Aromatic Compounds with Strongly Electron-Withdrawing Groups

Aromatic Compounds with Strongly Electron-Withdrawing Groups 

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Aromatic halides are normally quite inert to the types of nucleophiles that readily displace halide ions from allgrl halides. However, when an aromatic compound contains strong electron-withdrawing nitro groups ortho or para (or both) to the halogen, nucleophilic aromatic substitution occurs quite readily. For example, when 1 -chloro-2,4-dinitrobenzene is heated at reflux in aqueous sodium carbonate followed by treatment with aqueous acid, it is converted in nearly quantitative yield to 2,4-dinitrophenol. 

Thus by several criteria l,4-dihydro-l,4-diazocines without a strongly electron-withdrawing group at the 1,4-positions are aromatic compounds, whereas those with such groups are non-planar systems with localized bonding. Diene character in the latter compounds is seen in the cycloaddition of N- phenyltriazolinedione to give the adducts (234b and c). 

Of course, with atoms from lower periods of the periodic system, such as P or S, it is possible to have aromatic systems that are not simply homologs of pyridine or pyrylium, (i.e., phosphinine and thiopyrylium, respectively) but also systems with P(V) and S(IV), i.e. phosphabenzene (/. -phosphinine) and thiabenzene, respectively. However, in some cases the aromaticity is strongly perturbed, as for instance in selenabenzene derivatives stabilized by electron-withdrawing groups, when these compounds are shown to be zwitterionic. 

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