Substitution at Aromatic Rings

The 77 bonds in aromatic compounds are also reactive toward electrophiles, although not nearly so much as alkenes. The aromatic ring attacks an electrophile to give an intermediate carbocation. The carbocation then undergoes fragmenta-tive loss of H+ (sometimes another cation) from the same C to which the electrophile added to re-form the aromatic system and give an overall substitution reaction. Thus, the predominant mechanism of substitution at aromatic rings under acidic conditions is electrophilic addition-elimination, sometimes referred to as SpAr. The reaction of toluene and nitric acid is indicative. 

In addition, there is another type of reaction which does not change the aromatic character involving either a substitution at the ring or a substitution at the side group. As in the case of acyclic addition an electron transfer mechanism operates. 

Electrophilic substitution of aromatic compounds proceeds via a two-step sequence - addition (of then elimination (of H ), of which the former is usually the slower (rate-determining) step. Qualitative predictions of relative rates of substitution at different ring positions can be made by inspecting the strac-tures of the o-complexes (Wheland intermediates) formed in the first step, on the assumption that their relative stabilities reflect the relative energies of the transition states that lead to them. 

Vilan et al. demonstrated that molecules adsorbed at the Au/ -GaAs interface can tune the energetics of this metal-semiconductor diode, because of their chemical character, namely, by systematic substitution on aromatic rings [77]. Using a series of dicarboxylic acid derivatives whose dipole was varied systematically, they produced diodes in which the effective... 

Hauser found that benzyltrimethylammonium ion was rearranged by sodamide in liquid ammonia to give dimethyl-2-methylbenzylamine in 97% yield . Quatemization of this base followed by a second reaction with sodamide resulted in the substitution of a second methyl on the benzene ring, and this procedure could be continued until the ring was fully methylated (equation 28). Hauser proposed that the mechanism of the reaction was initial proton abstraction by base at a benzylic site, and equilibration to a methyl anion, followed by anionic substitution at the ring ortho position . The resulting eATo-methylene derivative would aromatize under the reaction conditions (equation 29), Support for this mechanism came when Hauser isolated the intermediate 8 from the reaction of trimethyl-(2,4,6-trimethylbenzyl)ammonium ion with sodamide in liquid ammonia (equation 30) . The anunonium ion contains an ortho... 

I have not examined a sufficient number of spectra to give some general rules as to how NMR spectra can be used to determine the substitution on aromatic rings, but in many instances it is possible to recognize at which position in the ring each substituent is located. Certainly, the combination of NMR and infrared spectroscopy is ideal for studying aromatic ring substitutions. 

Adaptations of this method have been proposed in order to take into account the band displacement due either to substitution on the aromatic ring, or to chains of different lengths. The variations consist, instead of measuring the absorbance at maximum absorption, of an integration of the absorbance curve over a specified range (Oelert s method, 1971). More exact, this method is used less often mainly because the Brandes method is simpler... 

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