Brpnsted acids, proton transfer from

The first step of this new mechanism is exactly the same as that seen earlier for the reaction of tert butyl alcohol with hydrogen chloride—formation of an alkyloxonmm ion by proton transfer from the hydrogen halide to the alcohol Like the earlier exam pie this IS a rapid reversible Brpnsted acid-base reaction... 

The mechanism of enolization involves two separate proton transfer steps rather than a one step process m which a proton jumps from carbon to oxygen It is relatively slow m neutral media The rate of enolization is catalyzed by acids as shown by the mechanism m Figure 18 1 In aqueous acid a hydronium ion transfers a proton to the carbonyl oxygen m step 1 and a water molecule acts as a Brpnsted base to remove a proton from the a car bon atom m step 2 The second step is slower than the first The first step involves proton transfer between oxygens and the second is a proton transfer from carbon to oxygen... 

The Brpnsted coefficient a represents the sensitivity of the rate to the acid strength of the catalyst. It is a measure of the degree of proton transfer from catalyst to substrate in the transition state. For nearly all reactions where BH+ contains acidic N-H or O-H groups, a is in the range 0-1. 

We used DFT to optimize the geometries of various Hammett bases on cluster models of zeolite Brpnsted sites. For p-fluoronitrobenzene and p-nitrotoluene, two indicators with strengths of ca. -12 for their conjugate acids, we saw no protonation in the energy minimized structures. Similar calculations using the much more strongly basic aniline andogs of these molecules demonstrated proton transfer from the zeolite cluster to the base. We carried out F and experimental NMR studies of these same Hammett indicators adsorbed into zeolites HY and HZSM-5. 

The crucial step in self-alkylation is decomposition of the butoxy group into a free Brpnsted acid site and isobutylene (proton transfer from the Fbutyl cation to the zeolite). Isobutylene will react with another t-butyl cation to form an isooctyl cation. At the same time, a feed alkene repeats the initiation step to form a secondary alkyl cation, which after accepting a hydride gives the Fbutyl cation and an -alkane. The overall reaction with a linear alkene CnH2n as the feed is summarized in reaction (10) ... 

The theoretical modeling of the activation and reaction of methanol by Brpnsted acid sites within zeolites has attracted a wide interest. This is in part a consequence of the industrial importance of the interaction—as the first step of the conversion of methanol into gasoline in the MTG process (213). However, a great deal of the theoretical interest has arisen because of the possibility of proton transfer from the zeolite lattice to methanol. An early investigation was that of Vetrivel et al. (214), who employed ab... 

Proton transfer from a Brpnsted acid to the oxygen of water is a single-step process and is very fast. It is a bimolecular, concerted process. 

Of course, protons cannot exist as free particles in molecular solvents they should react with the solvent molecules, and such an interaction in aqueous solutions results in the formation of H5Oj (H+-2H20 or [H20--H--0H2]+) particles. Similarly, hydroxide ions in aqueous solutions exist as H302 (OH2- -OH-). The stronger the acid Ai compared to the acid A2, the more complete is the shift of reaction (1.1.6) to the right. The Brpnsted-Lowry definition thus extends the term acid-base interaction to reactions accompanied with proton transfer from one base to another. For example,... 

Normalized p values for this reaction catalyzed by three different aryloxide ions are reported in Table 12 while Brpnsted coefficients for this type of catalysis are summarized in Table 13. The rather low values (high a(k )) suggest that the proton transfer from the attacking methanol nucleophile to the buffer base has made much less progress than the C-0 bond formation at the transition state (or that in the reverse direction protonation of the departing MeO by the buffer acid is ahead of C-0 bond cleavage), as shown in 82 on MeO group). 

A different organocatalytic approach to isoquinuclidine molecules has been described based on a cooperative double Brpnsted acid catalysis (Scheme 11.46) [127]. Thus, a proton transfer from the chiral Brpnsted acid to the aromatic imine would occur forming the chiral ion pair A ready to react with the dienol derived from... 

Stereochemical and kinetic analyses of the Brpnsted acid-catalysed intramolecular hydroamination/deuterioamination of the electronically non-activated cyclic alkene (13) with a neighbouring sulfonamide nucleophile have been found to proceed as an anh-addition (>90%) across the C=C bond to produce (15). No loss of the label was observed by and NMR (nuclear magnetic resonance) spectroscopies and mass spectrometry (MS). The reaction follows the second-order kinetic law rate = 2 [TfOH] [13] with the activation parameters being = 9.1 0.5 kcal moP and = -35 5 cal moP An inverse a-secondary kinetic isotope effect of d/ h = (1-15 0.03), observed for (13) deuteration at C(2), indicates a partial CN bond formation in the transition state (14). The results are consistent with a mechanism involving concerted, intermolecular proton transfer from an N-protonated sulfonamide to the alkenyl C(3) position coupled with an intramolecular anti-addition by the sulfonamide group. ... 

Over the past few years, a number of PILs have emerged offering properties that are often different from those offered by the better-known aprotic RTILs. PILs are simply formed by proton transfer from a Brpnsted acid to a Brpnsted base and can be synthesised relatively easily oti a large scale, at low cost, and without the need for further purification. PILs are also often more conductive than most aprotic RTILs [31]. The proton transfer reaction that yields a PIL can be represented... 

The adsorption complex formation involves the partial proton transfer from the Brpnsted acid site to the carbonyl oxygen. Thus, the extent of the proton transfer will lead to a H-bonded or to a protonated carbonyl, for example, a carbocation, depending on the acid strength of the Brpnsted site. In addition, the extent of the proton transfer defines the probability of condensation with the enol (64). 

Unlike the situation for Brpnsted acidity, manifest by complete or partial proton transfer from acid to base, Lewis acidity is a more general concept. It encompasses the Brpnsted-Lowry definition and thus there is no universal scale (in contrast to the Hammett acidity... 

NH adsorbed on H—MFI and all-silica MFI zeolites. Adsorption of NH3 has been widely used to assess the acidic strength of both Brpnsted and Lewis sites at solid surfaces [68, 70, 71]. In Fig. 1.12, the ads. I and ads. n volumetric (section a) and calorimetric (section b) isotherms of NH3 adsorbed on (i) the proton-exchanged H—MFI zeolite, (ii) one defective MFI-Silicalite (MFI—def) and (iii) the perfect (defect-free) all-silica MFl-Silicalite (MFT—perf), are reported. In H—MFI, the adsorption was only partially reversible in agreement with the proton-transfer from the Brpnsted acidic site Si(OH)+Al to NH3, as reported in Ref. [81]. Conversely,... 

The stepwise mechanism starts with protonation of the quinoline through the Brpnsted acid catalyst to form imi-nium ion I. Hydride transfer from the HEH neutralizes the substrate II and generates pyridinium salt A. Via a proton transfer from A to the Brpnsted acid catalyst, the latter is regenerated to protonate II again providing imine III for a second catalytic cycle. A subsequent hydride transfer leads to the final product IV and to pyridinium salt A that regenerates the catalyst as described above (Scheme 32.25). 

The result of the fast reactions in the ion source is the production of two abundant reagent ions (CH5+ and C2H5+) that are stable in the methane plasma (do not react further with neutral methane). These so-called reagent ions are strong Brpnsted acids and will ionize most compounds by transferring a proton (eq. 7). For exothermic reactions, the proton is transferred from the reagent ion to the neutral sample molecule at the diffusion controlled rate (at every collision, or ca. 10 9 s 1). 

Finally, in many cases the acidity equilibria cannot be measured but the rate of proton transfer or transmetallation can be measured to give an ionic or ion pair kinetic acidity. Studies using the rates of proton transfer have included the use of isotopes such as tritium and deuterium5,6. The rate is then used to calculate the Brpnsted slope, a, by plotting the logarithm of the proton transfer rate against the pK, as determined by the equilibrium acidity, for a series of compounds. From this plot, the approximate pKa of an unknown compound can be determined by comparison of the same type of compounds. 

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