Équation de Hammett

Résumé

L'équation de Hammett, en chimie organique, permet de relier les rendements ou les constantes d'équilibre d'un grand nombre de réactions impliquant des dérivés de l'acide benzoïque avec la nature des meta- ou para-substituants. Cette équation décrit la variation linéaire d'enthalpie libre comme une fonction de seulement deux paramètres, l'un caractéristique de la réaction et l'autre du substituant. Cette équation a été développée et publiée par Louis Plack Hammett en 1937 à la suite de ses observations qualitatives publiées en 1935. L'idée est que pour deux réactions sur deux composés aromatiques différents uniquement par le type de substituant, la variation d'énergie d'activation est proportionnelle à celle d'enthalpie libre. Cette notion ne découle pas des concepts élémentaires de thermochimie ou de cinétique mais a été introduite intuitivement par Hammett. Équation de Hammett L'équation de base est : \log \frac{K}{K_0} = \sigma\rho où

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Selective three-phase hydrogenation of aromatic nitro-compounds over beta-molybdenum nitride

Fernando Cardenas Lizana, Lioubov Kiwi, Daniel André Samuel Lamey

A tetragonal molybdenum nitride (beta-Mo2N) has been prepared by temperature programmed treatment of MoO3 in flowing N-2 + H-2 and for the first time shown to catalyze the liquid phase selective hydrogenation (T = 423 K; P-H2 = 11 bar) of a series of para-substituted (-H, -OH, -O-CH3, -CH3, -Cl, -I and -NO2) nitrobenzenes to give the corresponding aromatic amine. Reaction over Pd/Al2O3, as a benchmark catalyst (Pd particle size ca. 18 nm), resulted in a composite hydrodechlorination/hydrogenation of p-chloronitrobenzene (as a representative nitroarene) to generate nitrobenzene and aniline. beta-Mo2N has been characterized in terms of temperature-programmed reduction (TPR), H-2 chemisorption/temperature programmed desorption (TPD), BET surface area/pore volume, elemental analysis, powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning (SEM) and transmission (TEM) electronic microscopy. Elemental analysis, XRD, SEM and TEM have confirmed the formation of tetragonal beta-Mo2N, characterized by an agglomeration of flake-like crystallites. Post-synthesis, the nitride was passivated by contact with 1% (v/v) O-2/He at ambient temperature and XPS analysis has demonstrated the formation of a superficial passivating oxide overlayer without bulk oxidation. Pre-reaction, activation by TPR to 673 K was necessary to remove the passivating film. Hydrogen TPD has revealed significant hydrogen uptake (0.7 mu mol m(-2)) associated with beta-Mo2N. Nitro group reduction kinetics have been subjected to a Hammett treatment where the reaction constant (p = 0 4) is diagnostic of an increase in rate due to the presence of electron-withdrawing substituents on the aromatic ring, consistent with a nucleophilic mechanism. The results presented in this study establish the viability of beta-Mo2N to promote selective nitroarene hydrogenation. (C) 2011 Elsevier B.V. All rights reserved.

2011

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Selective three-phase hydrogenation of aromatic nitro-compounds over β-molybdenum nitride

Fernando Cardenas Lizana, Lioubov Kiwi, Daniel André Samuel Lamey

A tetragonal molybdenum nitride (β-Mo2N) has been prepared by temperature programmed treatment of MoO3 in flowing N2 + H2 and for the first time shown to catalyze the liquid phase selective hydrogenation (T = 423 K; PH2=11bar) of a series of para-substituted (-H, -OH, -O-CH3, -CH3, -Cl, -I and -NO2) nitrobenzenes to give the corresponding aromatic amine. Reaction over Pd/Al2O3, as a benchmark catalyst (Pd particle size ca. 18 nm), resulted in a composite hydrodechlorination/ hydrogenation of p-chloronitrobenzene (as a representative nitroarene) to generate nitrobenzene and aniline. β-Mo2N has been characterized in terms of temperature-programmed reduction (TPR), H2 chemisorption/temperature programmed desorption (TPD), BET surface area/pore volume, elemental analysis, powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning (SEM) and transmission (TEM) electronic microscopy. Elemental analysis, XRD, SEM and TEM have confirmed the formation of tetragonal β-Mo2N, characterized by an agglomeration of flake-like crystallites. Post-synthesis, the nitride was passivated by contact with 1% (v/v) O2/He at ambient temperature and XPS analysis has demonstrated the formation of a superficial passivating oxide overlayer without bulk oxidation. Pre-reaction, activation by TPR to 673 K was necessary to remove the passivating film. Hydrogen TPD has revealed significant hydrogen uptake (0.7 μmol m-2) associated with β-Mo2N. Nitro group reduction kinetics have been subjected to a Hammett treatment where the reaction constant (p = 0 4) is diagnostic of an increase in rate due to the presence of electron-withdrawing substituents on the aromatic ring, consistent with a nucleophilic mechanism. The results presented in this study establish the viability of β-Mo2N to promote selective nitroarene hydrogenation. © 2011 Elsevier B.V. All rights reserved.

Elsevier2011

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Mechanistic investigations into homogeneously catalyzed reactions are essential for the fur-ther development of synthetic methodologies. Insights in the overall reaction pathway and vital steps such as the rate-determining and selectivity-determining steps allow for rational design of improved catalysts. Moreover, mechanistic investigations may reveal the existence of unexpected intermediates in the catalytic cycle. The discovery of such intermediates may inspire new methodologies that would not have been considered if their formation was un-known. Because of economic advantages over heavier congeners, there has been a recent surge in the use of nickel -based homogeneous catalysts. Although much progress has been made into methodology development, the mechanistic understanding of the new reactivities is relatively primitive. In-depth mechanistic investigations into nickel catalyzed reactions are therefore of considerable interest. Chapters 2-4 deal with the mechanism of reductive coupling of nitroarenes and esters to form amides. In chapter 2, the initial reduction pathway that occurs before the reaction with ester is investigated, revealing the crucial intermediate to be azobenzene and the source of the proton in the final amide product. In chapter 3, kinetics, Hammett plots, competition experi-ments and stoichiometric reactions with a potential intermediate are described. These studies reveal azobenzene is reductively cleaved by low-valent nickel before the ester gets involved in the reaction. Moreover, ZnCl2, a byproduct of the reduction of the nickel precursor by zinc, is revealed to play an important role in several steps of the reaction. In chapter 4, the computa-tional investigation based on the experimental data is discussed. By combining the computa-tional exploration of the potential energy surface with qualitative kinetic analysis, a catalytic cycle for the reaction between azobenzene and ester could be proposed. The finding that the reaction between azobenzene and ester goes through nickel imide inter-mediates is intriguing as nickel imides are known to react with a variety of substrates and azo-benzene would be a relatively benign precursor to them. Chapter 5 reports a preliminary explo-ration of several types of reactivities can be expected to work on the basis that the reactivity with imides is already known. Several types of these reactivities seem to be inaccessible as they are incompatible with the highly reductive nature of the nickel catalyst that is required for the azobenzene activation. However, some other reactivities show promise. Chapter 6 reports an investigation in the alkene hydrosilylation catalyzed by a nickel pincer complex developed in our group. Previous investigations had indicated a nickel alkyl complex was the resting state. However, a combination of reaction progress analysis, transmission elec-tron microscopy and competition reactions reveal that this is merely a precatalyst. Under cata-lytic conditions, it slightly decomposes under to form the true catalyst, which is tentatively identified as nickel nanoparticles.

EPFL2020

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