Resonance Mesomerism

Tautomeric (3.43) and fluxional (3.44) equilibria both involve atom transfer and a change in the site of the multiple bonding, but should not be confused with resonance (mesomerism) (3.45). 

While benzene is the most celebrated example in carbon chemistry (3.46), the orthophosphate anion (3.47) is probably the most commonly encountered resonance structure in phosphorus chemistry. One of the simplest examples in carbophosphorus chemistry is phosphinine (see (3.48a) and (6.860a)). Cyclohexaphosphene (3.48b) may also exist under certain conditions (see (4.32a) and (8.259)). 

Valence bond tautomerism involves equilibria between configurationally similar structures which differ in the arrangement of their chemical bonding. It is related to the phenomenon of sigmatropic rearrangement encountered in carbon chemistry. 

A spectacular example is provided by the P anion (3.49a), whose behaviour is analogous to that of bullvalene, CioHjo (3.49b). 

In these three-fold axially symmetric fluxional molecules the P-P and C-C bonds are continually being broken and reformed between different pairs of atoms in such a way that the new structures all remain chemically identical (3.50). 

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Within the context of this book the quantitative relationships between structure and chemical reactivity are very informative. One of the early postulates of Ingold and his school in the 1930s (review see Ingold, 1969, p. 78) was that the electronic effects of substituents are composed of two main parts a field/inductive component and a mesomeric component. Hammett s work indicated clearly from the beginning that his substituent constants am and crp reflect Ingold s postulate in numerical terms. In particular, many observations indicated that the /7-substituent constant ap is the sum of a field/inductive component 0 and a resonance (mesomeric) component (Jr. 

Electron withdrawal arising from the inductive effect in the a and from the resonance (mesomeric) effect in the fi position of nitroalkenes (Table 4.48) induces deshieldings of similiar magnitudes and large two-bond carbon-proton coupling constants (up to 8 Hz). 

In Table 4.77 a small selection of carbanionic species has been compiled [502-510]. The carbanionic carbon shift of methyllithium is — 16.6 ppm (Table 4.71) in comparison to - 23.1 ppm, which is predicted for an sp2 carbon containing two electrons in a p orbital, following the empirical carbon-13 shift to charge density correlation [76, 507]. Carbanion carbon shifts become progressively more positive with increasing delocalization of the negative charge by resonance (mesomeric) effects, as shown for allyl and pentadienyl anions [503-505] in Table 4.77. 

What words should be used to describe delocaUzation is a vexed question. Terms such as resonance, mesomerism, conjugation, and delocaUzation are only a few of the ones you will find in books. You will already have noticed that we don t like resonance because it suggests that the structure vibrates rapidly between localized structures. We shall use conjugation and delocalization conjugation focuses on the sequence of alternating double and single bonds while delocalization focuses on the molecular orbitals covering the whole system. 

Electron density is influenced by resonance (mesomerism), as well as by inductive effects, as seen in unsaturated molecules such as alkenes and aromatics. The donation of electrons through resonance by a methoxy group increases the electron density at the p position of a vinyl ether (3-1) and at the para position of anisole (C6H5OCH3). Thus, the... 

In the following few chapters the electronic spectra of various types of molecules have been discussed. The electronic spectra of molecules are modified b> substitution with different groups. Both electronic and stcrie interactions of groups are important in electronic spectra. The term, electronic interaction, includes both inductive and resonance (mesomeric) interactions, although the influence of the latter may be more prominent. The substituent effects on the spectra of different chromophores and molecules will be discussed appropriately in the following chapters. Ster e effects on electronic spectra have been dealt with separately in Chapter S. 

The term resonance has also been applied in valency. The general idea of resonance in this sense is that if the valency electrons in a molecule are capable of several alternative arrangements which differ by only a small amount in energy and have no geometrical differences, then the actual arrangement will be a hybrid of these various alternatives. See mesomerism. The stabilization of such a system over the non-resonating forms is the resonance energy. 

The underlying principle of the PEOE method is that the electronic polarization within the tr-bond skeleton as measured by the inductive effect is attenuated with each intervening o -bond. The electronic polarization within /r-bond systems as measured by the resonance or mesomeric effect, on the other hand, extends across an entire nr-system without any attenuation. The simple model of an electron in a box expresses this fact. Thus, in calculating the charge distribution in conjugated i -systems an approach different from the PEOE method has to be taken. 

CjHsOOC)—CH—COOCjHj (I) + HOC Hj The carbanion (I) is a resonance hybrid (mesomeric anion) to which there are contributions carrying the negative charge on either carbon or oxygen ... 

The best-known equation of the type mentioned is, of course, Hammett s equation. It correlates, with considerable precision, rate and equilibrium constants for a large number of reactions occurring in the side chains of m- and p-substituted aromatic compounds, but fails badly for electrophilic substitution into the aromatic ring (except at wi-positions) and for certain reactions in side chains in which there is considerable mesomeric interaction between the side chain and the ring during the course of reaction. This failure arises because Hammett s original model reaction (the ionization of substituted benzoic acids) does not take account of the direct resonance interactions between a substituent and the site of reaction. This sort of interaction in the electrophilic substitutions of anisole is depicted in the following resonance structures, which show the transition state to be stabilized by direct resonance with the substituent ... 

Substituent effects (substituent increments) tabulated in more detail in the literature demonstrate that C chemical shifts of individual carbon nuclei in alkenes and aromatic as well as heteroaromatic compounds can be predicted approximately by means of mesomeric effects (resonance effects). Thus, an electron donor substituent D [D = OC//j, SC//j, N(C//j)2] attached to a C=C double bond shields the (l-C atom and the -proton (+M effect, smaller shift), whereas the a-position is deshielded (larger shift) as a result of substituent electronegativity (-/ effect). 

In 1955 Boyer d al challenged this formulation, and suggested a static, mesomeric system rather than a dynamic, tautomeric one, with Contributing structures of type 9 and 10 to a symmetrical resonance hybrid, proposing the name -o-dinitrosobenzene for the parent System. This notion, however, raised more problems than it solved,... 

Illuminati et al. have also investigated the methoxydechlorination of 4-substituted-2- and 2-substituted-4-chloroquinolines. The relation between the reaction site, the 2- or 4-position, and the substituent in the 4- or 2-position, respectively, is always meta. The authors found the two reaction series well correlated with one another, but diverging quite seriously from the Hammett correlation. They concluded that mesomerically electron-donating substituents, because of the importance of resonance structures like 12 and 13, are more deactivating than expected, while electron-withdrawing substituents, and even the methyl group, seem to follow normal a correlation. 

The amino form is usually much more favored in the equilibrium between amino and imino forms than is the hydroxy form in the corresponding keto-enol equilibrium. Grab and XJtzinger suggest that in the case of a-amino- and a-hydroxy-pyrroles, structure 89 increases the mesomeric stabilization and thus offsets the loss of pyrrole resonance energy, but the increase due to structure 90 is not sufficient to offset this loss. Similar reasoning may apply to furans and... 

The reported systems are always hydrogenated derivatives or compounds which exhibit full conjugation only in one mesomeric resonance structure, e.g. amides. 

Here we will not discuss these problems and the intriguing observation that am and strong correlation which is, however, difficult to explain (reviews Charton, 1981 Cook et al., 1989 Hansch et al., 1991). These questions were intensively studied in the 1970s and 1980s, leading gradually to the development of field and resonance parameters denoted by F and R respectively (after an original proposal of Swain and Lupton, 1968), which can be considered as independent of each other. The secondary parameters R + and R reflect the potential for an additional mesomeric donor-acceptor interaction (as in 7.7, and the opposite type with a donor instead of NQ2 and the reaction site as acceptor). 

Over the years, the Hammett equation has been modified many times, usually by defining an alternative scale of a constants, the better to allow for special features found in some mechanisms, such as resonance stabilization and mesomeric effects. Thus, there are substituent scales known at cr+, er , crj, etc. The reader is referred to specialized treatises for further details.5-811... 

The effects of structure on reactivity can be divided into three major types field, resonance (or mesomeric), and steric. In most cases two or all three of these are operating, and it is usually not easy to tell how much of the rate enhancement (or decrease) is caused by each of the three effects. 

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