Hydrolytic attack

As previously mentioned, some urethanes can biodegrade easily by hydrolysis, while others are very resistant to hydrolysis. The purpose of this section is to provide some guidelines to aid the scientist in designing the desired hydrolytic stability of the urethane adhesive. For hydrolysis of a urethane to occur, water must diffuse into the bulk polymer, followed by hydrolysis of the weak link within the urethane adhesive. The two most common sites of attack are the urethane soft segment (polyol) and/or the urethane linkages. Urethanes made from PPG polyols, PTMEG, and poly(butadiene) polyols all have a backbone inherently resistant to hydrolysis. They are usually the first choice for adhesives that will be exposed to moisture. Polyester polyols and polycarbonates may be prone to hydrolytic attack, but this problem can be controlled to some degree by the proper choice of polyol. 

The usual order found with halogenonitrobenzenes is F > Cl Br I, the order of Cl and Br being variable, just as in heteroaromatic reactivity. The position of fluorine is of interest the available data indicate that it is usually the same as for nitrobenzene derivatives. Thus, in acid hydrolysis the order F > Cl for 2-halogeno-quinolines can be deduced beyond doubt since the fluoro derivative appears to react in the non-protonated form and the chloro derivative to resist hydrolytic attack even in the protonated form under appropriate conditions (Section II,D, l,d). Furthermore, in the benzo-thiazole ring, fluorine is displaced by the CHgO reagent at a rate 10 times that for chlorine. ... 

For the most common series of corrosive agents, water, steam, acids, alkalis and salts, the hydrolytic processes peculiar to each determine the mechanism of attack. Thus, under the right circumstances, hydrolytic attack on the bridging oxygens can occur in the following way ... 

Many xenobiotics, both synthetic and natnrally occuring, are lipophilic esters. They can be degraded to water-soluble acids and bases by hydrolytic attack. Two important examples of esteratic hydrolysis in ecotoxicology now follow ... 

Enzymes catalyzing the hydrolysis of esters are termed esterases. They belong to a larger group of enzymes termed hydrolases, which can cleave a variety of chemical bonds by hydrolytic attack. In the classification of hydrolases of the International Union of Biochemistry (lUB), the following categories are recognized ... 

While catalysis by aspartic proteases involves the direct hydrolytic attack of water on a peptide bond, catalysis... 

The organic structures holding the ions, porphyrins, are themselves quite resistant to decomposition. They are not open to hydrolytic attack but note that they are sensitive to oxidation. 

Polyurethane elastomers are exceptionally tough, abrasion resistant, and resist attack by oil. The polyester types (AU) are susceptible to hydrolytic attack at above ambient temperatures, and certain polyester thermoplastic polyurethanes have been known to stress crack in cable jacketing applications when in contact with water at ambient temperatures this latter effect has sometimes, incorrectly, been ascribed to fungal attack. Polyether types (EU) are far more resistant to hydrolytic attack. Certain polyurethanes can be attacked by UV light, the resistance to this agency primarily being determined by the isocyanate used. 

Fig. 8.2. Hydrolytic pathways in the activation of prodrugs of carboxylic acids that contain a) an (acyloxy)methyl or b) an [(alkoxycarbonyl)oxy]methyl group. Liberation of the active acid occurs with comparable rates whatever the initial site of hydrolytic attack is. Formaldehyde or acetaldehyde is liberated when R = H or Me, respectively.

In Sect. 11.4, we examined hydrolytic attack at sp3 C-atoms rendered more electrophilic by the presence of one or more halogen substituents. In a logical follow-up, the present section presents the hydrolysis of Mannich bases [78], L e., compounds that contain the X-CH2-N moiety, where an sp3 C-atom is made more electrophilic by the presence of two flanking heteroatoms, one of which is an N-atom. 

The negative coefficient of E° and Ef indicates an enhancement of activity with an increase of steric bulk. Especially, the variation in activity against barnyard grass is mainly governed by steric factors. If steric bulkiness prevents the proper fit with the target site, these results mean that it inhibits the interaction with some enzymes which participate in the detoxication process(es) in the plant body. The negative slope of the cr term in Eq. 15 and 16 coincides with this view, i.e. a molecule with a more electron-withdrawing substituent becomes more susceptible to a hydrolytic attack. 

The hydrolytic attack of water at the ester linkages in epoxies is accelerated in alkaline media and is a mechanically activated process 35). Matrix hydrolysis is also enhanced in the presence of inorganic fillers35). 

We begin by considering the enzymes using the hydroxyl group of serine (Bruice et al., 1962 Fersht, 1985) or the thiol group of cysteine to initiate the hydrolytic attack ... 

The ability of Ca2+ to crosslink groups and so stabilize proteins against thermal denaturation or hydrolytic attack is well illustrated by thermolysin, which involves four Ca2+ and a catalytic Zn2+. The function of the calcium is to stabilize the protein structure.404 Calcium also protects surface protein of the neural retina from tryptic cleavage.405... 

Very important is the fact that the Si—C bond is stable against hydrolytic attack. Only in special cases (e.g. (3-functional substituted organosilicon compounds,... 

The hypothesis put forth concerning steric effects in acid hydrolysis would have as its corollary the proposal that the role of the Ci component in cellulase enzyme system complexes is to disrupt the engagement of the C6 oxygen in the bifurcated intramolecular hydrogen bond and thus permit rotation of the C6 group into positions more favorable to hydrolytic attack. 

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