Resonator equivalent

Resonance theory can also account for the stability of the allyl radical. For example, to form an ethylene radical from ethylene requites a bond dissociation energy of 410 kj/mol (98 kcal/mol), whereas the bond dissociation energy to form an allyl radical from propylene requites 368 kj/mol (88 kcal/mol). This difference results entirely from resonance stabilization. The electron spin resonance spectmm of the allyl radical shows three, not four, types of hydrogen signals. The infrared spectmm shows one type, not two, of carbon—carbon bonds. These data imply the existence, at least on the time scale probed, of a symmetric molecule. The two equivalent resonance stmctures for the allyl radical are as follows ... 

Some fundamental structure-stability relationships can be employed to illustrate the use of resonance concepts. The allyl cation is known to be a particularly stable carbocation. This stability can be understood by recognizing that the positive charge is delocalized between two carbon atoms, as represented by the two equivalent resonance structures. The delocalization imposes a structural requirement. The p orbitals on the three contiguous carbon atoms must all be aligned in the same direction to permit electron delocalization. As a result, there is an energy barrier to rotation about the carbon-carbon... 

Chemists traditionally represent benzene in terms of a pair of equivalent resonance structures, each with alternating single and double bonds. 

Active Figure 10.3 An orbital view of the allyl radical. The p orbital on the central carbon can overlap equally well with a p orbital on either neighboring carbon, giving rise to two equivalent resonance structures. Sign in afwww.thomsonedu.com to see a simulation based on this figure and to take a short quiz. 

Although five equivalent resonance structures can be drawn for all three species, Huckel s rule predicts that only the six-ir-electron anion should be aromatic. The four-77-electron cyciopentadienyl carbocation and the five-7r-electron cyciopentadienyl radical are predicted to be unstable and antiaromatic. 

The structures of S02 and S03 were referred to in Chapter 7. These molecules are often cited as examples of resonance sulfur trioxide, for example, has three equivalent resonance structures ... 

In 1999, Karl Christe synthesized and characterized a salt that contained the N,+ cation, in which the five N atoms are connected in a long chain. This cation is the first allnitrogen species to be isolated in more than 100 years. Draw the most important Lewis structure for this ion, including all equivalent resonance structures. Calculate the formal charges on all atoms. 

Draw the most important Lewis structure for each of the following ring molecules (which have been drawn without showing the locations of the double bonds). Show all lone pairs and nonzero formal charges. If there are equivalent resonance... 

In the examples presented so far, all the resonance structures are equivalent, but resonance structures are not always equivalent. Resonance structures that are not equivalent occur when Step 5 requires shifting electrons from atoms of different elements. In such cases, different possible structures may have different formal charge distributions, and the optimal set of resonance structures includes those forms with the least amount of formal charge. Example treats a molecule that has near-equivalent resonance structures. 

C09-0108. Carbon, nitrogen, and oxygen form two different polyatomic ions cyanate ion, NCO, and isocyanate ion, CNO". Write Lewis stmctures for each anion, including near-equivalent resonance structures and indicating formal charges. 

Figure 3.8 Two resonance structures that can be written for acetic acid and two that can be written for acetate ion. According to a resonance explanation of the greater acidity of acetic acid, the equivalent resonance structures for the acetate ion provide it greater resonance stabilization and reduce the positive free-energy change for the ionization.

Since A and B are equivalent resonance structures, the allyl radical should be much more stable than either, that is, much more stable than a 10 radical => the allyl radical is even more stable than a 3° radical. 

D and E are equivalent resonance structures => the allyl cation should be unusually stable. 

Equivalent resonance structures make equal contributions to the hybrid, and a system described by them has a large resonance stabilization. 

Radicals in which the odd electron is on a nitrogen next to an aromatic ring are stabilized by resonance analogous to that of tri-phenylmethyl. In the case of Wurster s salts, the nitrogen analogs of semiquinones, there are two equivalent resonance structures in the acid form, but in the less stable basic form one of the structures requires separation of charge. Evidence for the unpaired electron has been obtained by measurement of the paramagnetism.144... 

Superficially, it is also possible to write two equivalent resonance structures for cyclobutadiene (C4H4)... 

Table 3.37. Leading NRTresonance structures and weightings (with numbers of symmetry-equivalent resonance structures in brackets) for anions of common...

The pattern shown is only one of a large number of distinct ways of choosing the L/X labels in (4.122) and (4.123), corresponding to the large number of equivalent resonance structures that could be written. However, such an exercise adds little value to the simple picture of ferrocene bonding provided by the localized bonding units. 

It has become increasingly popular to represent [C5H5]- by the structure shown in Fig. 6d. This representation is a valence bond structure notation it is intended to represent the five equivalent resonance structures (three of these are shown as Figs. 6c, /, and g the other two are similar), in which the negative charge is located at each carbon in turn. Each double bond is thus only a partial double bond and, if the usual notation of writing a partial double bond by a dashed line were applied, the structure would be written as 6h. The solid circle (6d) is a rapid way to write the 66 structure. In a completely analogous... 

The even greater basicity of guanidine (p/iLa = 13-6) as compared with amidines is due to an even greater resonance stabilization of the cation, since three equivalent resonance structures enter into resonance [6]. The base itself resonates between three nonequivalent structures [37], and has according to Pauling (1939,... 

Equivalent resonance structures contribute equally to the hybrid. 

Equivalent resonance structures are equally important to the overall structure. 

Figure 7-15 shows that each attack yields three equivalent resonance structures. This implies that the simple electrophilic attack is not the key to what product will form. It must be the identity of the original substituent (G). 

One of the polymerization routes involves polymerization of one or the other of the double bounds in the usual manner. The other route involves the two double bonds acting in a unique and concerted manner. Thus addition of an initiating radical to a 1,3-diene such as 1,3-butadiene yields an allylic radical with the two equivalent resonance forms LI and LII... 

Figure 22.6. 1,4-Dehydrocubane (5), showing the nonbonding orbital that is essentially doubly occupied in the lowest singlet. Both A and B are two of the six, equivalent resonance structures that contribute to the low energy of this state, relative to the triplet.

Figure 3.24. Stages in the interaction between two molecules, and A, showing the effects of electron correlation and the equivalent resonance structures a) no interaction, the electrons are spin paired and confined to the orbital of B (b) weak interaction (e.g., a transition structure), the electrons can separate into a larger volume of space (c) strong interaction, a bond is formed and the electron distribution is again confined.

Benzene, C6H6, has two equivalent resonance structures, so all C—C bonds are equivalent. We often draw benzene rings with a circle in place of three double bonds. 

Nitro compounds are a very important class of nitrogen derivatives. The nitro group, —N02, like the carboxylate anion, is a hybrid of two equivalent resonance structures ... 

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