Chloride attack mechanism

The mechanism of the rearrangement is explained as shown in Scheme 19. Protonation of the 9-hydroxy group followed by its elimination and subsequent chloride attack at the 4a-carbon generates a chloroindolenine 126. Addition of water to the 9a-imine carbon atom of 126 gives 127. Concerted elimination of the chloride with rearrangement of the alkyl side chain attached to the 9a carbon atom results in 3,3-disubstituted oxindole structure 120a. 

My last comment concerns the reaction of palladium olefin complexes with carbon monoxide discovered by Tsuji. I agree that this is most likely to proceed by an insertion rather than an ionic mechanism. Chloride attack on coordinated olefin is rare however. Chloride ion is an inhibitor, for example in the palladous chloride catalyzed hydration of ethylene (0). I, therefore, wondered whether carbon monoxide was affecting the ease with which chloride attacks olefin. One can postulate that carbon monoxide participates in this insertion either as a gas phase reactant or by first forming a carbonyl olefin complex. Such complexes of the noble metals were unknown, but examining the reaction between carbon monoxide and the halogen bridged olefin complexes of platinum revealed that they are formed very readily... 

Heating with phosphoryl chloride converted 1 -hydroxy-3-phenyl-2( 1H )-pyrazinone (52) into the 2,5-dichloro derivative (53) via the 5-monochIoro species. It had been expected that chlorination would take place at C-6, but this occurred only to a minor extent. The observed chloride attack /3 to the oxygen function might be accounted for in terms of the sequence illustrated in Scheme 46 (86JHC149). Reaction mechanisms have been proposed to explain the observed a- and /3-chlorination when 2- and 3-substituted pyrazine Af-oxides are subjected to the Meisenheimer reaction. /3-Chlorination was rationalized in terms of electron withdrawal by the unoxidized nitrogen atom [84JCR(S)318] (Scheme 46). 

Previously proposed mechanisms of the biosynthesis of certain chlorinated compounds have invoked electrophilic bromination of alkenes followed by passive chloride attack [62], Although this mechanism could explain the origin of adjacent brominated and chlorinated carbons, it does not readily account for compounds containing chlorine only. Thus, with the discovery of chloroperoxidase activity of the vanadium enzyme, the origin of specific chlorinated marine natural products can now be addressed. 

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... 

It is clear, however, that other evidence supports the direct attack mechanism and makes a radical chain mechanism likely, at least with alkyl chlorides and bromides. 

Corrosion is often local, with a few centimetres of corrosion and then up to a metre of clean passive bar, particularly for chloride induced corrosion. This indicates the separation of the anodic reaction (2.1) and the cathodic reaction (2.2) to form a macrocell . Chloride induced corrosion gives rise particularly well defined macrocells. This is partly due to the mechanism of chloride attack, with pit formation and with small concentrated anodes being fed by large cathodes. It is also because chloride attack is usually associated with high levels of moisture giving low electrical resistance in the concrete and easy transport of ions so the anodes and cathodes can separate easily. 

In Chapter 2, we discussed the corrosion of steel in concrete and the effectiveness of the alkalinity in producing a passive layer of protective oxide on the steel surface which stops corrosion. In the previous section we observed that alkalinity is neutralized by carbonation. The depassivation mechanism for chloride attack is somewhat different. The chloride ion attacks the passive layer although in this case (unlike carbonation) there is no overall drop in pH. Chlorides act as catalysts to corrosion. They are not consumed in the process but help to break down the passive layer of oxide on the steel and allow the corrosion process to proceed quickly. This is illustrated in... 

We have considered the main mechanisms of corrosion in Chapter 2. We have seen that the chemical process is the same regardless of whether the cause is carbonation or chloride attack as described in Chapter 3. But if we are to perform an effective repair we must fully understand the cause and extent of damage or we risk wasting resources with an inadequate or unnecessarily expensive repair. This chapter explains how to evaluate the condition of corroding reinforced concrete structures. 

The chemical test for this type of corrosion is very straightforward. A Imnp of concrete is freshly broken out from the corroding area and sprayed with a 1% phenolphthalein solution in isopropyl alcohol. If the phenolphthalein remains colorless, then the alkalinity of the concrete has been neutralized. If the phenolphthalein goes purple (i.e., the concrete is sufficiently alkaline) but reinforcement corrosion is observed, then the far more serious problem of chloride attack is likely to be occurring. An explanation of the mechanism of chloride attack follows. 

There are two main causes of corrosion of steel in concrete. This chapter will discuss how chloride attack and carbonation lead to corrosion and how the corrosion proceeds once it has started. The mechanism of corrosion damage is explained. There will also be discussion of the variations that can be found when carrying out investigations in the field. 

Conversion to Acid Haiides (Section 17.8) Acid chlorides, the most common and widely used of the acid halides, are prepared by treating a carboxylic acid with thionyl chloride. The mechanism, similar to that of the conversion of alcohols to chloroalkanes, involves initial chlorosulfite formation, followed by nucleophilic attack of chloride ion on the carbonyl carbon to give a tetrahedral carbonyl addition intermediate, which decomposes to give the acid chloride, SOj, and chloride ion. 

The main causes of corrosion of steel in concrete are chloride attack and carbonation. These two mechanisms are unusual in that they do not attack the integrity of the concrete. Instead, aggressive chemical species pass through the pores in the concrete and attack the steel. This is unlike normal deterioration processes due to chemical attack on concrete. 

Both react through an 8 2 mechanism that includes chloride attack on the protonated hydroxy group. The conversion of phenylmethanol is accelerated relative to that of ethanol because of a delocalized transition state. 

Chloride attacks the highly polarized carbonyl carbon of the chlorosulfite ester. A tetrahedral intermediate forms. It can exist either as an anion, as shown, or it can be protonated by the acid formed in the first step of the mechanism. 

Fn some cases, r-allylpalladium complex formation by retention syn attack) has been observed. The reaction of the cyclic allyiic chloride 33 with Pd(0) affords the 7r-allylpalladium chlorides 34 and 35 by retention or inversion depending on the solvents and Pd species. For example, retention is observed in benzene, THF, or dichloromethane with Pd2(dba)3. However, the complex formation proceeds by inversion in these solvents with Pd(Ph3P)4, whereas in MeCN and DMSO it is always inversion[33]. The syn attack in this case may be due to coordination of Pd to chlorine in 33, because Pd is halophilic. The definite syn attack in complex formation has been observed using stereoche-mically biased substrates. The reaction of the cxoallylic diphenylphosphino-acetate 36 with phenylzinc proceeds smoothly to give 37. The reaction can be explained by complex formation by a syn mechanism[31]. However, these syn attacks are exceptional, and normally anti attack dominates. 

The major difference between the two mechanisms is the second step The second step m the reaction of tert butyl alcohol with hydrogen chloride is the ummolecular dis sociation of tert butyloxonium ion to tert butyl cation and water Heptyloxonium ion however instead of dissociating to an unstable primary carbocation reacts differently It IS attacked by bromide ion which acts as a nucleophile We can represent the transition state for this step as... 

Secondary alkyl halides react by a similar mechanism involving attack on benzene by a secondary carbocation Methyl and ethyl halides do not form carbocations when treated with aluminum chloride but do alkylate benzene under Friedel-Crafts conditions The aluminum chloride complexes of methyl and ethyl halides contain highly polarized carbon-halogen bonds and these complexes are the electrophilic species that react with benzene... 

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