Acid-catalyzed cleavage reactions

Ethers undergo an acid-catalyzed cleavage reaction when treated with the Lewis acid Blh at room temperature. Propose a mechanism for the reaction. 

Cleavage in aqueous solution in the presence of other anions, Y-, has been studied for Y- = N03-, SCN-, Cl-, and CH3S03 (355-358). In all these reactions variable amount of Co(NH3)5Y2+ are produced together with the aqua complex, and extensive competition studies have been reported for the uncatalyzed as well as for the acid-catalyzed cleavage reaction. The results of these competition experiments are in keeping with an essentially dissociative mechanism for bridge cleavage. 

Historically, the first cracking catalyst used was aluminum trichloride. With the development of heterogeneous solids and supported catalysts, the use of AICI3 was soon superseded, since its activity was mainly due to the ability to bring about acid-catalyzed cleavage reactions. 

Epoxides are highly reactive due to the ease of opening of the highly strained three-membered ring. Epoxides can undergo base and acid-catalyzed cleavage. Reactions in (a) - (c) are examples of such cleavages. 

Under conditions similar to those for (XMe2Si)20, the acid catalyzed cleavages with alcohols of linear and cyclic methylsiloxanes have been investigated. The rate constants of the primary reaction, the cleavage of the first Si-O-Si-bond, determined by GLC and HPLC, are given in Table 2. 

Values of pA"R for the addition of water to carbocations to give the corresponding alcohols. The equilibrium constants KR (m) were determined as the ratio Hoh/ h> where fcHOH (s 1) is the first-order rate constant for reaction of the carbocation with water and H (m 1 s ) is the second-order rate constant for specific acid-catalyzed cleavage of the alcohol to give the carbocation.9,12 13... 

Fig. 5 Logarithmic plots of rate-equilibrium data for the formation and reaction of ring-substituted 1-phenylethyl carbocations X-[6+] in 50/50 (v/v) trifluoroethanol/water at 25°C (data from Table 2). Correlation of first-order rate constants hoh for the addition of water to X-[6+] (Y) and second-order rate constants ( h)so1v for the microscopic reverse specific-acid-catalyzed cleavage of X-[6]-OH to form X-[6+] ( ) with the equilibrium constants KR for nucleophilic addition of water to X-[6+]. Correlation of first-order rate constants kp for deprotonation of X-[6+] ( ) and second-order rate constants ( hW for the microscopic reverse protonation of X-[7] by hydronium ion ( ) with the equilibrium constants Xaik for deprotonation of X-[6+]. The points at which equal rate constants are observed for reaction in the forward and reverse directions (log ATeq = 0) are indicated by arrows.

Fig. 8.21. Mechanisms of cleavage ofN-[(acyloxy)methyl] derivatives of primary amides (8.170, R = H) and secondary amides (8.170, R = methyl) via an acid-catalyzed pathway (.Reaction a), a pH-independent pathway (.Reaction b), and base-catalyzed pathways (Reactions c and d) [218].

For phloroglucinolysis, a solution of 0.1 N HCl in MeOH, containing 50 g/L phloroglucinol and 10 g/L ascorbic acid, is prepared. The PA of interest is reacted in this solution at 50°C for 20 min and then combined with 5 volumes of 40 mM aqueous sodium acetate to stop the reaction. After acid-catalyzed cleavage in the presence of phloroglucinol, the fraction is depolymerized and the terminal subunits released as flavan-3-ol monomers and the extension subunits released as phloroglucinol adducts of flavan-3-ol intermediates. These products are then separated and quantified by HPLC [25]. 

This acid-catalyzed cleavage of the glycosidic bonds is rather complex and often suffers from a lack of selectivity mainly due to side dehydration or recombination reactions of monosaccharides. In the existing literature, four different classes of solid catalysts are reported (1) cation-exchange resins, (2) siliceous-based materials, (3) metal oxides, and (4) sulfonated amorphous carbons. 

Thiolysis also proved useful for the analysis of derived tannins. Methylmethine-linked tannin-like compounds resulting from acetaldehyde-mediated condensation of flavanols (see Section 5.5.S.2) yielded several adducts when submitted to acid-catalyzed cleavage in the presence of ethanethiol, providing information on the position of linkages in the original ethyl-linked compounds. " Thiolysis of red wine extracts released benzylthioether derivatives of several anthocyanin-flavanol adducts, indicating that such structures were initially linked to proanthocyanidins. However, some of the flavonoid derivatives present in wine (e.g., flavanol-anthocyanins ) are resistant to thiolysis, while others (e.g., flavanol-ethyl anthocyanins) were only partly cleaved. Thiolysis, thus, appears as a rather simple, sensitive, and powerful tool for quantification and characterization of proanthocyanidins, but provides mostly qualitative data for their reaction products. 

Flavanols also react both as nucleophiles, through their A-ring, and as elecrophiles, through the carbocations formed after acid-catalyzed cleavage of the interflavanoid linkages. The latter reaction, restricted to oligomers and polymers, was shown to occur spontaneously at wine pH values. ... 

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