Acid-catalysed Degradation

For the addition polymers of interest in this review, the predominant foeus in the literature has been on using acid catalysts to degrade them. With their known ability to cleave carbon-carbon bonds and their widespread use in the hydro- 

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Epicatechin-(4a-6)-(-)-epicatechin (B5) Seeds Ricardo da Sdva et al. (1991c) HPLC, TLC, — H NMR, FAB-MS after enzymatic hydrolysis, acid hydrolysis, partial acid-catalysed degradation with phloroglucinol or phenylmethanethiol... 

Tables Selected product select ivities (wt%) for the thermal and acid-catalysed degradation ofpolystyrene at 350 °C after 60 min of batch reactioni ...

The pH-rate profile for the hydrolysis of benzylpenicillin is shown in Fig. 9 (Gensmantel et al., 1978). There is no significant spontaneous hydrolysis but the P-lactam does undergo an acid catalysed degradation. Also shown in Fig. 9 is the pH-rate profile for the hydrolysis of cephaloridine. There are two immediate differences cephalosporins exhibit a spontaneous pH-independent hydrolysis and are less reactive towards acid than penicillins by a factor of about 10 (Proctor et al., 1982). 

In addition to the expected hydrolysis product, benzylpenicilloic acid [30], the acid catalysed degradation of benzylpenicillin gives benzylpenicillenic... 

Electron-withdrawing substituents in the amine portion of the P-lactam decrease the rate of acid catalysed degradation of penicillins. The Bronsted P-value is ca 0.35 (Proctor et al., 1982) compared with —0.26 for acyclic anilides and amides (Giffney and O Connor, 1975). Although the effects of substituents are not large, they are significant and in the opposite direction for P-lactams compared with other amides which again is indicative of a different mechanism. 

There is a large dependence of the rate of the acid catalysed degradation of penicillins upon the nature of the acylamido side chain. Electron-with-drawing substituents decrease the rate of degradation and for substituted phenylpenicillins the Hammett p-value is — 1.60 (Yamana et ai, 1974b). 

The acid catalysed degradation of C(6) acylamido penicillins show a rate of enhancement of ca 10 compared with that predicted from the Hammett plot for C(6) substituents (Proctor et al., 1982). The mechanism of degradation must therefore incorporate the acylamido group in the rate-limiting step or in a pre-equilibrium step. 

The rate enhancement observed for the acid catalysed degradation of benzylpenicillin indicates neighbouring group participation by the acylamido side chain. 

The rate-limiting step for the acid catalysed degradation of all penicillins does not appear to involve water. 

The 6-amido side chain also does not provide a significant rate enhancement in the degradation of penicillins between pH 3 and 6. The second-order rate constants for the acid catalysed degradation of 6-ammonium penicilla-... 

The acid catalysed degradation of penicillins is inhibited in cationic micelles of cetyltrimethylammonium bromide (Tsuji et al., 1982) and, as expected, neither anionic micelles of sodium dodecylsulphate nor polyoxyethylene lauryl ether promote the hydroxide-ion catalysed hydrolysis of benzylpenicillin (Gensmantel and Page, 1982a). In the presence of cetyltrimethylammonium bromide (CTAB) the pseudo first-order rate constants for the alkaline hydrolysis increase rapidly with surfactant concentration once... 

In the 1950s, Ranby reported for the first time that colloidal suspensions of cellulose can be obtained by controlled sulphuric acid-catalysed degradation of cellulose fibers [10-12]. This work was inspired by the studies of Nickerson and Habrle [40], who observed that the degradation induced by boiling cellulose fibers in acidic solution reached a limit after a certain time of treatment. Transmission electron microscopy (TEM) images of dried suspensions revealed for the first time the presence of aggregates of... 

In the absence of atmospheric oxygen, acid-catalysed degradation of ascorbic acid is considered a major cause of the loss of vitamin C in dried fruits, canned fruit compotes and juices (pH value around 3.5), especially when stored at higher temperatures. For example, fruit juices lose 70-95% of ascorbic acid within 12 weeks of storage at 50 °C. The reaction rate is about ten times lower than that of autoxidation catalysed by metal ions. 

There are two main options for getting around the risk of acid-catalysed degradation of the PPI in the stomach after oral administration. 

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