Brpnsted acid-base systems

The real promise of this catalytic reaction is the eventual development of an efficient enantioselective allylboration catalyzed by chiral Lewis acids. A stereoselective reaction using a substoichiometric amount of a chiral director has been reported, but only modest levels of stereo-induction were achieved with an aluminum-BINOL catalyst system (Eq. 19)P Recently, a chiral Brpnsted acid catalyzed system has been devised based on a diol-tin(IV) complex (Eq. 80). In this approach, aliphatic aldehydes provide enantioselectivities (up to 80% ee) higher than those of aromatic aldehydes when using the optimal complex 114. Although the levels of absolute stereoselectivity of this method remain too low for practical uses, promising applications are possible in double diastereoselection (see section on Double Diastereoselection ). 

This classification has been broadened39,40 by replacing the Brpnsted acid (proton donor) with a Lewis acid (an electron acceptor) and the Brpnsted base with a Lewis base (an electron donor). (A Brpnsted acid is a Lewis acid but not necessarily vice versa.) Solvent-proton interactions are therefore included as one subdivision of this classification, but many solvation reactions of cations with solvents also will be included as reactions of Lewis acid-base systems. This approach still does not solve the problem of fitting specific solvation interactions into the classification scheme. For example, acetonitrile behaves as a good Lewis base toward silver ion, but a poor one toward hydronium ion. The broader scheme also does not specifically take into account hydrogenbonding effects in hydroxylic and other solvents, which affect both the dielectric... 

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... 

Lewis defined a base as an electron-pair donor and an acid as an electron-pair acceptor. This definition further expands the list to include metal ions and other electron pair acceptors as acids and provides a handy framework for nonaqueous reactions. Most of the acid-base descriptions in this book will use the Lewis definition, which encompasses the Brpnsted-Lowry and solvent system definitions. In addition to all the reactions discussed previously, the Lewis definition includes reactions such as... 

For each of the following reactions identify the acid and the base. Also indicate which acid-base definition (Lewis, solvent system, Brpnsted) applies. In some cases, more than one definition may apply. 

Zirconia and titania both contain Lewis-acid and Lewis-base sites, with the latter having stronger adsorption properties. The titania phase also has strong Brpnsted acidic sites. Basic compounds are less retained on zirconia and titania phases, due to their basic nature. Neutral compounds such as poly aromatic hydrocarbons (PAH), due to their rr-electron system, behave as Lewis bases and the interactions with Lewis acid sites on the zirconia and titania packing materials become dominant for retention. 

Although benzene has six 7i-electrons, it cannot donate electrons as well as a simple alkene with a single n-bond. Benzene is less reactive than a simple alkene. The reaction of an alkene with HCl is an acid-base reaction, where the 7i-bond of the alkene is the Brpnsted-Lowry base. If benzene does not react with HCl, benzene must be a weaker base than the alkene. Benzene is a weaker base (unable to donate electrons as efficiently) because the six 7t-electrons of benzene are delocalized on six carbons, whereas the two 7t-electrons of an alkene are only distributed between two carbons. In other words, benzene is resonance stabilized, making it less reactive. The more delocalized the electrons are, the lower is the net electron density for any point between the carbon atoms. If a reaction has to occur at one carbon in any reaction, then the net electron density of one carbon in benzene is less than that of one carbon in an alkene. Benzene is less reactive than alkenes because more energy is required to disrupt the 7t-system (an endothermic process see Chapter 7, Section 7.5) and the electron delocalization makes less electron density available for donation. 

A reversible chemical reaction is one where the compounds normally defined as products react to regenerate the compounds normally defined as starting materials, and the two reactions are competitive. All of the Brpnsted-Lowry acid-base reactions discussed in previous chapters are equilibrium reactions, defined by an equilibrium constant K. They are acid-base reactions, so is used, but the principle is fundamentally the same. The fundamental principles of equilibria and reversible reactions can be applied to other systems. 

In a well-known reaction of conjugated carbonyl compounds, a nucleophile adds to the C=C unit. The formation of 38 previously used the C=C unit of the conjugated system as a Brpnsted-Lowry base, donating electrons to the acidic proton. It is known from Chapter 16 that the acyl carbon will accept electrons in a reaction with a nucleophile. Imagine that the 7i-system effectively extends the reactivity of the carbonyl to the C=C unit then, the C-C should also react with a nucleophile. The extension of points of reactivity by conjugating n-bonds is called vinylogy. 

In the aqueous system, the Brpnsted-Lowry definitions of acids and bases are of general applicability. A Brpnsted acid a proton donor. Acidic oxides react with water to produce oxonium ions in solution, e.g. 

In some ways, this acid-base terminology is unfortunate, since there are other, more common descriptors (e.g. Brpnsted, Lewis, hard and soft). However, the terminology has been helpful in suggesting lines of research for the study of non-aqueous systems, and its use will probably continue. 

Acid Base Reactions Usinq the Brpnsted LovUrif System... 

Depending on the nature of the metals the M-OH units created in a metal fluoride system may either react as a base or as a (Brpnsted) acid. Thus, replacement of F by OH, or vice versa, should result in an alteration of the basicity of M-OH groups, e.g. in MgF2 x(OH)x derivatives as compared to Mg(OH)2 and in the same way of the (Brpnsted) acidity, e.g. in A1F3 x(OH)x phases as compared to Al(OH)3. 

Hydrogen bonding. Interaction between molecules or portions of a molecule resulting from the Lewis acid or base properties of the molecular units. Most commonly applied to water or hydroxyl containing systems (e.g., alcohols) in the sense of Brpnsted-Lowry acid-base theory, but also found in molecules having hydrogen bound to nitrogen (amines and amides). 

Conjugate acid (Section 1 13) The species formed from a Brpnsted base after it has accepted a proton Conjugate addition (Sections 1010 and 1812) Addition reaction in which the reagent adds to the termini of the con jugated system with migration of the double bond synony mous with 1 4 addition The most common examples include conjugate addition to 1 3 dienes and to a 3 unsaturated car bonyl compounds... 

On the Brpnsted theory (p. 51), solutions with concentrations of H3O+ greater than that in pure water are acids (proton donors), and solutions rich in OH are bases (proton acceptors). The same classifications follow from the solvent-system theory of acids and bases... 

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