π-Hydrogen Bonding Probes the Reactivity of Aromatic Compounds: Nitration of Substituted Benzenes

Abstract

The shifts of phenol O–H stretching vibration frequencies [Δν­(OH)exp] upon π-hydrogen bonding with aromatic compounds is proposed as a spectroscopic probe of the reactivity of aromatic substrates toward electrophiles. A single infrared spectrum reflecting the Δν­(OH)exp shift for an aromatic species in a reference solvent (CCl4 in this study) provides a good estimate of reactivity. The methodology is applied in rationalizing reactivity trends for the BF3 catalyzed nitration by methylnitrate in nitromethane of 20 aromatic reactants, including benzene, 11 methylbenzenes, several monoalkyl benzenes, the four halobenzenes, and anisole. Literature kinetic data are employed in the analysis. Very good correlations between relative rates of nitration and Δν­(OH)exp are obtained. The approach is best applied to reactions, where the initial interactions between the reactants controls the rates. A new theoretical quantity, the shifts (with respect to benzene) of the molecular electrostatic potential at 1.5 Å over the centroid of the aromatic ring [ΔV(1.5)] is defined and shown to provide a good description of substituent effects on properties of the aromatic species. B3LYP density functional and MP2 ab initio methods combined with the 6-311++G­(3df,2pd) basis set are employed in evaluating the ΔV(1.5) values.

Mini-Review on Structure–Reactivity Relationship for Aromatic Molecules: Recent Advances

“…Considering the nature of this initial step, we investigated the correlations between phenol Δν­(OH) exp shifts induced by π-hydrogen bonding with aromatic substrates. The experimentally recorded phenol Δν­(OH) exp shifts are juxtaposed to the relative rate constants (log k rel ) for the nitration of a series of aromatic derivatives (methylbenzenes, monoalkylbenzenes, halobenzenes, and anisole) by methyl nitrate in the presence of BF 3 in nitromethane solvent (literature kinetic data, see footnotes to Table in ref ). The reaction is illustrated in Scheme .…”

“… a Relative rate with respect to benzene ( k benzene = 1). See ref and references therein for the sources of experimental kinetic data and Δν­(OH) exp shifts. b Theoretical computations for modeled nitromethane solvent. c The correlation coefficients (absolute values are shown) refer to the linear regressions between the experimental log k rel values and the predicted values according the 2nd order polynomial expressions. …”

π‐Hydrogen Bonding Probes Chemical Reactivity: Bromination of a CC Double Bond and Electrophilic Aromatic Benzylation

“…In a previous research we have shown that the molecular electrostatic potential values, evaluated at some distance over the aromatic ring centroid, may provide a good description of the effect of aromatic substituents on the π‐hydrogen bonding and nucleophilic reactivity of the aromatic derivatives considered . We applied here a similar approach in the search for appropriate theoretical parameters to describe the nucleophilic reactivity of the alkenes.…”

“…Pursuant to computations of atomic charges, we also evaluated another theoretical quantity, recently introduced for studies of aromatic systems, the shifts of molecular electrostatic potential at a point in the molecular space in close proximity to the reaction center atoms . For the benzene derivatives studied we used the MESP shifts (relative to benzene) at 1.5 Å over the centroid of the aromatic ring [Δ V (1.5)].…”

“…Beginning with the pioneering work of Fukui, the reactivity of arenes is usually interpreted with the aid of theoretically estimated charge densities at the ring atoms . In recent research we proposed an efficient spectroscopic approach of quantifying reactivity in EAS reactions . The methodology is based on the expected dependence between shifts of O–H stretching frequencies [Δ ν (OH) exp ] of a selected proton donor molecule (phenol) upon π‐hydrogen bonding with the variations of the electron densities over the aromatic ring.…”