Acid 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. 

All these facts—the observation of second order kinetics nucleophilic attack at the carbonyl group and the involvement of a tetrahedral intermediate—are accommodated by the reaction mechanism shown m Figure 20 5 Like the acid catalyzed mechanism it has two distinct stages namely formation of the tetrahedral intermediate and its subsequent dissociation All the steps are reversible except the last one The equilibrium constant for proton abstraction from the carboxylic acid by hydroxide is so large that step 4 is for all intents and purposes irreversible and this makes the overall reaction irreversible... 

This case history illustrates the paradox so often encountered in microbiologically influenced corrosion. Clearly, two corrosion mechanisms were operating in the system, namely, acid attack and microbiologically influenced corrosion. To what degree each mechanism contributed to wastage is difficult to quantify after the fact. This was especially the case here, since other areas of the rolling oil system were attacked by a predominantly acidic form of corrosion. 

Nucleophilic substitution at RSO2X is similar to attack at RCOX. Many of the reactions are essentially the same, though sulfonyl halides are less reactive than halides of carboxylic acids. The mechanisms are not identical, because a tetrahedral intermediate in this case (148) would have five groups on the central atom. Though this is possible (since sulfur can accommodate up to 12 electrons in its valence shell) it seems more likely that these mechanisms more closely resemble the Sn2 mechanism, with a trigonal bipyramidal transition state (148). There are two major experimental results leading to this conclusion. 

In the pH range 2—3.5 the phosphonate (78) hydrolyses with loss of ROH at approximately 10 times the rate of comparable esters lacking the vicinal oxime function or in which this function is methylated on oxygen. An intramolecular general-acid catalysis mechanism was proposed, but it was not possible to exclude entirely an intramolecular nucleophilic attack at phosphorus. Intramolecular attack by the vicinal dimethylamino-group takes place preferentially at carbon rather than phosphorus in the phos-phonofluoridate (79). ... 

Cements based on phytic add set more quickly than their glass polyalkenoate or dental silicate cement cormterparts, but have similar mechanical properties (Table 8.2). They are unique among add-base cements in being impervious to acid attack at pH = 2-7. Unfortunately, they share with the dental silicate cement the disadvantage of not adhering to dentine. They do bond to enamel but this is by micromechanical attachment - the cement etches enamel - and not by molecular bonding. Lack of adhesive property is a grave weakness in a modern dental or bone... 

MIC almost always acts in concert with other corrosion mechanisms and may, at times, appear to be crevice corrosion, underdeposit acid attack, oxygen-concentration cell corrosion, ion-concentration cell corrosion, and CO2... 

Induction of elimination by nucleophilic attack of a solvent molecule on phosphorus (SN2(P)) would represent a mechanistic alternative. However, since not only ethanol but also sterically demanding alcohols such as cyclohexanol and tert-butanol are phosphorylated on alcoholysis of 2-chlorodecyl-l-phosphonic acid, this mechanism appears unlikely. Neither alcohol is a suitable nucleophile. 

Alkenes can be transformed into epoxides by hydroperoxides and a catalyst, which often is a high-valent titanium or molybdenum complex acting as a Lewis acid. The mechanism is not clear in great detail in Figure 2.34 a suggested mechanism is given. Coordination of the alkene to the metal prior to attack of the electrophilic oxygen is not considered as a necessary step. 

Scheme 16 Catalytic mechanism of ADP-ribosylation. The nucleophilic side chain of the amino acid attacks the anomeric carbon of ribose carrying nicotinamide to create ADP-ribosylated proteins.

In the acid-catalyzed mechanism (Figure 11-6), the protonation of the intermediate cation may produce either enantiomer because the hydrogen ion may attack the carbon from either side. ( One if by land, two if by sea ) Equal amounts of each enantiomer result and give a racemic mixture. 

Nickel is added to improve resistance to environmental and stress cracking corrosion. Molybdenum provides even greater resistance to corrosion and improves mechanical strength. Copper increases resistance to sulfuric acid attack. 

Phenol may be converted into a mixture of o- and p-nitrophenols (Expt 6.102) by reaction with dilute nitric acid the yield of p-nitrophenol is increased if a mixture of sodium nitrate and dilute sulphuric acid is employed. Upon steam distillation of the mixture of nitrophenols, the ortho isomer passes over in a substantially pure form the para isomer remains in the distillation flask, and can be readily isolated by extraction with hot 2 per cent hydrochloric acid. The mechanism of the substitution probably involves an electrophilic attack (cf. Section 6.2.1, p. 851) by a nitrosonium ion at a position either ortho or para to the activating hydroxyl group, to yield a mixture of o- and p-nitrosophenols, which are then oxidised by the nitric acid to the corresponding nitrophenols. The reaction depends upon the presence in the nitric acid of traces of nitrous acid which serve as the source of the nitrosonium ion. 

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. 

Although these additions to CO double bonds have some superficial similarities to the electrophilic additions to CC double bonds that were presented in Chapter 11, there are many differences. The acidic conditions mechanism here resembles the mechanism for addition to carbon-carbon double bonds in that the electrophile (the proton) adds first, followed by addition of the nucleophile. However, in this case the first step is fast because it is a proton transfer involving oxygen, a simple acid-base reaction. The second step, the attack of the nucleophile, is the rate-determining step. (Recall that it is the first step, the addition of the electrophile, that is slow in the additions to CC double bonds.) Furthermore, in the case of additions to simple alkenes there is no mechanism comparable to the one that operates here under basic conditions, in which the nucleophile adds first. Because the nucleophile adds in the slow step, the reactions presented in this chapter are termed nucleophilic additions, even if the protonation occurs first. In... 

As was the case for the addition reaction of Chapter 18, another version of the mechanism operates under acidic conditions. In this version the carbonyl oxygen is protonated before the nucleophile attacks. Because this makes the carbonyl carbon more electrophilic, weaker nucleophiles can be used (often the conjugate acids of those used under basic conditions). In addition, the leaving group may be protonated before it leaves. Examples of the acidic conditions mechanism are provided in later sections. 

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