Electrophilic addition, comparison

Addition as a preceding reaction. In comparison with the use of I2, the more reliable results of the two-step electrophilic addition of IC1 in glacial acetic acid with some CC14 (Wijs iodine number for edible oils and fats) according to... 

It is concluded that the selectivities of electrophilic additions are not directly related to the reactivities but to the transition-state positions. Extensive comparison with similar data on the bromination and hydration of other ethylenic compounds bearing a conjugated group shows that this unexpected reactivity-selectivity behaviour can arise from an imbalance between polar and resonance effects (Ruasse, 1985). Increasing resonance in the ground state would make the transition state earlier and attenuate the kinetic selectivity more strongly than it enhances the reactivity. Hydration and halogenation probably respond differently to this imbalance. 

It seems obvious that electron-withdrawing groups enhance nucleophilic addition and inhibit electrophilic addition because they lower the electron density of the double bond. This is probably true, and yet similar reasoning does not always apply to a comparison between double and triple bonds.70 There is a higher concentration of electrons between the carbons of a triple bond than in a double bond, and yet triple bonds are less subject to electrophilic attack and more subject to nucleophilic attack than double bonds.71 This statement is not universally true, but it does hold in most cases. In compounds containing both double and triple bonds (nonconjugated), bromine, an electrophilic reagent, always adds... 

This supposition is supported by the correlation of the rate constants for product formation from metMb, horseradish peroxidase, Cu, Zn SOD, Mn SOD, and GSH (147). Electrophilic addition of HNO is assumed to be the rate-limiting step in these reactions. In comparison, the addition of HNO to NH2OH is substantially slower under neutral conditions 1 01 M 1 s 1 (148)], which may be a function of amine pK.r... 

Electrophilic addition of sulphenyl halides to alkenes occurs, by all the evidence, via cyclic thiiranium ions (Mueller, 1969) and a comparison of the rates of addition to the double and triple bond would be quite interesting. Unfortunately, direct kinetic data for strictly comparable and typical cases are not available. Phenylacetylene has been reported (Kharasch and Yannios, 1964) to react 102 times slower than styrene (in acetic acid at 25°) with 2,4-dinitrobenzenesulphenyl chloride. On the other hand, Thaler (1969), by means of competitive experiments carried out in dilute paraffin solutions at — 70°, estimated that methane-sulphenyl chloride adds to mono- (and di-)alkylacetylenes 50-100 times more slowly than to the corresponding alkenes (cis) (but only ca. twice slower than to trans dialkylethylenes). The paucity of information does not allow generalizations and further work in this area seems desirable also with respect to the much larger rate differences observed in those bromine additions to triple and double bonds which also occur via bridged species. 

Norbomene adds to photolytically produced ethoxycarbonylnitrene specifically at the exo face the same aziridine is produced in the thermal addition of ethoxycarbonyl azide, but via the triazoline rather than the nitrene, with much imine by-product. There can be problems of selectivity and rearrangements when one reacts ethoxycarbonylnitrene with more complex substrates, e.g. alkenic steroids. Ethoxycarbonylnitrene via a-elimination) adds to vinyl chlorides to give 2-chloroaziridines, which can be rearranged thermally to yield 2-chloroallyl carbamates. This nitrene also adds to enamines, giving an array of rearranged products. A modem discussion of the reactivities of ethoxycarbonylnitrene (electrophilic) in comparison with phthalimidonitrene (nucleophilic) towards alkenes of different electronic properties has tqipeared. ... 

Comparison of the free energies of activation for the addition of an electrophile to an alkyne and to an alkene. Because an alkyne is less reactive than an alkene toward electrophilic addition, we know that A6 for the reaction of an alkyne is greater than the AG for the reaction of an alkene. 

Polyphosphazenes with ferrocenyl substituents 35 have also been synthesized via the functionalization of poly-(methylphenylphosphazene) and related polymers by means of a deprotonation-electrophilic addition strategy (e.g., see Equation (10)). This versatile reaction sequence has yielded materials with, for example, degrees of substitution of 45% and 36% for polymers 35 (R = H and Me), respectively. The molecular weights of the polymers were M = Z.O x 10 and 1.5 x 10 for 35 (R = H and Me), respectively (with PDI values of 1.4-2.0). The glass transition temperatures increased in comparison with the unsubstituted polymer (Tg = 37°G) for 35 with values of 92 °C (R = H) and 87 °G (R = Me). 

The plausibility of the rate-limiting formation of III can be assessed by examining the linear free-energy relationship of log k2 and for the epoxidation of substituted styrenes by (BrgTPP)CrV(0)(X)51 and (+-TMP)FeiV(0), 6c which provide a slope of p+ = -1.9. Comparison with known rate-limiting carbocation formations via electrophilic additions to substituted styrenes show greater negative p+ values (-3.58 for hydration and -4.8 for... 

---