Resonance hybrid major contributor

Examine the geometry and electrostatic potential map for acetone enolate. Are the CC and CO bonds in the enolate more similar to those in acetone or propen-2-ol precursors Is the negative charge primarily located on oxygen or on carbon Assuming this enolate is a hybrid of the two resonance contributors as shown above, which, if either, appears to be the major contributor ... 

A similar concept exists when comparing resonance structures. One compound might have three resonance structures, but all three resonance structures might not contribute equally to the overall resonance hybrid. One resonance structure might be the major contributor (like the peach), while another resonance structure might be insignificant (like the kiwi). In order to understand the true nature of the compound, we must be able to compare the resonance structures and determine which structures are major contributors and which structures are not significant. 

Recently, a variety of (3-silylated carboxonium ions have been prepared and characterized by NMR spectroscopy.541 Kira et al.631 used the Corey hydride transfer method, whereas Olah, Prakash, and co-workers applied triphenylmethyl tetrakis (pentafluorophenyl)borate to silylate esters,632 ketones, enones, and carbonates633 [Eq. (3.91)]. The ions thus produced are resonance hybrids of oxocarbenium (327b) and carboxonium (327a) ions with the latter as the major contributors. Calculated (DFT/IGLO) 29 Si NMR chemical shifts agree well with the experimental data. 

The first structure, with more bonds and less charge separation, is possible because sulfur is a third-row element with accessible d orbitals, giving it an expandable valence. For example, SFg is a stable compound with 12 electrons around sulfur. Theoretical calculations suggest that the last structure, with octets on all atoms, may be the major resonance contributor, however. We cannot always predict the major contributor of a resonance hybrid. 

When two resonance structures are different, the hybrid looks more like the better resonance structure. The better resonance structure is called the major contributor to the hybrid, and all others are minor contributors. The hybrid is the weighted average of the contributing resonance structures. What makes one resonance structure better than another There are many factors, but for now, we will learn just two. 

Comparing resonance structures X and Y, X is the major contributor because it has mote bonds and fewer charges. Thus, the hybrid looks more like X than Y. 

Although the resonance hybrid is some combination of all of its valid resonance structures, the hybrid more closely resembles the most stable resonance structure. Recall from Section 1.5C that the most stable resonance structure is called the major contributor to the hybrid, and the less stable resonance structures are called the minor contributors. Two identical resonance structures are equal contributors to the hybrid. 

Because A contains a positive charge and a ione pair on adjacent atoms, a second resonance structure B can be drawn. Because B has more bonds and aii second-row atoms have octets, B is more stabie than A, making it the major contributor to the hybrid C. Because the hybrid is more stable than either resonance contributor, the order of stability is ... 

The two resonance structures that contain an intact aromatic ring and place a negative charge on an O atom are major contributors to the hybrid. Resonance stabilizes phenoxide but not as much as... 

Structures with the lowest formal charges usually have the lowest energy (major contributors to the resonance hybrid). 

The proton NMR spectra of 50b and 50c revealed the C-3 proton at 6.05 and 5.95 ppm, respectively. These shifts led the authors to conclude that these munchnones are not aromatic and that resonance structure 50 is the major contributor to the hybrid. Compound 50a was too labile to be studied by NMR. The UV and IR spectra of 50a-c were also recorded, although with difficulty, since decomposition was occurring. The authors believe that the (CH2CI2) at 360 and 353 nm for 50a and 50b, respectively, are 71 71 transitions. The IR spectra of 50a-c aU show carbonyl bands at 1700-1708 cm and 1726-1747 cm , in addition to the acetoxy absorption at 1770 cm for 50c. 

In most cases, the various resonance structures of a molecule are not equivalent and do not contribute equally to the resonance hybrid. The electron distribution in the molecule resembles that of its major contributor more closely than any of its alternative resonance stmctures. Therefore, it is important that we develop some generalizations concerning the factors that make one resonance form more important (more stable) than another. Table 1.6 outlines the structural features that alert us to situations when resonance needs to be considered and lists criteria for evaluating the relative importance of the contributing structures. 

Judgment must be exercised in cases where two of the general resonance criteria are in conflict. For example, the structure B is an important contributor to the resonance hybrid for the intermediate in the nitration of methoxybenzene (anisole), even though the structure has a positive charge on oxygen. This unfavorable feature is compensated for by the additional covalent bond present in structure B compared with the other major contributor, structure A, in which the positive charge is on carbon. 

Consider the free radical nitrogen dioxide, NO2, a major contributor to urban smog that is formed when the NO in auto exhaust is oxidized. NO2 has several resonance forms. Two differ in terms of which O atom is doubly bonded, as in the case of ozone. Two others have the lone electron residing on the N or on an O, so the resonance hybrid has the lone electron delocalized over these two atoms ... 

Problem 1.17 Draw a second resonance structure for nitrous acid. Label each resonance structure as a major, minor, or equal contributor to the hybrid. Then draw the resonance hybrid. 

Draw two or three resonance forms for each of the following species. Indicate the major contributor or contributors to the hybrid in each case. 

For each species in Problem 32, indicate the resonance form that is the major contributor to the resonance hybrid. Explain your choices. 

A better resonance structure is one that has more bonds and fewer charges. The better structure is the major contributor and all others are minor contributors. To draw the resonance hybrid, use dashed lines for bonds that are in only one resonance structure, and use partial charges when the charge is on different atoms in the resonance structures. 

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