More Resonance

Problems that appear to be more complex can usually be broken down into these five motifs, provided that you isolate and focus on one bit of the molecule at a time. Until you have had more practice, looking at a small part of the molecule at a time is a good strategy. 

FIGURE 4.19 Key points from Section 4.8—motifs for resonance. 

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Alkylation of pyrimidin-2(or 4)-amine on a ring-nitrogen gives an imine, e.g. (8), of quite high basic strength (pjSTa 10.7) because its cation, e.g. (13 R = Me), has typical and effective resonance stabilization indeed, methylation of pyrimidine-2,4-diamine gives a still stronger base (pjSTa> 13) due to an even more resonance-stabilized cation (14). 

The 1,3-dipolar molecules are isoelectronic with the allyl anion and have four electrons in a n system encompassing the 1,3-dipole. Some typical 1,3-dipolar species are shown in Scheme 11.4. It should be noted that all have one or more resonance structures showing the characteristic 1,3-dipole. The dipolarophiles are typically alkenes or alkynes, but all that is essential is a tc bond. The reactivity of dipolarophiles depends both on the substituents present on the n bond and on the nature of the 1,3-dipole involved in the reaction. Because of the wide range of structures that can serve either as a 1,3-dipole or as a dipolarophile, the 1,3-dipolar cycloaddition is a very useful reaction for the construction of five-membered heterocyclic rings. 

The situation is more eomplieated when the set of reasonable eontributing struetures are not all equivalent. Examine the geometry and atomie eharges forphenoxide anion. Do these data fit any one of the possible resonanee struetures (draw all reasonable possibilities), or is a eombination of two or more resonance contributors necessary ... 

Delocalized cations, represented by two or more resonance contributors, are usually more stable than localized cations. However, the fact that several resonance contributors can be drawn for a molecule does not guarantee that the molecule will actually be resonance stabilized (see also Chapter 12, Problem 10). 

Two independent molecular orbital calculations (HMO method) of delocalization energies for isoindole and isoindolenine tautomers agree that the isoindole form should possess the more resonance stabilization. The actual difference calculated for isoindole-isoindolenine is about 8 kcal/mole, but increases in favor of the isoindole with phenyl substitution at position 1 (Table VI).Since isoindole and isoindolenine tautomers have roughly comparable thermodynamic stabilities, the tautomeric proce.ss is readily obser-... 

Resonance hybrid (Section 2.4) A molecule, such as benzene, that can t be represented adequately by a single Kekule structure but must instead be considered as an average of two or more resonance structures. The resonance structures themselves differ only in the positions of their electrons, not their nuclei. 

Not all resonance structures are equally significant. A compound might have many valid resonance structures (that do not violate the two conunandments), but it is possible that one or more resonance structures might be insignificant. To understand what we mean when we say insignificant, let s revisit the analogy we used in the beginning of the chapter. 

There are many other molecules in which some of the electrons are less localized than is implied by a single Lewis structure and can therefore be represented by two or more resonance structures. For example, the three bonds in the carbonate ion all have the same length of 131 pm, which is intermediate between that of the C—O single bond in methanol (143 pm) and that of the C=0 double bond in methanal (acetaldehyde) (121 pm). So the carbonate ion can be conveniently represented by the following three resonance structures ... 

The need to use two or more resonance structures to describe the bonding in a molecule is a reflection of the inadequacy of Lewis structures for describing the bonding in molecules in which some of the electrons are not as localized as a Lewis structure implies. 

By extension one may say that the power laws (5-7) which determine the magnitude of the linear and nonlinear optical coefficients are consequences of this strong electron-lattice coupling. We now make the conjecture that the time response of these coefficients is severely affected by the dynamics of the electron-lattice coupling in conjugated chains when two or more resonant chemical structures can coexist this is the case for many of the organic chains of Figure 2. 

The statistical prior distribution predicts for triatomic molecules a more resonant behavior of energy-transfer processes than in the diatomic cases. 

The maximum in the CM X p(ECM) spectra is predicted at around 0.28 eV, as compared to 0.6 eV in the diatomic case. Unfortunately, the experimental observation of this maximum in the triatomic case is obscured by the elastic scattering. But clearly a much more resonant process is observed in the Na quenching by C02 and NzO illustrated in Fig. 27. A similar observation has been made for HzO, however, the latter does not quench... 

It is also generally true that the greater the number of contributing resonance forms, the greater will be the resonance stabilization. For this reason die enolate of a yS -diketone has much more resonance stabilization than die enolate of a simple ketone (three resonance forms versus two). The electrons are delocalized over five atoms in the former versus three atoms in the latter. In addition, the electron density on the carbon atom is less in the diketone enolate than in a simple methyl ketone enolate. 

A-9. Write one or more resonance structures that represent the delocalization of the following carbocation. 

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