Electrophilic and free radical nitration of benzene and toluene with various nitrating agents

Abstract

Electrophilic nitration of toluene and benzene was studied under various conditions with several nitrating systems. It was found that high ortlopara regioselectivity is prevalent in all reactions and is independent of the reactivity of the nitrating agent. The methyl group of toluene is predominantly ortho-para directing under all reaction conditions. Steric factors are considered to be important but not the sole reason for the variation in the ortho/para ratio. The results reinforce our earlier views that, in electrophilic aromatic nitrations with reactive nitrating agents, substrate and positional selectivities are determined in two separate steps. The first step involves a ir-aromatic-NO2 ion complex or encounter pair, whereas the subsequent step is of arenium ion nature (separate for the oftho, meta, and para positions). The former determines substrate selectivity, whereas the latter determines regioselectivity.Thermal free radical nitration of benzene and toluene with tetranitromethane in sharp contrast gave nearly statistical product distributions. Stable nitronium salts were introduced as new nitrating agents by Olah and coworkers (1) in 1956. In the course of these studies (2-5), the competitive nitration of benzene and toluene, as well as other aromatics, was carried out in organic solvents.Under usual conditions of electrophilic nitration, toluene reacts about 20 times more rapidly than benzene whereas, with nitronium salts, toluene was found to react only 1.7 times faster than benzene (2). The practical disappearance of intermolecular (substrate) selectivity was accompanied by no significant alteration of isomer distribution (regioselectivity). This observation led to the suggestion that the transition state of highest energy (which determines substrate selectivity) is of starting aromatic (i.e., 7r-complex) nature, which is then followed by separate a-complex formation (for the individual positions), determining positional selectivity.In a series of studies, we have found that, in electrophilic aromatic substitutions, the position of the transition state of NO2Y + R-Xhighest energy is not rigidly fixed (6) but can shift from "early" (r-complex-like) to "late" (u-complex-like) nature, depending upon the reactivity of the electrophiles and the basicity of the aromatic substrates.In order to further explore electrophilic nitration, we carried out a comprehensive study of nitration of benzene and toluene under various conditions. RESULTS AND DISCUSSION With Nitronium Salts. Although we had previously examined competitive nitration using high-speed mixing (7), it was considered of interest to extend the studies by using more advanced methods such as the mixing chamber of an efficient Durrum-Gibson stopped-flow apparatus. Competitive nitrations, with nitronium hexafluorophosphate in nitromethane, provided the data in Table 1. Whereas mixing still can be incomplete before reaction, with the nitration rates being very fast (or reaching the encounter-controlled limit), the data seem to indicate that, ...

Nonaromatic diazonium ions

“…In contrast, dediazoniation of 02N-N ~ ~ to nitrogen and NO~ ~ is thermodynamically favorable and it is possible that dediazoniation to form NO~ precedes nitration of toluene. In fact, the observed isomer distribution (71% ortho, 3.5% meta, 25.5% para) closely resembles that of electrophilic nitration of toluene with strongly electrophilic nitrating agents such as nitronium salts [45].…”

An artificial self‐sufficient cytochrome P450 directly nitrates fluorinated tryptophan analogs with a different regio‐selectivity

“…Aromatic nitration is one of most widely studied and used organic reactions . Industrial scale reactions us ually include a mixture of nitric acid and sulfuric acid, and the nitronium ion, NO 2 + , is believed to be the active species, albeit with the potential minor contribution of a radical mechanism . The practicality and economics of this nitration method have made it predominant in producing nitroaromatics.…”

Electronic effect of ionic‐pair substituents

“…Conventional organic substituents are extensively investigated due to their importance in tuning the chemical, physicochemical, and biochemical properties of chemical species . Various substituent groups on catalysts ligands result in different catalytic effects, while multiple substituent groups on substrates can also change their reactivities . All these can be understood in terms of the electronic effect of organic substituents.…”