Chlorobenzene resonance

The darkest regions in the slices indicate the greatest electron density. The meta form of nitrated chlorobenzene and the para form of nitrated nitrobenzene retain the resonance structure to a much greater degree throughout the extent of the electron density. In contrast, the density in the less-favored conformations becomes more localized on the substituent as one moves outward from the plane of the carbon atoms. 

Problem 16.8 Write resonance structures for chlorobenzene to show the electron-donating resonance effect of the chloro group. 

For example, in benzene, all H- and C-atoms are constitutionally equivalent. In chlorobenzene, however, only the o- and m-atoms are pairwise equivalent. Note here that the individual resonance formulas of delocalized bond systems are not distinguishable. 

Halogen substituents withdraw electron density from the aromatic nucleus but direct olp-through resonance effects. The result is that halobenzenes undergo nitration with more difficulty relative to benzene. The nitration of chlorobenzene with strong mixed acid gives a mixture of 2,4- and 2,6-isomeric dinitrochlorobenzenes in which the former predominates." The nitration of 2,4-dinitrochlorobenzene to 2,4,6-trinitrochlorobenzene (picryl chloride) requires an excess of fuming nitric acid in oleum at elevated temperature. Both are useful for the synthesis of other polynitroarylene explosives but only 2,4-dinitrochlorobenzene finds industrial importance (Sections 4.8.1.2 and 4.8.1.3). 

The energy balance of photodissociation the importance of stabilization of the free radicals. When chlorobenzene or chloro-Np loses the halogen atom, a phenyl or a naphthyl radical is formed with the odd electron localized in an sp2 orbital which is orthogonal to the aromatic zr orbitals such a radical is not stabilized through resonance, unlike the benzyl- or the methyl-Np radicals for which several resonance structures can be drawn (Figure 4.32). 

Exercise 14-11 a. Write resonance structures analogous to structures 5a through 5d to show the activating effect of—CsN and —S02R groups in nucleophilic substitution of the corresponding 4-substituted chlorobenzenes. 

The resonance parameter, o°R, is the resonance contribution of a substituent when it is not directly conjugated with the reaction center. The dataset is composed of benzene, chlorobenzene, bromobenzene, and iodobenzene ... 

Friedel-Crafts type reactions of strongly deactivated arenes have been the subject of several recent studies indicating involvement of superelectrophilic intermediates. Numerous electrophilic aromatic substitution reactions only work with activated or electron-rich arenes, such as phenols, alkylated arenes, or aryl ethers.5 Since these reactions involve weak electrophiles, aromatic compounds such as benzene, chlorobenzene, or nitrobenzene, either do not react, or give only low yields of products. For example, electrophilic alkylthioalkylation generally works well only with phenolic substrates.6 This can be understood by considering the resonance stabilization of the involved thioalkylcarbenium ion and the delocalization of the electrophilic center (eq 4). With the use of excess Fewis acid, however, the electrophilic reactivity of the alkylthiocarbenium ion can be... 

It is clear that in the aromatic halogen compounds, for example chlorobenzene, the same phenomena will occur, since the same resonance possibilities exist in this case ... 

The contribution from the last form, however, will be small. A similar series of structures occurs in chlorobenzene although in this case additional structures are possible in view of the resonance of the benzene ring. 

The difference between the chemical behaviour of the substitution products of benzene and the corresponding aliphatic derivatives are well known and are reflected in the values of their dipole momcnxs [Table XCVI), In chlorobenzene, in addition to the bond resonance structures, I and //, there are three additional structures ///, IV and V contributing to the molecular resonance and the dipole moment is lowered in comparison with the alkyl halides. 

However, we see that this is not the case. Chlorobenzene and bromobenzene have dipole moments of only 1.7 d, and vinyl chloride and vinyl bromide have dipole moments of only 1.4 d. This is consistent with the resonance picture of these molecules. In the structures that contain doubly-bonded halogen (III, IV,... 

Figure 5-2 A stack plot for the inversion recovery experiment of the carbon-13 resonances of chlorobenzene at 25 MHz. The time t in the pulse sequence 180°-t-90° is given in seconds at the right. (From R. K. Harris, Nuclear Magnetic Resonance Spectroscopy Pitman Publishing, Ltd., London, 1983, p. 82. Reproduced with permission of Addison Wesley Longman, Ltd.)...

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