Nitration with nitronium salts in organic

It can be concluded, as already stated above, that the diminution in intermolecular selectivity observed in these nitrations with nitronium salts in organic solvents does not of itself require any special mechanistic considerations as regards the process of substitution. 

The nitronium ion is the electrophile in nitrations with nitronium salts in organic solvents. 

The work of Olah et al. on nitration with nitronium salts in organic solvents has already been discussed in some of its aspects ( 4.4). It will... 

NITRATIONS WITH SOLUTIONS OF NITRONIUM SALTS IN ORGANIC SOLVENTS... 

Even so, the results were claimed to show a greater resemblance to nitrations with nitronium salts than to nitrations in organic solvents. However, reaetion at the eneounter rate ( 3.3) imposes a limit to the rate of reaetion in these media, whieh deereases from 40 times the rate for benzene in 68 % sulphuric acid to 6 times the rate in 80 % sulphurie acid. Therefore it is reasonable to expeet that in stronger solutions even under homogeneous eonditions, the rates of these eompounds would approximate to that of benzene. 

Olah and his co-workers compared the behaviour of nitronium salts in competitive nitrations with the behaviour of other nitrating systems. The results are given in table 4.1, columns (a)-(j), and also in table 4.2. The results obtained from competitive nitrations using solutions of nitric acid in organic solvents (table 4.1, columns (6)-(e)) are in line with those obtained by earlier workers. The evidence that in nitromethane,... 

Nitronium salts are efficient and powerful nitrating agents. Nitronium tetrafluoroborate is the most commonly used nitronium salt for nitration and is commercially available as a solid or as a solution in sulfolane in which the ions are highly solvated and exist as an ion pair. Nitronium tetrafluoroborate shows poor solubility in most organic solvents and so the more soluble nitronium hexafluorophosphate is sometimes preferred for nitrations. Olah and co-workers - have studied the nitration of a vast array of aromatics with nitronium salts. Solutions of nitronium salts in aprotic organic solvents are useful for the nitration of acid sensitive or readily oxidized substrates. Nitronium tetrafluoroborate has been used for the... 

Kinetic studies on the nitration of nitrobenzene by nitronium borofhioride in the polar solvents sulphuric acid, methane-sulphuric acid, and acetonitrile show the reaction to be first-order in both nitronium salt and aromatic110. With the first two solvents, the rate coefficients are similar for nitration by nitric acid and by the nitronium salts, indicating a common nitrating entity. With acetonitrile the rate coefficients are very much lower, consistent with a much lower concentration of free nitronium ions in this medium and thus with the nitronium salts existing as ion pairs in organic solvents (see Table 25). 

Stable nitronium salts, which are readily prepared from nitric acid (or nitrates) with HF and BF3 (and other Lewis acids such as PF5, SbF5, etc.) [Eqs. (5.171) and (5.172)], will nitrate aromatics in organic solvents generally with close to quantitative yield. Because HF and PF5 (or BF3) can be easily recovered and recycled, the method can be considered as a nitric acid nitration using a superacid catalyst [Eq. (5.173)]. 

Nitronium salts are thermodynamically stable, but very reactive chemically. They are rapidly hydrolyzed by moisture in addition, NO CIO , for example, reacts violently with organic matter, but it can actually be used to carry out nitrations in nitrobenzene solution. 

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