The Resonance Hybrid

Sample Problem 16.1 Draw two more resonance structures for each species. 

Mentally breaking a molecule into two- or three-atom units can make it easier to draw additional resonance structures. 

Problem 16.7 Draw additional resonance structures for each ion. 

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

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. 

Use the following three rules to evaluate the relative stabilities of two or more valid resonance structures. 

Rule[1] Resonance structures with more bonds and fewer charges are more stable. 

Rule [2] Resonance structures in which every atom has an octet are more stable. 

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Now add more dilute sulphuric acid drop by drop the colour almost completely fades, as salt formation occurs on both nitrogen atoms with suppression of the resonance hybrid formation. 

Support for this suggestion comes from many quarters. Reduction of the jS-carboline anhydro-bases with sodium and alcohol or with tin and hydrochloric acid gives the 1,2,3,4-tetrahydro derivatives, as does catalytic reduction over platinum oxide in an alkaline medium. On the other hand, catalytic reduction with platinum oxide in acetic acid results in the formation of the 5,6,7,8-tetrahydro-j3-carbolinium derivatives (see Section III,A,2,a). It should be noted, however, that reduction of pyrido[l,2-6]indazole, in which the dipolar structure 211 is the main contributor to the resonance hybrid, could not be effected with hydrogen in the presence of Adams catalyst. 

When two resonance forms are nonequivalent, the actual structure of the resonance hybrid is closer to the more stable form than to the less stable form. I bus, we might expect the true structure of the acetone anion to be closer to the resonance form that places the negative charge on an electronegative oxygen atom than to the form that places the charge on a carbon atom. 

Rule 5 The resonance hybrid is more stable than any individual resonance form. 

Valence bond theory (Chapter 7) explains the fact that the three N—O bonds are identical by invoking the idea of resonance, with three contributing structures. MO theory, on the other hand, considers that the skeleton of the nitrate ion is established by the three sigma bonds while the electron pair in the pi orbital is delocalized, shared by all of the atoms in the molecule. According to MO theory, a similar interpretation applies with all of the resonance hybrids described in Chapter 7, including SO S03, and C032-. 

On the basis of these values one can conclude that, with increasing bond orders, the force constants rise, suggesting that the S—O bond of sulphoxides should have more semipolar character than that of sulphones. Furthermore, molecular diffraction measurements20 and Parachors21 for sulphoxides also suggest that the S—O bond in sulphoxides should have a semipolar single-bond representation while the S—O bond in sulphones is described by double bonds or better as the resonance hybride shown in Scheme 1. 

Self-Test 2.7A Write Lewis structures contributing to the resonance hybrid for the acetate ion, CH ,CC)2. The structure of CH COOH is described in Example 2.4 the acetate ion has a similar structure, except that it has lost the final H atom while keeping both electrons from the O -H bond. 

Self-Test 2.7B Write Lewis structures contributing to the resonance hybrid for the nitrite ion, N02. ... 

Benzene, C6H(l, is another molecule best described as a resonance hybrid. It consists of a planar hexagonal ring of six carbon atoms, each one having a hydrogen atom attached to it. One Lewis structure that contributes to the resonance hybrid is shown in (11) it is called a Kekulc structure. The structure is normally written as a line structure (see Section C), a simple hexagon with alternating single and double lines (12). 

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... 

Write the Lewis structures that contribute to the resonance hybrid of the guanadinium ion, C(NH2)3+. 

It is often asked whether or not the constituent structures of a resonating system, such as the Kekul4 structures for the benzene molecule, are to be considered as having reality. There is one sense in which this question may be answered in the affirmative but the answer is definitely negative if the usual chemical significance is attributed to the structures. A substance showing resonance between two or more valence-bond structures does not contain molecules with the configurations and properties usually associated with these structures. The constituent structures of the resonance hybrid do not have reality in this sense. 

Resonance effects are also important in aromatic amines. m-Nitroaniline is a weaker base than aniline, a fact that can be accounted for by the —7 effect of the nitro group. But p-nitroaniline is weaker still, though the —I effect should be less because of the greater distance. We can explain this result by taking into account the canonical form A. Because A contributes to the resonance hybrid, " the electron density of the unshared pair is lower in p-nitroaniline than in m-nitroaniline, where a canonical form such as Ais impossible. The basicity is lower in the para compound for two reasons, both... 

In the case of an unsymmetrical diene such as isoprene, different orientations of the structural units are possible depending on which end of the diene unites with the chain radical. Of the two competing reactions (6) and (7) shown on page 241, the former would appear to be the more probable one on account of the influence of the methyl substituent in stabilizing to some extent one of the resonance hybrid structures which are shown. 

These observations are compatible with the model for the carbene complex presented in Section II,A. Both metal and w-donor substituents compete to donate electron density to unfilled carbenepz orbitals, and with good 7r-donors such as nitrogen, the metal is less effective. In terms of resonance formalism, the resonance hybrid 39 makes a more significant contribution than 40 to the structure of the carbene ligands in these compounds. Similar conclusions are reached when the structures of Group 6, 7, and other Group 8 heteroatom-substituted carbene complexes are considered. 

The first kinetically stable dibenzosilafulvene (7), whose structure and properties should more correctly be described by the resonance hybrid 7a 7b with a great contribution of the ylide form 7a, reacts with phosphorus ylide to form betaine (8), which is rearranged, under thermodynamically controlled conditions, into the salt (9) (Scheme 4).24,25... 

When anisole is reduced by an alkali metal in liquid ammonia either a stable, conjugated diolefin or a less stable, unconjugated diolefin can be isolated, the latter being the product when the reaction mixture is neutralized with a strong acid such as ammonium chloride. The free ions, whether they are present to any great extent or not, are the resonance hybrid LXVI. 

Neither structure is consistent with the observation that the two S-0 bond lengths in SO2 are equal, and in fact, the true Lewis structure for SO2 is neither (A) or (B) but rather an equal blend of the two individual contributors called the resonance hybrid, (see below)... 

In NBO language, the resonance hybrid (5.29a) corresponds to a two-electron intermolecular donor-acceptor interaction of the form... 

The H-bonded H HOH product species was previously depicted in Fig. 5.16, while the structure and leading n— a interaction for the corresponding H2 OH-reactant species are shown in Fig. 5.33. Figure 5.34 similarly depicts the structure of the transition-state species and principal n—a interaction for the reactant-like Lewis structure that better describes the resonance hybrid (see below). 

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