Nitrations with nitronium ions special cases

3 NITRATIONS WITH NITRONIUM IONS SPECIAL CASES 

As outlined above, the process of substitution by the nitronium ion is satisfactorily described by an mechanism in which k. In 

Another circumstance which could change the most commonly observed characteristics of the two-stage process of substitution has already been mentioned it is that in which the step in which the proton is lost is retarded because of a low concentration of base. Such an effect has not been observed in aromatic nitration ( 6.2.2), but it is interesting to note that it occurs in A -nitration. The A -nitration of A -methyl-2,4,6-trinitroaniline does not show a deuterium isotope effect in dilute sulphuric acid but does so in more concentrated solutions ( 60 % sulphuric acid kjj/kjj = 4 8).  

The cases of pentamethylbenzene and anthracene reacting with nitronium tetrafluoroborate in sulpholan were mentioned above. Each compound forms a stable intermediate very rapidly, and the intermediate then decomposes slowly. It seems that here we have cases where the first stage of the two-step process is very rapid (reaction may even be occurring upon encounter), but the second stages are slow either because of steric factors or because of the feeble basicity of the solvent. The course of the subsequent slow decomposition of the intermediate from pentamethylbenzene is not yet fully understood, but it gives only a poor yield of pentamethylnitrobenzene. The intermediate from anthracene decomposes at a measurable speed to 9-nitroanthracene and the observations are compatible with a two-step mechanism in which k i k E and i[N02 ] / i. There is a kinetic isotope effect (table 6.1), its value for the reaction in acetonitrile being near to the 

It is probable that the nitration of anthracene with nitric acid in 7-5 % aqueous sulpholan proceeds through the rapid formation of a complex.  

As outlined above, the process of substitution by the nitronium ion is satisfactorily described by an SE2 mechanism in which k2 E k v In certain circumstances the process could be changed so that this condition did not hold, and the step in which the proton is lost could become kinetically important. One such circumstance is that in which the hydrogen atom being replaced is situated between bulky substituents steric hindrance would then make it difficult for the nitro group to move from its position in the intermediate complex to that between the bulky substituents k2 would be diminished, and a kinetic isotope effect might appear. It is for this reason that 1,3,5-tri-f-butylbenzene and its derivatives are interesting (table 6.1) whilst the hydrocarbon undergoes 

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Our special concern is with those nitrating systems for which mechanistic studies have established, or made probable, the identity of the electrophile through which they effect nitration. In most cases, though not quite all, this has proved to be the nitronium ion. The important nitrating systems formed by mixing nitric acid with acetic anhydride stand out in having so far resisted attempts to identify beyond reasonable doubt the electrophile or electrophiles through which they operate ( 5-3)-... 

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