Electrophile mechanism

Iodine azide, 1N3, adds to alkenes by an electrophilic mechanism similar to that of bromine. If a monosubstituted alkene such as 1-butene is used, only one product results ... 

As we ve seen, aromatic substitution reactions usually occur by an electrophilic mechanism. Aryl halides that have electron-withdrawing substituents, however, can also undergo nucleophilic aromatic substitution. For example. 2,4,6-trinitrochlorobenzene reacts with aqueous NaOH at room temperature to give 2,4,6-trinitrophenol. The nucleophile OH- has substituted for Cl-. 

Bromination and chlorination of alkanes and cycloalkanes can also take place by an electrophilic mechanism if the reaction is catalyzed by AgSbFs. " Direct chlorination at a vinylic position by an electrophilic mechanism has been achieved... 

Aromatic compounds can also be arylated by aryllead tricarboxylates. Best yields ( 70-85%) are obtained when the substrate contains alkyl groups an electrophilic mechanism is likely. Phenols are phenylated ortho to the OH group (and enols are a phenylated) by triphenylbismuth dichloride or by certain other Bi(V) reagents. O-Phenylation is a possible side reaction. As with the aryllead tricarboxylate reactions, a free-radical mechanism is unlikely. ... 

This species is similar to the bromonium ion that is responsible for stereospecific anti addition in the electrophilic mechanism. Further evidence for the existence of such bridged radicals was obtained by addition of Br- to alkenes at 77 K. The ESR spectra of the resulting species were consistent with bridged structures. ... 

The addition of hydrogen halides to simple alkenes, in the absence of peroxides, takes place by an electrophilic mechanism, and the orientation is in accord with Markovnikov s rule. " When peroxides are added, the addition of HBr occurs by a free-radical mechanism and the orientation is anti-Markovnikov (p. 985). It must be emphasized that this is true only for HBr. Free-radical addition of HF and HI has never been observed, even in the presence of peroxides, and of HCl only rarely. In the rare cases where free-radieal addition of HCl was noted, the orientation was still Markovnikov, presumably beeause the more stable product was formed. Free-radical addition of HF, HI, and HCl is energetically unfavorable (see the discussions on pp. 900, 910). It has often been found that anti-Markovnikov addition of HBr takes place even when peroxides have not been added. This happens because the substrate alkenes absorb oxygen from the air, forming small amounts of peroxides... 

This type of polyhalo alkane adds to halogenated alkenes in the presence of AICI3 by an electrophilic mechanism. This is called the Prins reaction (not to be confused with the other Prins reaction, 16-53). ... 

The observed stereoselectivity with the alkenes utilized in this particular study led the authors to postulate that the reaction proceeded via a synchronous mechanism. However, there was also the possibility of a stepwise electrophilic mechanism involving carbocationic intermediates, as represented in Scheme 15. 

The alternative electrophilic mechanism for the nitration of ESE with TNM requires a close approach of a hindered ESE to a N02 group on the quaternary carbon center of TNM. However, this transition state is sterically very demanding, and it will not readily account for the observed reactivity. Furthermore, the observed lack of regioselectivity in the nitration of the isomeric enol silyl ethers of 2-methylcyclohexanone that leads to the same 2-methyl-2-nitrocyclohexanone (in thermal as well as photochemical nitration) is not readily reconciled by a concerted (electrophilic) mechanism (equation 18). 

The negative value of the p-coefficient means that this reaction proceeds via the electrophilic mechanism. 

The reaction of peroxynitrite with the biologically ubiquitous C02 is of special interest due to the presence of both compounds in living organisms therefore, we may be confident that this process takes place under in vivo conditions. After the discovery of this reaction in 1995 by Lymar [136], the interaction of peroxynitrite with carbon dioxide and the reactions of the formed adduct nitrosoperoxocarboxylate ONOOCOO has been thoroughly studied. In 1996, Lymar et al. [137] have shown that this adduct is more reactive than peroxynitrite in the reaction with tyrosine, forming similar to peroxynitrite dityrosine and 3-nitrotyrosine. Experimental data were in quantitative agreement with free radical-mediated mechanism yielding tyrosyl and nitric dioxide radicals as intermediates and were inconsistent with electrophilic mechanism. The lifetime of ONOOCOO was estimated as <3 ms, and the rate constant of Reaction (42) k42 = 2 x 103 1 mol 1 s 1. 

Aurations, mediated by Au(m), take place at relatively low temperature compared with the corresponding pallada-tion processes. Kinetic products are often obtained via an electrophilic mechanism, displaying high levels of, steric-controlled, regioselectivity (Equation (71)).70... 

Iodine (or IC1, NBS, IpyrgBF promotes similar cyclizations, probably via an electrophilic mechanism (Equation (181)).147 147a... 

For the C-H activation sequence, the different possibilities to be considered are shown in Scheme 5 (a) direct oxidative addition to square-planar Pt(II) to form a six-coordinate Pt(IV) intermediate and (b, c) mechanisms involving a Pt(II) alkane complex intermediate. In (b) the alkane complex is deprotonated (which is referred to as the electrophilic mechanism) while in (c) oxidative addition occurs to form a five-coordinate Pt(IV) species which is subsequently deprotonated to form the Pt(II) alkyl product. 

The redistribution reaction of MeSnCl3 to give Me2SnCl2 and S11CI4 in solution at 50 °C follows second-order kinetics, and is faster in coordinating solvents and in the presence of alcohols or amines, suggesting a nucleophile-assisted electrophilic mechanism.330... 

An electrophilic mechanism is plausible for these reactions, on the basis of the dependence on para-electron-releasing groups and the enhancement by F" ions (forming ArTeCljFj " or Ar2TeCl2F2 species which make the electrophilic attack by halogens easier). 

We have here a mixture of electrophilic and radical addition reactions to alkenes. Remember the guidelines that radical reactions are characterized by the inclusion of radical initiators, such as light or peroxides. In the absence of such initiators, consider only the alternative electrophilic mechanisms. 

Condensation reactions exhibit several characteristics such as (typically) expulsion of a smaller molecule on reaction leading to a repeat unit containing fewer atoms than the sum of the two reactants, and most reactions can be considered in terms of polar (Lewis acid-base nucleophilic-electrophilic) mechanisms. The reaction site can be at the metal atom (that is adjacent to the atom)... 

Addition of bromine to quinolizinium bromide results in formation of the perbromide salt (13). The formation of this salt is reversible, for treatment with an easily brominated solvent like acetone regenerates the original bromide. Van der Plas and coworkers (81H(15)213) have shown that if the perbromide salt (13) is heated at 200 °C it is converted into 1-bromoquinolizinium bromide (Scheme 3). While the high temperature suggests that this may be a radical reaction, the orientation (at a carbon atom of lower positive charge, although not the 3-position predicted above) is consistent with an electrophilic mechanism. 

Henry B, Grant SG, Klopman G, et al. 1998. Induction of forward mutations at the thymidine kinase locus of mouse lyphoma cells evidence for electrophilic and non-electrophilic mechanisms. Mutat Res 397(2) 313-335. 

Bromination and chlorination of alkanes and cycloalkanes can also take place by an electrophilic mechanism if the reaction is catalyzed by AgSbF. "2 Direct chlorination at a vinylic position by an electrophilic mechanism has been achieved with benzenescleninyl chloride PhSe(0)Cl and AIC13 or AIBr3. n However, while some substituted alkenes give high yields of chloro substitution products, others (such as styrene) undergo addition of Cl2 to the double bond (5-26).113 Electrophilic fluorination has already been mentioned (p. 690). 

Aromatic compounds can also be arylated by aryllead tricarboxylates.33 Best yields (—70 to 85%) are obtained when the substrate contains alkyl groups an electrophilic mechanism... 

H2S and thiols add to olefins by electrophilic, nucleophilic, or free-radical mechanisms.192 In the absence of initiators the addition to simple olefins is by an electrophilic mechanism, similar to that in 5-4, and Markovnikov s rule is followed. However, this reaction is usually very slow and often cannot be done or requires very severe conditions unless a proton or Lewis acid catalyst is used. For example, the reaction can be performed in concentrated... 

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