Attack mechanism

Reverse osmosis membrane separations are governed by the properties of the membrane used in the process. These properties depend on the chemical nature of the membrane material, which is almost always a polymer, as well as its physical stmcture. Properties for the ideal RO membrane include low cost, resistance to chemical and microbial attack, mechanical and stmctural stabiHty over long operating periods and wide temperature ranges, and the desired separation characteristics for each particular system. However, few membranes satisfy all these criteria and so compromises must be made to select the best RO membrane available for each appHcation. Excellent discussions of RO membrane materials, preparation methods, and stmctures are available (8,13,16-21). 

Type of attack Environmental Cause of attack Mechanical factors Corrosion product... 

For n-alkanes, n-alcohols, 1-chloroalkanes, n-ethers, and chloroethenes, the carbon chain length influences the reactivity, and the clear linear correlations indicate that the attack mechanism of these pollutants by OH or Cl radicals occurs via the same pathway. However, such correlations do not hold true for aromatics, ketones, and aldehydes, for reasons discussed in our previous paper [3]. We also estimated missing values of kci by analogy for ethylbenzene, we take kci = 1.5e-10 cm molecule S, greater than that for m-xylene, but smaller than the 2.0e-10 cm molecule- s-i value for very reactive compoxmds. Also we estimate a similar value for butyraldehyde kci = le-10 cm molecule- s-, only 10% larger than kci of acetaldehyde to remain consistent with the equivalent koH value. 

Regenerated cellulose Hydrophilic, low-fouling Sensitive to temperature, pH, CI2, microbial attack, mechanical creep... 

Waymouth and coworkers used chiral zirconocene complexes such as 56 with Et3Al as the stoichiometric reductant to enantioselectively desymmeter-ize oxabicyclic compounds (Scheme 9) [29]. A reductive coupling mechanism to give 57 followed by (i-alkoxidc ring opening and transmetallation is consistent with the experimental results. Neither direct insertion of the alkene into the M - C bond nor nucleophilic attack mechanisms can be ruled out, however [12]. 

Kinetic studies of the 5-methyl analogs 5-methyldibenzothiophenium tetrafluoroborate (72) and 5-methyldiphenylsulfonium tetrafluoroborate (73) were reported. The difference in AS between heterocyclic S —CH3 salt 72 AS = -13.4 eu for ethanolysis) (91JOC1845) and nonheterocyclic S —CH3 salt 73 (A5 = -10.3 eu at 25°C for A-methylation of pyridine-ds) (83TL4859) was small. This is in accord with the conventional 5n2 attack mechanism of a nucleophile on the CH3 carbon along the S—CH3 bond. Thus, the great difference in AS found between heterocyclic S—CF3 salt 17 and nonheterocyclic S —CF3 salt 29 suggests a different reaction mechanism, probably an attack on the S — CF3 bond. 

Fujita S (1984) Radiolysis of nucleosides in aqueous solutions base liberation by the base attack mechanism. Int J Radiat Biol 45 371-377... 

In a nucleophilic attack mechanism similar to Grignard synthesis of an alcohol hydrides (H") will react with carbonyls to form alcohols. Unlike Grignard synthesis of... 

Mittal postulated that radical formation is likely due to the chemical reaction of H2 and 02 on Pt surface, this reaction is chemical in nature and shows strong dependence on the surface properties of Pt particles, and the sulfonic acid groups in the PFSA membrane maybe the key to the radical attack mechanisms.27 Cipollini28 in... 

Since the products are the same chemical species as the reactants, the over-all reaction is substantially thermoneutral except for activation energy, the problem of energetics is thus side-stepped. The apparent ter-molecular reaction required by Eq. (43) is also no problem, as the dissolved molecules are essentially in constant collision with water molecules. Wilmarth, Dayton, and Flournoy questioned the adequacy of this concerted attack mechanism, however, as they believed that it would predict acid catalysis of the exchange reactions, as well as base catalysis. 

The cobalt (III) complex, (1,2-C2B9Hn)2Co, undergoes bromination in glacial acetic acid solution to afford a hexabromo derivative, (1,2-C2B9HsBr3) 2Co 54). An X-ray crystallographic study of this product 14) showed that bromination occurred on the boron atoms farthest from the polyhedral carbon atoms, those boron atoms expected to have the greatest electron density, as predicted for an electrophilic attack mechanism. 

The active-sites of the glucoamylases are pockets that do not have a flexible, mobile loop that can act as a trap door to let the product (glucose) out. After the glycosidic bond is cleaved by glucoamylase, the remaining starch chain must dissociate and leave the active-site before glucose can leave. This explains why glucoamylase has such a low turnover number, and why it does not have a multiple attack mechanism. 

The CNDO/2 quantum chemistry calculations suggest that acid attacks are performed at the oxygen sites which are attached to aluminum atoms of the framework. The experiments of acid extraction of Al atoms indicate that the theoretically calculation reaults are achieved in assuming the acid attack mechanism. 

Figure 3.4.5 Suggested routes for 0-0 bond formation via a nucleophilic-attack mechanism (I) and radical attack (II) as described in the text. The proposed positions of the two substrate waters in the resting, S i and catalytically active S4 states are shown (green).

We established in Chapter 12 a hierarchy for the electrophilic reactivity of acid derivatives that should by now be very familiar to you—acyl chlorides at the top to amides at the bottom. But what about the reactivity of these same derivatives towards enolization at the a position, that is, the CH2 group between R and the carbonyl group in the various structures You might by now be able to work this out. The principle is based on the mechanisms for the two processes, mechanism of nucleophilic attack mechanism of enolate formation... 

Nimlos et al. (1993) have foimd that the dependence of the gas-phase oxidation of TCE on the UV intensity, which was linear at high concentrations of TCE, changed its dependence into a square root law when the concentrations were low. This was explained (Upadhya and Ollis, 1998) by a change in the mechanism at high concentrations the mechanism was a chain reaction mechanism induced by chlorine atoms, whereas at low concentrations the mechanism is the common holes /OH attack mechanism. This h)q5othesis correlated well with the fact that the quantum efficiency at high concentrations of TCE was 4-10 times higher than at low concentrations. 

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