Chloride attack

Chlorides can come from several sources. They can be cast into the concrete or they can diffuse in from the outside. Chlorides cast into 

Source Parrott, LJ.A Review of Carbonation in Reinforced Concrete. A review carried out by the Cement and Concrete Association under a BRE Contract PuM. BRE Garston,Watford, UK 1987, Jul. 

Much of our discussion will centre on the diffnsion of chlorides into concrete as that is the major problem in most parts of the world either dne to marine salt spray or use of deicing salts. However, the cast in chlorides mnst not be overlooked especially when they are part of the problem. This often happens in marine conditions where sea water contaminates the original concrete mix and then diffnses into the hardened concrete. 

Like carbonation, the rate of chloride ingress is often approximated to Pick s law of diffusion. There are further complications here. The initial mechanism appears to be suction, especially when the surface is dry, that is, capillary action. Salt water is rapidly absorbed by dry concrete. There is then some capillary movement of the salt laden water throngh the pores followed by true diffusion. There are other opposing mechanisms that slow the chlorides down. These include chemical reaction to form chloroaluminates and absorption onto the pore surfaces. The detailed transport mechanisms of chloride ions into concrete are discussed in Kropp and Hilsdorf (1995). 

The different mechanisms of chloride transport into concrete. 

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Carbocations usually generated from an alkyl halide and aluminum chloride attack the aromatic ring to yield alkylbenzenes The arene must be at least as reactive as a halobenzene Carbocation rearrangements can occur especially with primary alkyl hal ides... 

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. 

Some alicyclic 1,2-diamine derivatives have recently been shown to have interesting CNS properties. For example, eclanamine (34) is an antidepressant with a rapid onset of action. The reasons for its potency are not as yet clear but pharmacologists note that the drug desensitizes adrenergic alpha-2 receptors and antagonizes the actions of clonidine. The synthesis of eclanamine starts with attack of cyclopentene oxide (30) by dimethylamine (to give 31). This product is converted to the mesylate by reaction with sodium hydride followed by mesyl chloride. Attack of... 

Pitting Corrosion Pitting corrosion is a general term for a visible sign of concentration cell corrosion, where further induced-corrosion processes develop, as when chloride attack occurs (although pits also may occur with simple acid corrosion processes). 

In the absence of solvent participation, chloride attack on the chlorosulfite ester leads to product with inversion of configuration. 

If the potential leaving group is attached to unsaturated carbon, as in vinyl chloride or phenyl chloride, attack by nucleophiles is also extremely difficult, and these compounds are very unreactive in Sn2 reactions compared with simple alkyl halides. In these cases, the reason is not so much steric but electrostatic, in that the nucleophile is repelled by the electrons of the unsaturated system. In addition, since the halide is attached to carbon through an 5p -hybridized bond, the electrons in the bond are considerably closer to carbon than in an 5/ -hybridized bond of an alkyl halide (see Section 2.6.2). Lastly, resonance stabilization in the halide gives some double bond character to the C-Hal bond. This effectively strengthens the bond and makes it harder to break. This lack of reactivity is also tme for SnI reactions (see Section 6.2). 

Mixed inhibitors may simultaneously affect both anodic and cathodic processes. A mixed inhibitor is usually more desirable because its effect is all-encompassing, covering corrosion resulting from chloride attack as well as that due to microcells on the metal surface. Mixed inhibitors contain molecules in which electron density distribution causes the inhibitor to be attracted to both anodic and cathodic sites. They are aromatic or olefinic molecules with both proton-forming and electron acceptor functional group such as NH2 or SH, as in... 

The most widely used anodic inhibitors are calcium and sodium nitrite, sodium benzoate and sodium chromate. With the exception of calcium nitrite, no other chemical is available in North America as a proprietary product. Nitrites have been used in the USA for more than 14 years and for nearly 40 years in Europe. Calcium nitrite is marketed as a non-chloride accelerator, as well as a corrosion inhibitor. For 25-30% solids in solution, dosage rates range from 2 to 4% by weight of cement depending on the application [50]. Calcium nitrite has been used in bridges, parking and roof decks, marine and other prestressed concrete structures that are exposed to chloride attack. 

In the acetylation of carbazole using dimethylacetamide-phosgene in methylene chloride, attack was shown to occur mainly at nitrogen (93 7, N C-3) trifluoroacetylation proceeded exclusively at nitrogen. Carbazole was more reactive in this sense than indole and much more reactive than pyrrole, a result paralleling their respective basicities at nitrogen (see Section II,A,3). N-Acylation under these conditions is thought to involve... 

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... 

By consuming the Ca(OH)2 the pozzolan makes it unavailable for sulphation and carbonation reactions and increases the density of the concrete, which lowers its permeability and reduces the risk of chloride attack. 

Aryl sulfenyl chlorides attack the acetylacetonate ring without catalysis. The resulting aryl sulfide chelates (XV) are unusual in that they form remarkably stable clathrate complexes with benzene (26). 

Addition of HC1 to 1,3-butadiene in the gas phase was shown to be a surface-catalyzed reaction occurring at the walls between a multilayer of adsorbed HC1 and gaseous or weakly adsorbed 1,3-butadiene.131 The initial proton transfer to the terminal carbon and the chloride attack occur almost simultaneously. Chloride... 

Sulphuryl chloride is without action on the dioxide, but thionyl chloride attacks it on warming, according to the equation 9... 

Anilide 8 is transformed under Vihineitr conditions into the quinoline derivative19 9. In the process, phosphoryl chloride attacks the carbonyl group of anilide 8 and initially forms inline 24, which is in equilibrium with the corresponding enamine 25. 

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. 

The first step is an allylic chloride displacement on a chlorosulfite ester generated from the tertiary vinyl carbinol and thionyl chloride. Attack of chloride on an allylic carbocation intermediate can equally well be envisaged. The subsequent conversion is simply an S 2 displacement by acetate ion (Scheme 10.6). 

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