Resonance structure substitution

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. 

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... 

Gronowitz et al. have discussed the effects of substituents on chemical reactivity and on ultraviolet (XJV), infrared (IR), and nuclear magnetic resonance (NMR) spectra in terms of simple resonance theory,They assume resonance structures (1-5) to contribute to a —I—M (Ingold s terminology) 2-substituted thiophene, resonance forms (6-10) to the structure of a drI-fM 2-substituted thiophene, forms (11-16) to a —I—M 3-substituted thiophene, and forms (17-22) to a I -M 3-substituted thiophene. 

Physical and chemical evidence supporting the theory mentioned in the foregoing will be given in the appropriate sections. Some predic-. tions may be mentioned here. From resonance structures (2, 3, 7, 8) it is inferred that the substituent effect in 2-substituted thiophenes should be parallel to that in the corresponding benzenes, the 3- and 5-positions may be considered as ortho and para positions and the 4-position as a meta position. It is, however, obvious that the effect of a —M- and a d-M-substituent are not simply reversed, as reso-... 

It can be seen from resonance structures (2) to (4) that a — I — M-substituent deactivates the 3- and 5-position most strongly in electrophilic substitution. If this deactivation of the 5-position is strong enough to overcome the activating effects of the sulfur in the 5-position, substitution will be directed to the 4-position to an increasing extent. Tirouflet and Fournari studied the nitration of 2-substituted thiophenes of this type. The analysis was carried out polarographically, and the percentage of 4-isomer was as follows ... 

From resonance structure (12) it is obvious that a —I—M-substit-uent strongly deactivates the 2-position toward electrophilic substitution, and one would thus expect that monosubstitution occurs exclusively in the 5-position. This has also been found to be the case in the chlorination, bromination, and nitration of 3-thiophenecarboxylic acid. Upon chlorination and bromination a second halogen could be introduced in the 2-position, although further nitration of 5-nitro-3-thiopheneearboxylic acid could not be achieved. Similarly, 3-thiophene aldehyde has been nitrated to 5-nitro-3-thiophene aldehyde, and it is further claimed that 5-bromo-3-thiopheneboronic acid is obtained upon bromination of 3-thiopheneboronic acid. ... 

Probably the most important development of the past decade was the introduction by Brown and co-workers of a set of substituent constants,ct+, derived from the solvolysis of cumyl chlorides and presumably applicable to reaction series in which a delocalization of a positive charge from the reaction site into the aromatic nucleus is important in the transition state or, in other words, where the importance of resonance structures placing a positive charge on the substituent - -M effect) changes substantially between the initial and transition (or final) states. These ct+-values have found wide application, not only in the particular side-chain reactions for which they were designed, but equally in electrophilic nuclear substitution reactions. Although such a scale was first proposed by Pearson et al. under the label of and by Deno et Brown s systematic work made the scale definitive. 

It is interesting to speculate why the basicities of the substituted pyridines are well correlated by normal a-values, rather than by a+-values. Writing the resonance structures for the conjugate acid (5<->6)... 

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. 

Draw resonance structures of the intermediate carbocations in the bromillation of naphthalene, and account for the fact that naphthalene undergoes electrophilic substitution at Cl rather than C2. 

You knowr the mechanism of HBr addition to alkenes, and you know the effects of various substituent groups on aromatic substitution. Use this knowledge to predict which of the following two alkenes reacts faster with HBr. Explain your answer by drawing resonance structures of the carbocation intermediates. 

The reaction is less selective than the related benzoylation reaction (/pMe = 30.2, cf. 626), thereby indicating a greater charge on the electrophile this is in complete agreement with the greater ease of nuclophilic substitution of sulphonic acids and derivatives compared to carboxylic acids and derivatives and may be rationalized from a consideration of resonance structures. The effect of substituents on the reactivity of the sulphonyl chloride follows from the effect of stabilizing the aryl-sulphonium ion formed in the ionisation step (81) or from the effect on the preequilibrium step (79). 

Long before their theories were supported by computations, organic chemists found a way to use resonance structures to explain the product distribution in electrophilic substitution. Thus, the Lewis structure for phenol is regarded as a resonance hybrid of the following structures ... 

The second of the resonance structures is the source of the radical reactivity displayed during oxidation and the Mn(III) in this structure must be low-spin to preserve multiplicity. Substitution at the meso-position could provide steric hindrance to analogous decompositions and reactions of the Mn(IV) complexes of ethyl- and benzylmalonic acids, and a conventional one-equivalent oxidation step becomes dominant. 

As a result of the contribution by resonance structures having some double bond character, the boron atom in BF3 is not as electron deficient as it is in BC13. To answer the second question, we will write a general substitution reaction as... 

Hydrocarbons containing one or more triple bonds in addition to double bonds have been excluded from the tile, as have been radicals (e.g. the allyl radical C3H5 ) and aromatic molecules, i.e. molecules for which more than one unexcited resonance structure (Kekule structure) can be written. Consequently, hydrocarbons such as phenyl-substituted polyenes, or annulenes — bridged or unbridged—have not been included. 

C-NMR spectroscopic studies on a-substituted tris(ethynyl)methyl cations 49 prepared from alcohols 50 (equation 18) provided evidence for the participation of resonance structures with allenyl cationic character38. The parent tris(ethenyl)methyl cation (49, R = H) cannot be generated under stable carbocation conditions (SbFs/FSOsH) presumably due to the highly reactive unsubstituted termini of the three ethynyl groups and the resulting low kinetic stability. The chemical shift data (Table 1) give evidence that in all cases Ca and CY are deshielded more than Cg (relative to their precursor alcohols). 

The stabilization mechanism operating in aryl-substituted vinyl cations such as 9 can be qualitatively depicted by the Lewis resonance structures A-C (Scheme 4). There is hyperconjugation between the carbon atom C° which is described by the no-bond resonance structure B. In addition, -delocalization between the aryl ring and the C+ carbon atom is indicated by structures such as C. 

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