Base (chemistry) | EPFL Graph Search

In chemistry, there are three definitions in common use of the word "base": Arrhenius bases, Brønsted bases, and Lewis bases. All definitions agree that bases are substances that react with acids, as originally proposed by G.-F. Rouelle in the mid-18th century. In 1884, Svante Arrhenius proposed that a base is a substance which dissociates in aqueous solution to form hydroxide ions OH−. These ions can react with hydrogen ions (H+ according to Arrhenius) from the dissociation of acids to form water in an acid–base reaction. A base was therefore a metal hydroxide such as NaOH or Ca(OH)2. Such aqueous hydroxide solutions were also described by certain characteristic properties. They are slippery to the touch, can taste bitter and change the color of pH indicators (e.g., turn red litmus paper blue). In water, by altering the autoionization equilibrium, bases yield solutions in which the hydrogen ion activity is lower than it is in pure water, i.e., the water has a pH higher than 7.0 a

Characterization of surface functional groups present on laboratory-generated and ambient aerosol particles by means of heterogeneous titration reactions

Michel Guillemin, Michel Rossi

A Knudsen flow reactor has been used to quantify surface functional groups on aerosols collected in the field. This technique is based on a heterogeneous titration reaction between a probe gas and a specific functional group on the particle surface. In the first part of this work, the reactivity of different probe gases on laboratory-gene rated aerosols (limonene SOA, Pb(NO3)(2), Cd(NO3)(2)) and diesel reference soot (SRM 2975) has been studied. Five probe gases have been selected for the quantitative determination of important functional groups: N(CH3)(3) (for the titration of acidic sites), NH2OH (for carbonyl functions), CF3COOH and HCl (for basic sites of different strength), and O-3 (for oxidizable groups). The second part describes a field campaign that has been undertaken in several bus depots in Switzerland, where ambient fine and ultrafine particles were collected on suitable filters and quantitatively investigated using the Knudsen flow reactor. Results point to important differences in the surface reactivity of ambient particles, depending on the sampling site and season. The particle surface appears to be multi-functional, with the simultaneous presence of antagonistic functional groups which do not undergo internal chemical reactions, such as acid-base neutralization. Results also indicate that the surface of ambient particles was characterized by a high density of carbonyl functions (reactivity towards NH2OH probe in the range 0.26-6 formal molecular monolayers) and a low density of acidic sites (reactivity towards N(CH3)(3) probe in the range 0.01-0.20 formal molecular monolayer). Kinetic parameters point to fast redox reactions (uptake coefficient y(0) > 10(-3) for O-3 probe) and slow acid-base reactions (gamma(0) < 10(-4) for N(CH3)(3) probe) on the particle surface. (C) 2009 Elsevier Ltd. All rights reserved.

2009

How Robust Is the Reversible Steric Shielding Strategy for Photoswitchable Organocatalysts?

Raimon Fabregat I De Aguilar-Amat, Simone Gallarati, Théo Pierre Jaffrelot Inizan, Veronika Juraskova

A highly appealing strategy to modulate a catalyst's activity and/or selectivity in a dynamic and noninvasive way is to incorporate a photoresponsive unit into a catalytically competent molecule. However, the description of the photoinduced conformational or structural changes that alter the catalyst's intrinsic reactivity is often reduced to a handful of intuitive static representations, which can struggle to capture the complexity of flexible organocatalysts. Here, we show how a comprehensive exploration of the free energy landscape of N-alkylated azobenzene-tethered piperidine catalysts is essential to unravel the conformational characteristics of each configurational state and explain the experimentally observed reactivity trends. Mapping the catalysts' conformational space highlights the existence of false ON or OFF states that lower their switching ability. Our findings expose the challenges associated with the realization of a reversible steric shielding for the photocontrol of Bronsted basicity of piperidine photoswitchable organocatalysts.

AMER CHEMICAL SOC2022

Organocatalysts in C-N Bond Forming Reactions of Amines with CO2

Martin Hulla

The thesis describes a simple approach for N-formylation of amines with CO2 and hydrosilane reducing agents, the use of organic salts as N-formylation catalysts, their physical properties required for high catalytic activity and their role in the catalytic cycle. The investigation is then extended to the synthesis of quinazoline-2,4(1H,3H)-dione from 2-aminobenzonitrile and CO2 as well as other C-N bond forming reactions. Basicity, as measured by pKb, is identified as the key property required for high catalytic activity of organic salt catalysts. Minor variations in catalytic activity among salts of equal basicity are assigned to the ion pairing energy, hydrogen bonding ability and steric factors. The N-formylation reaction catalysed by organic salts proceeds by the activation of the hydrosilane reducing agent. Although, the catalytic activity was originally assigned to the nucleophilicity of the anion, a detailed investigation of the reaction mechanism and physical parameters of numerous salt catalysts revealed that they are more active as bases. Extensive substituent exchange around the hydrosilane silicon centre confirms its activation and supports the proposed reaction mechanism, where in-situ formed carbamate salt acts as the reaction nucleophile. Accounting for small detrimental effect of ion paring, a linear relationship between the pKa of the salt and catalytic activity is observed. The onset of the base catalysed reaction mechanism is substrate dependent with further variations for amines of high basicity. Nevertheless, highly basic salt catalysts, such as tetra-n-butylammonium fluoride ([TBA]F), promote rapid N-formylation of all types of amines under extremely mild conditions. A linear relationship between the basicity of salt catalysts and their catalytic activity was also observed in the synthesis of quinazoline-2,4(1H,3H)-dione. Similarly, the reaction proceeds by in-situ formation of a carbamate salt, which requires a base catalyst. However, here the reaction mechanism is limited by the acidity of the quinazoline-2,4(1H,3H)-dione product. Quinazoline-2,4(1H,3H)-dione is deprotonated by more basic catalysts (pKa of conjugate acid > 14.7 in DMSO) leading to the neutralization of the base catalyst and the formation of the quinazolide anion, which then acts as the reaction catalyst. Analysis of the reported literature reveals that the findings are directly applicable to the vast majority of C-N bond forming reactions of amines with CO2 and to almost all types of organocatalysts. The reactions proceed by carbamate salt formation in the form [BaseH][RR'NCOO]. The anion of the carbamate salt then acts as a nucleophile in hydrosilane reductions of CO2 towards formamides, N-methylamines and aminals or internal cyclization reactions to quinazoline-2,4-diones, oxazolidinones and oxazin-2-ons or after dehydration as an electrophile in the synthesis of urea derivatives. The role of organocatalysts in the reactions indicates that all bases of sufficient strength should be able to catalyse the reactions.