Electrophiles, in aromatic

Among the variety of electrophilic species present in concentrated sulfuric acid, sulfur trioxide (Figure 12.4) is probably the actual electrophile in aromatic sulfonation. We can represent the mechanism of sulfonation of benzene by sulfur trioxide by the sequence of steps shown in Figure 12.5. 

The general point that should be drawn from this example is that the actual electrophile in aromatic substitution is usually a highly reactive intermediate formed under the conditions of the reaction. The formation of the true electrophile may or may not be the rate-determining step. Scheme 9.1 indicates the structure of some of the electrophilic species involved in typical electrophilic aromatic substitution processes and the reactions involved in their formation. 

Hammett correlations also permit some insight into the reactivity and selectivity of electrophiles in aromatic substitution reactions. In general, the standard Hammett O substituent constant gives poor correlations with reactions involving electrophilic... 

Draw all reasonable electron-dot formulas for the nitronium ion (N02 ), the electrophile in aromatic nitrations. Show any formal charges. Which structure is favored and why (See Secs. 1.11 and 1.12.)... 

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