Resonance, structural

Structurally benzene is the simplest stable compound having aromatic character, but a satisfactory graphical representation of its formula proved to be a perplexing problem for chemists. Kekule is usually credited with description of two resonating structures which. 

Dubai H-R and Quack M 1984 Tridiagonal Fermi resonance structure in the IR spectrum of the excited CH chromophore in CFgH J. Chem. Phys. 81 3779-91... 

Segall J, Zare R N, Dubai H R, Lewerenz M and Quack M 1987 Tridiagonal Fermi resonance structure in the vibrational spectrum of the CM chromophore in CHFg. II. Visible spectra J. Chem. Phys. 86 634-46... 

Mandelshtam V A, Taylor H S, Jung C, Bowen H F and Kouri D J 1995 Extraction of dynamics from the resonance structure of H + H2 spectra J. Chem. Phys. 102 7988... 

Clearly such bonding would produce two different carbon-oxygen bond distances (p. 48) but in fact all bonds are found to be identical and intermediate in length between the expected C=0 and C—O bond distances. We conclude, therefore, that the true structure of the carbonate ion cannot be accurately represented by any one diagram of the type shown and a number of resonance structures are suggested (p. 50). 

The carbonate ion is planar and can be regarded as a resonance structure between the three forms given below (see also p. 44) ... 

Benzene has already been mentioned as a prime example of the inadequacy of a connection table description, as it cannot adequately be represented by a single valence bond structure. Consequently, whenever some property of an arbitrary molecule is accessed which is influenced by conjugation, the other possible resonance structures have to be at least generated and weighted. Attempts have already been made to derive adequate representations of r-electron systems [84, 85]. 

Figure 2-51. a) The rotational barrier in amides can only be explained by VB representation using two resonance structures, b) RAMSES accounts for the (albeit partial) conjugation between the carbonyl double bond and the lone pair on the nitrogen atom. 

Figure 2-52. a) Two semipolar resonance structures are needed in a correct VB representation of the nitro group, b) Representation of a nitro group by a structure having a pentavalent nitrogen atom, c) The RAMSES notation of a nitro group needs no charged resonance structures. One jr-system contains four electrons on three atoms. 

Our first approach took resort in simple resonance theory [36, 37]. For each conjugated nr-system aU resonance structures were generated, such as those shown in Figure 7-5. 

For singlet spin molecules at the equilibrium geometry, RHF and UHF wave functions are almost always identical. RHF wave functions are used for singlets because the calculation takes less CPU time. In a few rare cases, a singlet molecule has biradical resonance structures and UHF will give a better description of the molecule (i.e., ozone). 

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 ... 

The more extensive problem of correlating substituent effects in electrophilic substitution by a two-parameter equation has been examined by Brown and his co-workers. In order to define a new set of substituent constants. Brown chose as a model reaction the solvolysis of substituted dimethylphenylcarbinyl chlorides in 90% aq. acetone. In the case ofp-substituted compounds, the transition state, represented by the following resonance structures, is stabilized by direct resonance interaction between the substituent and the site of reaction. 

As apparent from the contributing resonance structures, both mesoionic systems contain an azomethinylide contribution, accounting for the reaction with representative dipolarenophiles to give cycioadducts such as 3 or 4 (Scheme 4). The cydoadditions and extrmsion reactions of the adducts have been the mam object of investigation. since previous reviews on me.soionic thiazoles (2.9V Results appearing since 1969 and before June 1976 are reported for each type of compound in this chapter. Tables VIIRl-5 contain all mesoionic thiazoles described before June 1976. 

The rules to be followed when writing resonance structures are summarized m Table 1 5... 

Electron delocalization can be important in ions as well as in neutral molecules Using curved arrows show how an equally stable resonance structure can be generated for each of the following anions... 

Atomic positions (connectivity) must be the same in all resonance structures only the electron posi tions may vary among the various contributing structures... 

These are the most important rules to be concerned with at present Additional aspects of electron delocalization as well as additional rules for Its depiction by way of resonance structures will be developed as needed in subsequent chapters... 

In Section 1 9 we introduced curved arrows as a tool to systematically generate resonance structures by moving electrons The mam use of curved arrows however is to show the bonding changes that take place in chemical reactions The acid-base reactions to be discussed in Sections 1 12-1 17 furnish numer ous examples of this and deserve some preliminary comment... 

Show by writing appropriate resonance structures that the two... 

Write a second resonance structure for each of the following ... 

Allyl radical is a conjugated system in which three electrons are delocalized over three carbons The resonance structures indicate that the unpaired electron has an equal probability of being found at C 1 or C 3 C 2 shares none of the unpaired electron... 

In general the most stable resonance structure for a polycyclic aromatic hydro carbon is the one with the greatest number of rings that correspond to Kekule formula tions of benzene Naphthalene provides a fairly typical example... 

Write resonance structures for tropylium cation sufficient to... 

Write resonance structures for cyclopentadienyl anion suffi cient to show the delocalization of the negative charge over all five carbons J... 

Each of the following may be represented by at least one alternative resonance structure in which all the six membered nngs correspond to Kekule forms of benzene Write such a resonance form for each... 

Write the principal resonance structures of o methylbenzyl cation and rn methylbenzyl cation Which one has a tertiary carbocation as a contnbuting resonance form ... 

Because the carbon atom attached to the ring is positively polarized a carbonyl group behaves m much the same way as a trifluoromethyl group and destabilizes all the cyclo hexadienyl cation intermediates m electrophilic aromatic substitution reactions Attack at any nng position m benzaldehyde is slower than attack m benzene The intermediates for ortho and para substitution are particularly unstable because each has a resonance structure m which there is a positive charge on the carbon that bears the electron withdrawing substituent The intermediate for meta substitution avoids this unfavorable juxtaposition of positive charges is not as unstable and gives rise to most of the product... 

In resonance terms electron delocalization map unsaturated carbonyl compounds IS represented by contributions from three principal resonance structures... 

Principal resonance structures of the anion of a 3 keto ester... 

Protonation of imidazole yields an ion that is stabilized by the electron delocalization represented in the resonance structures shown... 

Write the most stable resonance structure for the cyclohexa dienyl anion formed by reaction of methoxide ion with o fluoronitrobenzene J... 

The structure of the conjugate base is more like resonance structure B than A because the nega tive charge is on the more electronegative atom (O versus S)... 

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