophilic attack does take place, the chlorine atom stabilizes the arenium ions resulting from ortho and para attack relative to that from meta attack. The chlorine atom does this in the same way as amino groups and hydroxyl groups do—by donating an unshared pair of electrons. These electrons give rise to relatively stable resonance structures contributing to the hybrids for the ortho- and para-substituted arenium ions. 

Each of these structures has the same fundamental weakness it implies that the central sulfur atom somehow interacts more strongly with one oxygen atom than with the other. The explanation for this contradiction is that the structure of the molecule cannot be accurately represented by a single Lewis strucmre. Instead we say that the real strucmre corresponds to an average of all the valid Lewis strucmres, and we call this average a resonance hybrid of the contributing Lewis strucmres. It is very important to emphasize that the double-headed arrow does not mean that the structure alternates between the two possibilities it indicates that the two structures shown for SO2 are resonance structures, contributing to a resonance hybrid. In resonance structures, the positions of all atoms are identical only the positions of the electrons are different. 

The unhybridized 2p orbital on the N atom can be used to make ir bonds. For any one of the three resonance structures shown, we might imagine the formation of a single localized N—Ott bond, formed between the unhybridized 2p orbital on N and a 2p orbital on one of the O atoms. Because each of the three resonance structures contributes equally to the observed stmeture of NOs , however, we represent the it bonding as spread out, or delocalized, over the three N—O bonds, as shown in Figure 9.29 . 

It is important to realize that if resonance structures contribute unequally, the actual structure of the hybrid most resembles the structure that contributes most. The electrostatic potential map of acetone shows the negative charge (red) on oxygen and the positive charge (blue) on carbon in agreement with the results we derive from the resonance treatment. 

Benzylic positions also exhibit the same pattern, with several resonance structures contributing to the overall resonance hybrid. 

Figure 9.8 Resonance structures contributing to n electron densities in acridine dyes (X = N) and rhodamine dyes (X = 0). The resonance structures containing =X < are dominant in acridines, but are less favorable in rhodamines.

The carbonate ion is not correctly described by any of these structures, but exists as a form, known as a resonance hybrid, which is a blend of all three structures (Figure 14.44). Each of the three structures, known as resonance structures, contributes to the resonance hybrid depending on its energy the lower the energy of the resonance structure, the greater its contribution to the hybrid. In the case of the carbonate ion all three resonance structures are of equal energy (due to their symmetry) and make an equal contribution to the resonance hybrid. The concept of resonance was introduced in Chapter 2. 

In the examples of resonance hybrids that we have examined so far, the contributing structures have been equivalent (or equally valid) Lewis structures. In these cases, the true structure is an equally weighted average of the resonance structures. In some cases, however, we can write resonance structures that are not equivalent. For reasons we cover in the material that follows—such as formal charge—one possible resonance structure may be somewhat better than another. In such cases, the true structure is still an average of the resonance structures, but the better resonance structure contributes more to the true structure. In other words, multiple nonequivalent resonance structures may be weighted differently in their contributions to the true overall structure of a molecule (see Example 9.8). 

This resonance structure contributes little to the hybrid since Cl is a weak base. Thus, the C-CI bond has little double bond character, making it similar in length to the C-CI bond in CH3CI. 

These resonance structures contribute very httle character to the overall resonance hybrid, and as a result, the oxygen atom of this C=0 bond is less electron rich as compared with most C=0 bonds. 

Therefore, this resonance structure contributes significant character to the overall resonance hybrid, which gives the azulene a considerable dipole moment. 

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