Acids Brpnsted

Still another type of adsorption system is that in which either a proton transfer occurs between the adsorbent site and the adsorbate or a Lewis acid-base type of reaction occurs. An important group of solids having acid sites is that of the various silica-aluminas, widely used as cracking catalysts. The sites center on surface aluminum ions but could be either proton donor (Brpnsted acid) or Lewis acid in type. The type of site can be distinguished by infrared spectroscopy, since an adsorbed base, such as ammonia or pyridine, should be either in the ammonium or pyridinium ion form or in coordinated form. The type of data obtainable is illustrated in Fig. XVIII-20, which shows a portion of the infrared spectrum of pyridine adsorbed on a Mo(IV)-Al203 catalyst. In the presence of some surface water both Lewis and Brpnsted types of adsorbed pyridine are seen, as marked in the figure. Thus the features at 1450 and 1620 cm are attributed to pyridine bound to Lewis acid sites, while those at 1540... 

Fig. XVIII-20. Spectra of pyridine adsorbed on a water-containing molybdenum oxide (IV)-Al203 catalyst L and B indicate features attributed to pyridine adsorbed on Lewis and Brpnsted acid sites, respectively. (Reprinted with permission from Ref. 191. Copyright 1976 American Chemical Society.)...

Water can also be a Brpnsted acid donating a proton to a base Sodium amide (NaNH2) for example is a source of the strongly basic amide ion which reacts with water to give ammonia... 

Consider first the case of adding a strong acid such as HBr to water The equation for the Brpnsted acid-base reaction that occurs between them is... 

Clearly the two reactions are analogous and demonstrate that the reaction between hydroxide ion and hydrogen bromide is simultaneously a Brpnsted acid-base reaction and a Lewis acid Lewis base reaction Br0nsted acid-base reactions constitute a sub category of Lewis acid Lewis base reactions... 

We saw m Chapter 1 especially m Table 1 7 that alcohols resemble water m respect to their Brpnsted acidity (ability to donate a proton/rom oxygen) They also resemble water m their Brpnsted basicity (ability to accept a proton on oxygen) Just as proton transfer... 

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 second stage is a Brpnsted acid-base reaction and is fast... 

Step 2 A water molecule acts as a Brpnsted acid to transfer a proton to the oxygen of the enolate ion... 

Conjugate base (Section 1 13) The species formed from a Brpnsted acid after it has donated a proton... 

As Olah et al. have reported (81JOC2706), iV-nltropyrazole in the presence of Lewis or Brpnsted acid catalysts is an effective nitrating agent for aromatic substrates. The greater lability of the N—NO2 bond in iV-nltropyrazole compared with aliphatic nltramines was discussed on the basis of its molecular structure as determined by X-ray crystallography. 

The complementary ABO ester (2,7,8-trioxabicyclo[3.2.1]octyl ester) is prepared from the epoxy ester by rearrangement with Cp2ZrCl2/AgC104. The OBO ester is more easily cleaved by Brpnsted acids than is the ABO ester, but the ABO ester is cleaved more easily by Lewis acids, thus forming an orthogonal set. The ABO ester can be cleaved with PPTS (MeOH, H2O, 22°, 2 h LiOH) the OBO ester is cleaved at 0° in 2 min. ... 

B5H9 also acts as a weak Brpnsted acid and, from proton competition reactions with other boranes and borane anions, it has been established that acidity increases with increasing size of the borane cluster and that arachno-boranes are more acidic than nido-horancs ... 

The great electron-pair acceptor capacity (Lewis acidity) of SbFs has been utilized in the production of extremely strong proton donors (Brpnsted acids, p. 48). Thus the acidity of anhydrous HF is substantially increased in the presence of SbF ... 

Although trityl perchlorate is used to accomplish the glycosidation of the C-8 hydroxyl in 44 with acetoxy glycoside 49, control experiments have demonstrated that no reaction takes place in the presence of 4 A molecular sieves or 2,6-di-terf-butylpyridine. This observation suggests that the actual catalyst is not trityl perchlorate, but perchloric acid. Consistent with this conclusion is the observation that catalytic amounts of a strong Brpnsted acid such as triflic or perchloric acid can catalyze the glycosidation of 44 with 49 in the absence of trityl perchlorate. 

The higher the proton affinity, the weaker the Brpnsted acid. 

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). 

In the Brpnsted picture, the acid is a proton donor, but in the Lewis picture the proton itself is the acid since it has a vacant orbital. A Brpnsted acid becomes, in the Lewis picture, the compound that gives up the actual acid. The advantage of Lewis theory is that it correlates the behavior of many more processes. For example, AICI3 and BF3 are Lewis acids because they have only 6 electrons in the outer shell and have room for 8. Both SnCU and SO3 have eight, but their central elements, not being in the first row of the periodic table, have room for 10 or 12. Other Lewis acids are simple cations, like Ag. The simple reaction A + B- A—B is not very common in organic chemistry, but the scope of the Lewis picture is much larger because reactions of the types... 

Periodic Table Correlations. When comparing Brpnsted acids and bases that differ in the position of an element in the periodic table ... 

Note that Lewis acidity decreases, whereas Brpnsted acidity increases, going down the table. There is no contradiction here when we remember that in the Lewis picture the actual acid in all Brpnsted acids is the same, namely, the proton. In comparing, say, HI and HF, we are not comparing different Lewis acids but only how easily F and 1 give up the proton. The effect discussed here is an example of a symmetry factor. For an extended discussion, see Eberson, L. in Patai The Chemistry of Carboxylic Acids and Esters, Wiley NY, 1969,... 

---