Bronsted acids, reaction with

A wide variety of solutes behave as weak acids that is, they react reversibly with water to form H30+ ions. Using HB to represent a weak acid, its Bronsted-Lowry reaction with water is... 

Enthalpies of formation of the magnesium bromide salts of the Bronsted acids are calculated from the measured enthalpies of the reaction of the Bronsted acid, HB, with pentylmagnesium bromide and the known enthalpies of formation of pentyhnagnesium bromide and pentane according to equation 11. Because some of the HB enthalpies of formation have been revised and others newly measured since the original publication, the BMgBr enthalpies of formation have been recalculated and appear in Table 4. ... 

These dianions on stoichiometric protonation, e.g. with H20 in THF, give hydrido anions which, in turn, with an excess of a Bronsted acid, or with an equivalent of benzoic acid, are transformed into labile dihydrogen complexes finally loosing dihydrogen according reaction sequence (19a, b, c) (M = Ru, Os P = OEP, TMP, L = THF] [261, 262],... 

Although Bronsted proton transfer reactions appear to belong to a unique category not described by Scheme 14, they are examples of polar-group transfer reactions and are not different in principle from nucleophilic displacement reactions. Deprotonation by hydroxide ion can be regarded as the shift of an electron from HO to the Bronsted acid synchronously with the transfer of a hydrogen atom from the Bronsted acid to the incipient HO- radical, with the reaction driven by covalent bond formation between the HO- radical and the H- atom to form water (equation 161). 

What are the molecular pathways for metal or ligand adsorption (4) How can we infer molecular information from dissolution experiments (5) Do the macroscopic properties of a surface in water, such as Bronsted acidity, change with size of the molecule (6) How do the microscopic properties of a molecule, such as the rates of bridge dissociation, relate to calculable bond properties, such as electronic charge densities (7) Over what time scales are different types of bond dissociations complete (8) Can geochemists predict rates of multi-step reactions ... 

Despite the transformation of a certain number of amino groups into urea groups, the qualitative shape of the zeta-potential plots as a function of pH is not changed significantly. That indicates that PVFA-co-PVAm can be used as a precursor polymer for the gradual functionalization of silica particles and other Bronsted-acidic surfaces with amino functionality. Chemical reactions of PVFA-co-PVAm/silica particles with isocyanates or other electrophilic reagents offer a wide field of applications, because of the simple experimental procedure this material combination seems promising for the construction of tailor-made polyelectrolyte networks on Bronsted-acidic surfaces or for other multicomponent systems. 

Meanwhile, Leitner and coworkers monitored the aza MBH reaction of methyl vinyl ketone (MVK) with N tosylated imine in the presence of P Ph 3 in TH F dg at room temperature by NMR spectroscopy [10]. The rate law was derived by analyzing the initial rates as a function of concentration for the individual components. The broken order of 0.5 in imine indicated that the rate determining step was partially influenced by proton transfer. A variety of Bronsted acidic additives with different p/<., values were examined with 3,5 bis(CF3)phenol at pKa 8 corresponding to a 14 fold rate enhancement as compared to the reaction without additive. Examination of the kinetics in the presence of phenol as prototypical additive revealed that the rate law of the reaction changed in the presence of the Bronsted acid, showing first order dependence on imine. Thus, the elimination step was not involved in the... 

A great number of theoretical works have been published on Diels-AIder reactions. Some of them deal with the interpretation of the increased reaction rate produced by Lewis acid catalysts (16-18). For the reaction reported in this paper, molecular orbital calculations were computed using the MNDO method (19) implemented in MOP AC (20) and performing full geometry optimization. The modelling of the catalytic activity was achieved by complexation of one reactant with either a proton (Bronsted acid) or with AICI3 (Lewis acid). The calculated reaction enthalpies showed that, for a Lewis acid catalyst, acrolein (-25.2 kcal/mol) is more complexed than dihydropyran (-8.2 kcal/mol). The same conclusion is obtained by comparison of protonation enthalpies of acrolein (-196.4 kcal/mol) and dihydropyran (-177.5 kcal/mol). 

Protonation with Bronsted acids proceeds with retention (Table 9). Alkylation, silylation, stannylation, carbon dioxide, and carbon disulfide addition (entries 3-10,12,25) are accompanied by inversion. This is also true for the reaction with acid chlorides (entry 13), but esters react with clean retention (entries 14-16). Aliphatic aldehydes and ketones add to the Hthium compound 216a with complete retention of configuration. A notable exception was found for benzaldehyde it is added with inversion (entry 23). 

Acidic aluminosilicate-based catalysts are of major industrial importance. In terms of product quantity, the most important catalytic process is the cracking of crude oil. The reaction is initiated by the reaction of a Bronsted acidic surface with alkenes in which addition of a proton to the double bond gives chemisorbed carbe-nium ions (Eq. 5-70). 

Substituted aromatics are essential chemical feedstocks. Among the xylenes, for example, p-xylene is in great demand as a precursor to terephthalic acid, a polyester building block. The pura-isomer is therefore more valuable than the o- and m-xylenes, so there is a powerful incentive for conversion of o- and m-xylene to p-xylene. Isomerisation over solid acids occurs readily as a result of alkyl shift reactions of the carbenium-ion-like transition state. The initial protonation occurs by interaction of the Bronsted acid site with the aromatic 71 system, by an electrophilic addition. Over non-microporous solid acids, at high conversion, xylenes are produced at their thermodynamically determined ratios, which favour the meta rather than the ortho or para isomers. In addition, unwanted transalkylation reactions occur, giving rise, for example, to toluene and trimethylbenzenes. Zeolite catalysts can be much more selective. 

In the same year, Zhou and List reported a novel one-pot tandem reaction which, for the first time, combined chiral Bronsted acid catalysis with enamine and iminium catalysis. Later, on the basis of control experiments and ESI-MS/MS analysis, a reasonable mechanism was proposed (Scheme 2.2). The initial step of this tandem reaction was mediated by achiral /j-ethoxyaniline (PEP-NH2) and chiral phosphoric acid (R)-TRIP either reagent alone was inefficient in promoting this aldol condensation to afford the first iminium intermediate. The following step was a conjugate reduction which was also Bronsted acid and amine co-catalysed, and no further conversion took place in the absence of either catalyst. The final step was an acid-catalysed reductive amination. This novel sequence allowed the highly enantioselective synthesis of pharmaceutically active chiral ds-3-substituted cyclohejyl or heterocyclohexyl amines in high diastereo- and... 

As detailed in Sections 42.2 and 42.3, both covalent and non-covalent organocatalytic activation modes have provided efficient strategies to design asymmetric MCRs. A quite recent study by Zhou and list has demonstrated the feasibility of combining asymmetric Bronsted acid catalysis with aminocatalysis to design even more sophisticated reaction sequences toward the synthesis of useful complex molecules. Specifically, they developed a highly enantioselective approach to pharmaceutically relevant 3-substituted cyclohexylamines from 2,6-diketones 223 via an aldolization/dehydration/conjugate reduction/reductive amination domino... 

Apart from Bronsted acid activation, the acetyl cation (and other acyl ions) can also be activated by Lewis acids. Although the 1 1 CH3COX-AIX3 Friedel-Crafts complex is inactive for the isomerization of alkanes, a system with two (or more) equivalents of AIX3 was fonnd by Volpin to be extremely reactive, also bringing abont other electrophilic reactions. 

Possible role of the induced acidity and basicity in catalysis and environmental chemistry is discussed. The suggested mechanism explains the earlier reported promotive effect of some gases in the reactions catalyzed by Bronsted acid sites. Interaction between the weakly adsorbed air pollutants could lead to the enhancement of their uptake by aerosol particles as compared with separate adsoi ption, thus favoring air purification. 

Lateral interactions between the adsorbed molecules can affect dramatically the strength of surface sites. Coadsorption of weak acids with basic test molecules reveal the effect of induced Bronsted acidity, when in the presence of SO, or NO, protonation of such bases as NH, pyridine or 2,6-dimethylpyridine occurs on silanol groups that never manifest any Bronsted acidity. This suggests explanation of promotive action of gaseous acids in the reactions catalyzed by Bronsted sites. Just the same, presence of adsorbed bases leads to the increase of surface basicity, which can be detected by adsorption of CHF. ... 

Reaction of 1 mole of aminals 352 with 4 mol of methyl 3-aminocrotonate in the presence of the solid acids montmorillonte clay (Kio) and ZF520 zeolite as strong Bronsted acidic catalysts, gave 1,4-dihydropyridines 353 and 2-methyl-4//-pyrido[l, 2-n]pyrimidin-4-one (99MI8). 

This section deals with Bronsted acid and Lewis acid catalyzed reactions, excluding Friedel-Crafts reactions, but including reactions such as nitrations, halogenations, and Claisen rearrangements. Friedel-Crafts reactions are discussed in the subsequent Sections 5.1.2.2 and 5.1.2.3. 

As pointed out in Chapter 13, strong acids ionize completely in water to form H30+ ions strong bases dissolve in water to form OH- ions. The neutralization reaction that takes place when any strong acid reacts with any strong base can be represented by a net ionic equation of the Bronsted- Lowry type ... 

Bronsted acid/base catalysis (equation, a, p) 113, 210, 355ff.,360ff., 392 Buckminsterfullerene, reaction with ArN 188... 

Thomas and Long488 also measured the rate coefficients for detritiation of [l-3H]-cycl[3,2,2]azine in acetic acid and in water and since the rates relative to detritiation of azulene were similar in each case, a Bronsted correlation must similarly hold. The activation energy for the reaction with hydronium ion (dilute aqueous hydrochloric acid, = 0.1) was determined as 16.5 with AS = —11.3 (from second-order rate coefficients (102At2) of 0.66, 1.81, 4.80, and 11.8 at 5.02, 14.98, 24.97, and 34.76 °C, respectively). This is very close to the values of 16.0 and —10.1 obtained for detritiation of azulene under the same condition499 (below) and suggests the same reaction mechanism, general acid catalysis, for each. 

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