Reversible acids

The THP group can be removed by dilute aqueous acid. The chemistry involved in both the introduction and deprotection stages is the reversible acid-catalyzed formation and hydrolysis of an acetal (see Part A, Section 7.1). 

The reversible acid-catalyzed transacetalation of the cyclophane formal 3 has been shown to undergo a ring-fusion/ring-fission process to generate a mixture of polymer cyclic formaldehyde acetals by means of oxonium ion intermediates <06CEJ8566>. The stepwise... 

Matsuura, J.E., A.E. Morris, R.R. Ketchem, E.H. Braswell, R. Klinke, W.R. Gom-botz, and R.L. Remmele, Jr. 2001. Biophysical characterization of a soluble CD40 ligand (CD154) coiled-coil trimer evidence of a reversible acid-denatured molten globule. Arch Biochem Biophys 392 208-218. 

This protective group is introduced by an acid-catalyzed addition of the alcohol to the vinyl ether moiety in dihydropyran. />-Toluenesulfonic acid or its pyridinium salt is used most frequently as the catalyst,3 although other catalysts are advantageous in special cases. The THP group can be removed by dilute aqueous acid. The chemistry involved in both the introduction and deprotection stages is the reversible acid-catalyzed formation and hydrolysis of an acetal (see Part A, Section 8.1). 

Forward and reverse acid-base reactions proceed simultaneously and can therefore be represented as occurring at the same time by using two oppositely facing arrows ... 

Theoretical analysis indicated that the first goal, reduced energy consumption, was realistic. A reversible acid gas removal process separating acid gases from a 1000 psia crude gas containing 30 mol % CO2 would reject carbon dioxide at ambient temperature and pressure, and be a net energy producer. 

The appearence of a negative power of concentration, [OH-], introduces a new concept, the role of chemical equilibrium in regulating the concentrations of reactants or reaction intermediates. In this case, in Step 1 HOC1 is formed in a straightforward reversible acid-base reaction. 

Mechanism of acid-base catalysis It is accepted that acid-base catalysis involves a reversible acid-base reaction between the substrate and catalyst. This is in agreement with the protonic concept of acids and bases, since acid catalysis depends on the tendency of the acid to lose a proton, while base catalysis depends upon the tendency of the base to gain a proton. The mechanism of reaction involving H and OH- ion catalysis may be expressed as follows, by taking the example of hydrolysis of esters. 

The most important example of the reactions in this section s title is the second reaction of the urea synthesis (B) (Figure 8.15). It starts from ammonium carbamate (A) generated in situ. In a reversible acid/base reaction, A—to a minor extent—is transformed into NH3 and the unsubstituted carbamic acid C. In a reversal of its formation reaction the latter decomposes—... 

One very important thing to remember is that an acid and base are always present in these reactions. In order for a molecule or ion to donate a proton, there has to be another ion or molecule to receive it. In addition, in reversible acid-base reactions, the roles of the substances as proton donor or acceptor will switch in the reverse reaction. In the example below, note how water (a Bronsted-Lowry base in this reaction) accepts a proton to form a hydronium ion in the forward reaction. In the reverse reaction, the hydronium ion (Bronsted-Lowry acid) donates a proton and, after losing the proton, becomes the water molecule once again ... 

An opposite reaction to the acid-catalyzed hydrolysis is the above-mentioned reversion. Acids can also catalyze the formation of anhydro sugars (see Section 2.3.4). Reversion tends to result in formation of (1—>6)-glycosidic bonds. The degradation of pentoses and uronic acids into furfural and of hexoses into hydroxymethylfurfural, levulinic, and formic acids are also important acid-catalyzed reactions, which, however, require concentrated acid and higher temperatures (Fig. 2-31). 

Fig. 7 Molecular-level plug/socket system for energy transfer based on the reversible acid-base driven threading-dethreading motions in the hydrogen bonded pseudorotaxane 4d5H+ (CH2C12 room temperature). The acid-driven threading of compound 6, incorporating a bulky benzyl group, through the macrocyclic cavity of 4 does not occur [57]...

Further examples of hybrid pseudorotaxanes containing a linear component incorporating two dialkylammonium centres and a 4,4 -bipyridinium unit associated with an assortment of macrocyclic polyethers have been reported. Both multi-component rotaxanes and polyrotaxanes based on the above tetracationic thread were observed to form. The reversible acid-base-controlled dethreading/rethread-ing of such [n]-rotaxanes has been demonstrated using a combination of NMR, absorption and fluorescence spectroscopies. ... 

The total number of Bronsted sites is lov ered by increasing calcination temperature. This results primarily from a sharp reduction in strong sites, as reflected by a decrease in the irreversible portion of acidity. The reversible acidity tends to increase. This is true for both the Bronsted and Lewis types. 

For the reversible acid unfolding of pmAAT, a stock solution of the enzyme in 2 mM Tris HCl, pH 7.5 was denatured by addition of diluted HCl to pH 2.0, followed by incubation for 90 min at room temperature (15). Refolding of pmAAT was performed by rapid dilution of the unfolded protein to 1.8 nM final concentration in refolding buffer (40 mM... 

Pavia, A, Ung-Chhun, S N, Mecanisme de la reaction de glycosylation par reversion acide en presence d anhydride trifluoromethane sulfonique agent de condensation. Can. J. Chem., 59,482-489, 1981. 

The latter reaction (Scheme 1.9) is a reversible, acid-catalysed reaction and may occur to the extent of 10%-20% loss of potential unsaturation. 

Criegee proposed a two-step mechanism (Eq. 1) for the Baeyer-Villiger oxidation which is widely accepted, at least for the reaction with organic peracids as oxidizing agent [5] as first step the reversible acid- or base-catalyzed addition of... 

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