Resonance and hybridization

In this case, the ionic structures make only a small contribution to the resonance hybrid, and we regard the hond as almost purely covalent. Moreover, the two ionic structures have the same energy and make equal contributions to the hybrid so the average charge on each atom is zero. However, in a molecule composed of different elements, such as HC1, the resonance... 

Addition to conjugated systems can also be accomplished by any of the other three mechanisms. In each case, there is competition between 1,2 and 1,4 addition. In the case of nucleophilic or free-radical attack, the intermediates are resonance hybrids and behave like the intermediate from electrophilic attack. Dienes can give 1,4 addition by a cyclic mechanism in this way ... 

The formation of indanthrone and flavanthrone, as well as alizarin, during the alkali fusion of 2-aminoanthraquinone can be explained mechanistically on the basis of the initial loss of a proton. The resulting anionic species can be represented as a resonance hybrid and is also tautomeric (Scheme 6.12). Primary 1-hydroxylation of 2-aminoanthraquinone is probably the first step in the formation of the alizarin by-product (compare Scheme 6.8). Such an attack may initiate the formation of flavanthrone [31 ]. It is also possible to envisage the formation of all three species by a radical mechanism [32]. 

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. 

The resonance hybrid and its contributing resonance structures resulting from nucleophilic attack in a conjugate addition reaction. 

A few words should be said about the difference between resonance and molecular vibrations. Although vibrations take place, they are oscillations about an equilibrium position determined by the structure of the resonance hybrid, and they should not be confused with the resonance among the contributing forms. The molecule does not resonate or vibrate" from one canonical structure to another. In this sense the term resonance is unfortunate because it has caused unnecessary confusion by invoking a picture of vibration. The term arises from a mathematical analogy between the molecule and the classical phenomenon of resonance between coupled pendulums, or other mechanical systems. 

The apparent fickleness of the acyl-pyrroles and -indoles in their reaction with carbanions to form new C—C bonds arises from the contribution made by the zwitterionic structure, e.g. (410b), to the resonance hybrid and the choice of the reaction conditions is critical for a successful nucleophilic reaction. Thus, formyl-pyrroles and -indoles do not normally undergo the Cannizzaro reaction nor do they form stable cyanohydrins or undergo benzoin-type reactions. However, surprisingly, 2-formylpyrrole reacts with arylaldehydes in the presence of potassium cyanide to yield (428), which is easily oxidized to (429) (B-77MI30505). It is noteworthy that the presence of an ester substituent adjacent to the formyl group modifies the mesomeric interaction to such an extent to allow the formation of (430) in low yield, as a result of an initial benzoin-type self-condensation (Scheme 76) (68BSF637). 

In contrast, the mechanism of cycloaddition of isothiocyanates is quite different. The -NCS group reacts with appropriate reagents to form 1,2-, 1,3-, and 1,4-cycloadducts. It may be assumed that one of the polar resonance-structures of the NCS group contributes predominantly to the resonance hybrid, and the compounds react through either the C=S or the C=N bond. 

Draw the electron-dot formulas that show all important contributors to a resonance hybrid and show their electronic relationship using curved arrows. 

When several structures may be assumed to contribute to the true structure of a molecule, but no one of them can be said to represent it uniquely, the molecule is referred to as a resonance hybrid and the phenomenon is termed as resonance. 

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. 

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. 

Benzothiadiazole is nitrated in the 4- and 7-positions (48JCS-1006). The former result is difficult to explain without involving 8.79 as a significant contributor to the overall resonance hybrid, and thereby deactivating the 5-position. 

The bond between carbons 1 and 2 is represented as a double bond in two of the three resonance structures, but the bond between carbons 2 and 3 is represented as a double bond in only one resonance structure. The C1-C2 bond thus has more double-bond character in the resonance hybrid, and it is shorter than the C2-C3 bond. The C3-C4, C5-C6, and C7-C8 bonds also have more double-bond character than the remaining bonds. 

There are a number of rules that distinguish meaningful contributory resonance structures. Firstly, the atoms involved must not move between resonance structures secondly, the same number of paired electrons should exist in each structure contributing to the resonance hybrid and thirdly, structures that have adjacent like charges will not make a major contribution to the overall resonance hybrid, neither will those involving multiple isolated charges. Finally, it is important that the a-bond framework, and in particular steric factors, permit a planar relationship between the contributory resonance structures. 

The electronic structures of the oxides of nitrogen are shown below. Most of these molecules are resonance hybrids, and the contributing structures are Sot all shown for nitrogen pentoxide, for example, the various single and double bonds may change places. 

A second possibility would be to move the electrons of a double bond to just one of the terminal carbons this leads to a structure like 1-20. However, when more than one neutral resonance structure can be written, doubly charged resonance structures, like 1-20 and 1-21, contribute an insignificant amount to the resonance hybrid and are usually not written. 

Valence-bond theory is over 90% successful in explaining much of the descriptive chemistry of ground states. VB theory is therefore particularly popular among chemists, since it makes use of familiar concepts such as chemical bonds between atoms, resonance hybrids and the like. It can perhaps be characterized as a theory which explains but does not predict. Valence-bond theory fails to account for the triplet ground state of O2 or for the bonding in electron-deficient molecules such as diborane, B2H6. It is not very useful in consideration of excited states, hence for spectroscopy. Many of these deficiencies are remedied by molecular orbital theory, which we take up in the next two chapters. 

FIGURE 11.18. Resonance hybrids and bond lengths in aromatic compounds. For benzene, (a) Kekule forms and a common mode of depicting the aromatic character of benzene, (b) bond character and predicted bond lengths, and (c) experimental bond lengths and angles. 

The chemistry and physics of nitroxides derived from hindered piperidines attracted enormous attention for years [64,108,176,177]. Information on their behaviour were available prior to the commercial boom of HAS. EPR spectral characteristics of NO have been well defined. The structure was determined by mass spectroscopy. HAS derived nitroxides react in two canonical forms [178]. The contribution of the dipolar double bonded canonical structure 123b (perhaps nearly 50%) makes the nitroxide 123a a resonance hybrid and a species having a substantial dipole moment. 

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