Bransted acids

Click Coached Problems for a self-study module on Bransted acids and bases. 

The basic reactions of the sol-gel procedure are shown in Eqs. (1-3), in which the species of Si—OH are the key intermediates. These Si—OH groups are known as Bransted acids. On the other hand, the amide carbonyl groups are... 

Bransted acid-catalyzed reactions with imines... 

The reactions of acylzirconocene chlorides with imines also proceed under Bransted acid-catalyzed conditions, even with aqueous acids (Table 5.3) [23],... 

Entry Bransted acid pKa Yield (%) a Entry Bransted acid pKa Yield (%) a... 

Scheme 3.3 Bransted acid promoted dehydrative glycosylations.

The trichloroacetimidate method for glycoside synthesis extended its versatility right at the outset (51,52a) by exhibiting an especially smooth reaction of 0-(glycosyl)trichloroacetimidates with Bransted acids. Without the addition of any catalyst, simple Bransted acids are able to substitute the trichloroacetimidate group at room temperature in high yields, as shown (17) for la-a in Scheme 6. Because of anomerization of possible / products... 

Scheme 6.—Substitution of the Trichloroacetimidate Group by Simple Bransted Acids.

Ring opening reaction of alkylidenecyclopropanone acetals readily proceeds in the presence of Lewis or Bransted acids to produce l-alkylidene-2-oxyallyl cation, which is provided for the reaction with nucleophiles such as chloride, alcohols, siloxyalkenes, and furans. The reaction of this cation with the carbon nucleophiles gives products of [4 + 3] and [3 + 2] cycloaddition as well as those of nucleophilic addition. The modes of addition reactions are controlled by the oxy group of the cation and by the reaction conditions including solvent. 

Reaction 3.2 is a prime example of the use of the terms Lewis acid and Lewis base. G. N. Lewis suggested the usage such that a donor of an electron pair is a base and the acceptor molecule is an acid. The classical Bransted acid and base pair, H+(aqueous) and OH (aqueous), are encompassed by the Lewis definitions as they combine to give water, the hydroxide ion supplying both electrons. 

Amphiprotic solvents have both acidic and basic properties in terms of the Bransted acid-base concept. If we denote an amphiprotic solvent by SH, it donates a proton by SH S +H+ and accepts a proton by SH + H+ = SH2. Overall, the autoprotolysis (autoionization) occurs by 2SH = SH2 + ST The extent of autoprotolysis is expressed by the autoprotolysis constant, Kjh = aSH2 aS, the values of which are also included in Table 1.5 as pKSH values (for more details, see Table 6.6). 

Bransted acids (proton donors, HA) have significant influences on the reduction of organic compounds in aprotic solvents. If a weak Bronsted acid like water is added step-wise to the electrolytic solution, the height of the first polarographic wave increases at the expense of that of the second wave (Fig. 8.12). By the addition of a weak acid, the following reactions occur at or near the electrode ... 

The influence of hydrothermal treatment on the concentration of Bransted sites, which are directly related to tetrahedrally framework aluminum, has been studied by FTIR spectroscopy in dependence on the Al content. A previously calcined and dehydrated H,A1-MCM-41, containing only ca. 20% of the initial Bransted acid sites, was in situ treated with water vapour at 80°C in the IR cell. After subsequent removal of loosely bound water by evacuation, which was controlled by the disappearance of the deformation mode of water at 1640 cm 1, ammonia was loaded to determine the change of the Bransted acidity. Based on the intensity of the BS band at 1450 cm 1, a distinct increase of the Bransted site content is found (Fig. 2). In comparison with the ammonium exchanged MCM-41, the amount of Bransted sites rises to 53% of the initial value observed with the ammonium exchanged MCM-41. 

The restructuring of the framework in connection with the re-coordination of Al in MCM-41 is also reflected in the v(OH) vibration spectra of the calcined sample. Two vibration bands appear in the range of the vibrations of Bransted acid bridging OH groups at 3628 and 3600 cm 1 (Fig. 3). The bands are absent in the spectra of NH4-MCM-41. These bands disappear immediately after adsorption of ammonia and re-appear after ammonia desorption at 500°C. This behaviour additionally supports the assignment of these bands to acidic OH groups. The bridging hydroxyl vibration bands are well resolved and broad. After hydrothermal treatment... 

After evacuation at 25°C (spectrum C), the physisorbed fraction of acetonitrile-d3 and also the H-bonding effects with the silica surface -OH have disappeared, while the Bronsted acidity is still present (2309 cm 1). Subsequent evacuation at 60°C does not change the intensity of the Bransted acidity (spectrum D). Even at 120°C and at 150°C Bransted acid sites are still detected (spectra E, F). Therefore, it can be concluded that the Al-PCH is characterized by an important Bransted surface acidity. This type of acidity is expected since the initial Na+ ions on saponite have been replaced by surfactant cations and then by protons upon destruction of the surfactant through calcination. Besides, by the grafting of Al-species onto the support, Si-(OH)-Al bonds have been created, giving rise to the band at 2309 cm 1 indicative of Bransted acidity [10]. 

Nb-containing MCM-41 sieves represent Lewis acidity proven by FTIR study conducted after pyridine adsorption [3,4], Hydrogen forms of niobium-containing MCM-41 materials exhibit lower Bransted acidity than that in hydrogen aluminosilicate mesoporous molecular sieves (see the band at 1549 cm 1 in Figure 6 [3]). The dehydroxylation of H-NbMCM-41 samples causes the formation of the following lattice species ... 

FIGURE 10.1 A Bransted acid is a proton donor, and a Bronsted base is a proton acceptor. In this context, a proton is an H+ ion. 

Self-Test 10.2A Identify (a) the Bransted acids and bases in both reactants and products in the proton transfer equilibrium HNOz(aq) + HP042 (aq) ... 

As for type (1), Niiyama et al. (42) proposed that protons are generated by dissociation of water and that the equilibrium of the dissociation is a function of the electronegativity of the metal cations. Formation of Bransted acid sites in the aluminum salt of H3PW12O40 as a result of exposure to water vapor at 573 K was confirmed by IR spectra of sorbed pyridine (138). 

The presence of Lewis acidity [type (2)] or Bransted acidity [type (1)] is revealed by the IR spectrum of pyridine sorbed on AI1.5PW12O40 (138, 139). IR spectra of sorbed of NH3 show that Cui.5PW]204o has Lewis acidity as well as Bransted acidity (140). Ghosh and Moffat (141) measured the acidities of several salts of H3PW12O40 with Hammett indicators. The acid strength increases with an increase in the calculated charge on the peripheral oxygen atom of the polyanion Zr > Al > Zn > Mg > Ca > Na (141). 

Typical results, exemplified by the dehydration of n-butyl alcohol over HZSM-5 (8j), are shown in Fig. 3, a characteristic feature being the gradual (ca. 8 min at 399 K) saturation of the catalyst with the alcohol. At saturation there are 8 X 1020 mol of adsorbed butyl alcohol per gram, which is more than 3.5 times the concentration of active (Bransted acid) sites. Under the same conditions, our amorphous A AS sample adsorbs 3 X 1020 mol of n-butyl alcohol per gram, which exceeds the active site concentration by ca. 2.7 times. 

Substituent Effects on Strengths of Bransted Acids and Bases 151... 

The problem is that a substance HA undergoing a reaction in which it behaves as a Bransted acid is not behaving as an acid in the Lewis sense it is, however, behaving as a Lewis acid-base adduct. The issue is further clouded by the fact that HA can enter into reactions in which it does behave as a Lewis acid, as for example in the formation of a hydrogen bond (Equation 3.61) in this case it is not, however, behaving as a Bransted acid, since the proton remains... 

There is no doubt that Pearson did not suggest something entirely new, as we have already seen, but he generalized the concepts derived from the mass-action law (formation constants and Bransted acidity) in a variety of new ways. Many chemists have felt that hard-hard interactions are a new name for electrovalent (ionic) bonding and soft-soft interactions for covalent bonding37. This is also a part of the truth, but other aspects are far more sophisticated and deserve detailed discussion. Other chemists sharply criticize the short and colloquial words hard and soft . In the writer s... 

Finally, in an interesting and unique transformation, the de-insertion of isonitrile from the nominally eight-coordinate molybdacyclopentenediimine complex 181 is induced upon protonation with strong Bransted acid (Equation 92) it is not obvious why the ring-contracted molybdacyclobutenimine product 182 should be thermodynamically more stable than the simple product of protonation <2001JCD1284>. 

A traditional acid-catalyzed cyclisation to piperidines can control the stereo-outcome by the judicious choice of acid (Scheme 43) <2002OL3727>. Stereoselectivities of up to 98 2 were achieved with a Lewis acid giving the trans-isomer and Bransted acids yielding the air-isomers. 

Acyl-2//-chromenes can be synthesized from O-propargylic salicylaldehydes in the presence of a Bransted acid (HBF4). The reaction proceeds via a formal [2+2]-cycloaddition of the alkyne and carbonyl group to form intermediate oxete 38, which undergoes cycloreversion to afford 3-acyl-2//-chromenes in excellent yield (Scheme 12) <2005OL2493>. 

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