Mechanisms electrophilic addition

Direct Chlorination of Ethylene. Direct chlorination of ethylene is generally conducted in Hquid EDC in a bubble column reactor. Ethylene and chlorine dissolve in the Hquid phase and combine in a homogeneous catalytic reaction to form EDC. Under typical process conditions, the reaction rate is controlled by mass transfer, with absorption of ethylene as the limiting factor (77). Ferric chloride is a highly selective and efficient catalyst for this reaction, and is widely used commercially (78). Ferric chloride and sodium chloride [7647-14-5] mixtures have also been utilized for the catalyst (79), as have tetrachloroferrate compounds, eg, ammonium tetrachloroferrate [24411-12-9] NH FeCl (80). The reaction most likely proceeds through an electrophilic addition mechanism, in which the catalyst first polarizes chlorine, as shown in equation 5. The polarized chlorine molecule then acts as an electrophilic reagent to attack the double bond of ethylene, thereby faciHtating chlorine addition (eq. 6) ... 

The most common method of epoxidation is the reaction of olefins with per-acids. For over twenty years, perbenzoic acid and monoperphthalic acid have been the most frequently used reagents. Recently, m-chloroperbenzoic acid has proved to be an equally efficient reagent which is commercially available (Aldrich Chemicals). The general electrophilic addition mechanism of the peracid-olefin reaction is currently believed to involve either an intra-molecularly bonded spiro species (1) or a 1,3-dipolar adduct of a carbonyl oxide, cf. (2). The electrophilic addition reaction is sensitive to steric effects. 

Double bonds in a,/3-unsaturated keto steroids can be selectively oxidized with alkaline hydrogen peroxide to yield epoxy ketones. In contrast to the electrophilic addition mechanism of peracids, the mechanism of alkaline epoxidation involves nucleophilic attack of hydroperoxide ion on the con-... 

HC1, HBr, and HI add to alkenes by a two-step electrophilic addition mechanism. Initial reaction of the nucleophilic double bond with H+ gives a carbo-cation intermediate, which then reacts with halide ion. Bromine and chlorine add to alkenes via three-membered-ring bromonium ion or chloronium ion intermediates to give addition products having anti stereochemistry. If water is present during the halogen addition reaction, a halohydrin is formed. 

Note that if we choose not to put in all the curly arrows, we could write the mechanism in two ways either considering the radical as the attacking species or the double bond as the electron-rich species. The first version is perhaps more commonly used, but it is much more instmctive to compare the second one with an electrophilic addition mechanism (see Section 8.1). The rationalization for the regiochemistry of addition parallels that of carbocation stability (see Section 8.2). 

Alkynes react with bromine via an electrophilic addition mechanism. A bridged bromonium ion intermediate has been postulated for alkyl-substituted acetylenes, while vinyl cations are suggested for aryl-substituted examples.119 1-Phenylpropyne gives mainly the anti addition product in acetic acid, but some of the syn isomer is formed.120 The proportion of dibromide formed and stereoselectivity are enhanced when lithium bromide is added to the reaction mixture. 

In contrast, reaction of electron acceptor-substituted phenols exhibits p = +1.72, indicating the development of negative charge at the phenolic oxygen in the rate-determining step for reaction of relatively acidic, weakly nucleophilic phenols with 110, and the addition of 112f exhibits a large primary deuterium kinetic isotope effect of k /kv = 5.3. This is consistent with the electrophilic addition mechanism of equation 85, in which full or partial protonation at silicon precedes nucleophilic attack. 

The appearance of these products supports the two-step electrophilic addition mechanism, in which an intermediate carbocation is formed. 

With CF3C=CCF3 or olefins with an internal double bond such as cis- or fraras-F-2-pentene, F-cyclobutene, or l,2-dichloro-l,2-difluoro-ethylene, or with a geminally disubstituted olefin such as 1,1-dichloro-2,2-difluoroethylene, only fully fluorinated products were obtained, likely through a nucleophilic fluorination mechanism. This fact, plus the necessity for a Lewis acid catalyst for addition to proceed, is evidence for an electrophilic addition mechanism. Others have suggested polar addition of BF3 to the olefin with RfBF2 as a reactive intermediate (294). 

Many of the addition reactions listed in Review Table 1 take place by an electrophilic addition mechanism. The electrophile can be H, X, Hg, but the basic process is the same. The remaining addition reactions in Review Table 1 occur by other mechanisms. 

With the exception of hydrogenation, the addition reactions of alkenes presented in this text occur by an electrophilic addition mechanism. The electrophile (H+ or X+) attacks the electron-rich pi-bond of the double bond. The pi electrons are used to form a single bond between the carbon and attacking species the other carbon becomes a carbocation. The carbocation is then neutralized by halide ion or water the addition is complete. In bromination reactions, the bromine adds in a trans fashion. 

Electrophilic nitrogen, in the forms of NO2, NO+, or ArN., has been well authenticated in aromatic substitution. However, while there is evidence for electrophilic addition mechanisms involving nitrogen , very little kinetic work has been carried out on such systems. Of the two which have been studied, the addition of 2A-dinitmbenzenediazonium ion to olefins seems to... 

Electrophilic addition Mechanism of addition in which the species that first reacts with the multiple bond is an electrophile ( electron seeker ). 

SAM alkylates alkenes by an electrophilic addition mechanism, adding a single carbon atom (Cj) and forming an intermediate carbocation (Figure 1.14a). 

Electrophilic Addition to Aromatics. Aromatic compounds are fluorinated with 1 by heating without a solvent to about 100-150 °C, or, for low-boiUng compounds, up to reflux temperature. Activated aromatics, such as anlsole or N-acetylaniline, react smoothly, whereas toluene requires considerably longer reaction times and gives low yields (eqs 9 11). In all cases, the regioselectivities are consistent with an electrophilic addition mechanism ortho- and para-substituted products predominate over the meta-isomers. 1 has recently been used in orthometalation-directed fluorination of aromatics. ... 

Electrophilic addition Mechanism of addition in which the species... 

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