Delocalization, of charge

In general, five-membered heteroaromatic ring systems with one heteroatom all undergo preferential a rather than /3 electrophilic substitution. This is rationalized in terms of the more effective delocalization of charge in the intermediate (36) leading to a substitution than in the intermediate (37) leading to /3 substitution. 

Draw the Lewis structure of each carbocation (show all formal charges). Do these structures provide any indication of which carbocation will form most readily Explain. Examine the electrostatic potential map of each carbocation. Which carbocation shows the greatest localization of positive charge Which shows the greatest delocalization of charge Where does the positive charge go in the most delocalized ion Do these observations provide any indication of which caibocation will form most readily ... 

A similar delocalization of charge which stabilizes the relevant intermediate occurs in 4-mercaptopteridine and pyrido[4,3-d]-P3Timidine-4(3 r)-thione. Both these compounds are cleaved under similar conditions. Such a delocalization is not possible with either pyrido[3,2-d]- or pyrido[3,4-d]pyTimidine-4(3 r)-thione and although the latter compound could not be prepared, the pjTido[3,2-d]pyrimi-dine was quite stable under these conditions. 

Tire fulvalenium salts 35 and 38 react reversibly in aqueous buffer solutions to yield the corresponding triarylcyclopropenols, as evidenced by the well-defined isosbestic points in their UV spectra. Tire pK values derived from spectra are compatible with extensive delocalization of charge in the heterocyclic ring (68TL5541). 

For azapentatriafulvalenium salts of type 35 the charge distribution was made evident by a comparison of the NMR spectra of these heteroful-valenes and of 3-benzhydrylidene-3H-indolium salts and their common indole precursors (68TL5541).The results suggested that the best description involves extensive delocalization of charge throughout the cyclopropene and indole rings. 

Thus, while the enhancement of the electron-attracting effect of S02Me by hydrogen bonding to water is fairly small in the meta-position, it is much larger in the para- position because of the delocalization of charge from O - into the substituent in the anion. The situation may be represented schematically as 28. It should be noted, however, that for... 

In fused ring systems, the positions are not equivalent and there is usually a preferred orientation even in the unsubstituted hydrocarbon. The preferred positions may often by predicted as for benzene rings. Thus it is possible to draw more canonical forms for the arenium ion when naphthalene is attacked at the a position than when it is attacked at the p position, and the a position is the preferred site of attack,though, as previously mentioned (p. 682), the isomer formed by substitution at the p position is thermodynamically more stable and is the product if the reaction is reversible and equilibrium is reached. Because of the more extensive delocalization of charges in the corresponding arenium ions, naphthalene is more reactive than benzene and substitution is faster at both positions. Similarly, anthracene, phenanthrene, and other fused polycyclic aromatic hydrocarbons are also substituted faster than benzene. 

One additional point should be discussed here, concerning the substantial emphasis that has been placed on the differences between alkyl and aryl isocyanides. It has been suggested, primarily on the basis of infrared evidence, that aryl isocyanides are better 7r-acceptors than alkyl isocyanides (90). Qualitatively this difference is easily rationalized. One can see that delocalization of charge into 7r -orbitals on an aryl ring in aryl isocyanide-metal complexes should be possible, whereas no such possibility exists for alkyl isQcyanide-metal complexes this means that aryl isocyanides should be better ir-acceptors. Of course, the simple qualitative model gives one no measure of the relative importance of this effect. 

Hydrolyses of p-nitrophenyl and 2,4-dinitrophenyl sulfate are accelerated fourfold and eightfold, respectively, by cycloheptaamylose at pH 9.98 and 50.3° (Congdon and Bender, 1972). These accelerations have been attributed to stabilization of the transition state by delocalization of charge in the activated complex and have been interpreted as evidence for the induction of strain into the substrates upon inclusion within the cycloheptaamylose cavity. Alternatively, accelerated rates of hydrolysis of aryl sulfates may be derived from a microsolvent effect. A comparison of the effect of cycloheptaamylose with the effect of mixed 2-propanol-water solvents may be of considerable value in distinguishing between these possibilities. 

There has long been interest in the structure of l,6-dioxa-6aA4-thiapentalene 174 and l,6,6aA4-trithiapentalene 175, for which the resonance forms 176 and 177 (with further delocalization of charges) can be included (see Figure 9). 

The delocalization of charge and the order of stability of carbocations parallel the number of attached methyl groups. 

Extensive theoretical studies have been carried out to probe the nature of the allyl anion. These studies supplement and extend the experimental results. Allyl anion is of special interest because it is the simplest 7r-delocalized carbanion with 4 electrons and 3 Pjr-centers. Much recent theoretical discussion has concerned the role of resonance in the stabilization of such conjugated systems, a stabilization defined as the enthalpy difference between the localized double-bonded system and its conjugated state. The stabilization of allyl anion has generally been attributed to the delocalization of charge associated... 

It is therefore improbable that homoaromaticity in neutral radicals will be of any importance. However, as already discussed, the driving force for delocalization of charge is high, and hence homoconjugative interactions in charged radicals are significant (Roth, 1987). 

Finally, the solvent also interacts with Lewis acid and Lewis base sites that are not directly involved in mutual coordination, thereby altering the electronic properties of the complex. For example, delocalization of charges onto the surrounding solvent molecules causes ions in solution to be softer than in gas phase241. Again, water is particularly effective in this respect because it can act as an efficient electron-pair acceptor and donor. 

Transition state imbalance in the deprotonation of substituted 2-tetralones by hydroxide ions has been described. A Brpnsted plot of logA versus substrate pAfa is linear, with slope —a) of —0.60 0.01 but the negative deviation of the point for the 6-nitro derivative suggests that delocalization of charge lags behind proton transfer. ... 

Delocalization of charge density Scheme 1 General features of ylides... 

Delocalization of charge in the conjugate base anion through resonance is a stabilizing factor and will be reflected by an increase in acidity. Drawing resonance structures allows us to rationalize that the negative charge is not permanently localized on a particular atom, but may be dispersed to other areas of the structure. We should appreciate that a better interpretation is that the electrons are contained in a molecular orbital that spans several atoms. 

The pATa of ethanol is 16, and that of acetic acid is 4.8. The increased acidity of acetic acid relative to ethanol can be rationalized in terms of delocalization of charge in the acetate anion, whereas in ethoxide the charge is localized on oxygen. Even more delocalization is possible in the methanesulfonate anion, and this is reflected in the increased acidity of methanesulfonic acid (pAla — 1-2). 

The alkane propane has pATa 50, yet the presence of the double bond in propene means the methyl protons in this alkene have pATa 43 this value is similar to that of ethylene (pATa 44), where increased acidity was rationalized through sp hybridization effects. 1,3-Pentadiene is yet more acidic, having pATa 33 for the methyl protons. In each case, increased acidity in the unsaturated compounds may be ascribed to delocalization of charge in the conjugate base. Note that we use the term allyl for the propenyl group. 

Resonance stabilization is also responsible for the increased acidity of a C-H group situated adjacent to a carbonyl group. The anion is stabilized through delocalization of charge, similar to that seen with the allyl anion derived from propene but this system is even more favourable, in that delocalization allows... 

With substituted phenols, there can be similar delocalization of charge into the aromatic ring as with phenol, but substituents will introduce their own effects, be it inductive or resonance related. It can be seen that the nitro group allows further... 

Delocalization of charge in the conjugate base anion contributes to stabilization of the anion, and thus ionization of the acid is enhanced. Delocalization effects in bases are more likely to stabilize the base... 

Now let us look at guanidines, which are even stronger bases. Guanidine itself has pATa 13.6. It can be seen that there is delocalization of charge in the conjugate acid, such that in each resonance structure the charge is favourably associated with... 

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