Electrophilic species definition

The proposal of the general-acid catalysed expulsion of the leaving group (28) raises the question of possible catalysis by other electrophilic species. No definitive case of catalysis by metal ions has been reported (Lam and Miller, 1966 Giles and Parker, 1970), which may be a consequence of competition from the uncatalysed step which is believed to occur via the entropically favoured transition states [13] or [14] (Kirby andjencks, 1965 Bemasconi, 1973). 

The electrophilic attack of nitric oxide on a bent nitrosyl is now realized to be the path by which hyponitrite-bridged Co species are formed. Reaction (93) was known since the time of Werner (217), but the black and red isomers of [Co(NO)(NH3)5]2+ obtained from this reaction defied definitive characterization for many years. It has now been established that the black isomer is a mononuclear, octahedral complex of Co(III) and NO- (218) while the red isomer is a hyponitrite bridged system containing two Co(III) ions (219). 

Although no definitive study of scavengers in photoaffinity labeling experiments has yet been made, the present indications are that thiols are the most suitable reagents. It is expected that as strong nucleophiles they should react with most photogenerated species which are electrophiles as well as with free radicals which can abstract hydrogen atoms from the... 

From Fig. 1 a value of g = -(-1.05 0.05 is derived, demonstrating definitely that 4 c is a nucleophilic species jhis beautiful result justifies the calculations and considerations described on p. 105. For comparison, the g value for the electrophilic ICCI2 is somewhat smaller but as expected, it shows the opposite sign (g = — 0.61) 2). Unfortunately, no simiUar data exist for 7. There is, however, some hint that 7 also has nucleophilic properties 7 does not add to electron-rich olefins but with electron-deficient olefins, such as dimethyl-fumarate, reacts very smoothly to afford the highly strained spirocyclopentenes 38 22-24) ... 

Adds and electrophiles are electron-deficient species. According to the Lewis concept, all electrophiles (e.g., cations, carbenes, metal ions) are acids by definition. However, from long usage the term acid is frequently used to refer to a proton donor, whereas the term Lewis acid usually refers to charged electrophiles in general. 

Structure was definitely established for two crystalline lithium enolates and is strongly indicated for several others by dieir stoichiometry. In aprotic solvents only aggregated species are detected by NMR spectroscopy even during reactions with electrophiles these aggregates are preserved and appear to be tte actual reacting species, as indicated by reaction rates, which are first-order and not broken-order in enolate concentration. (R roduced by permission of the Swiss Chemical Society fiom Helv. Chim. Acta, 1981,64,2617.)... 

In view of the fact that in this low-potential region the surface is predominantly covered by CO with very small coverage of the electron acceptor surface oxygen species, this L-H reaction is, by definition, electrophilic and is thus expected to be promoted by electron donor species, such as Na+ present at the OHP, as experimentally observed [117]. 

There is clearly a conceptual relationship between the properties called nucleophilicity and basicity. Both describe a process involving formation of a new bond to an electrophile by donation of an electron pair. The pKa values in Table 5.5 refer to basicity toward a proton. There are many reactions in which a given chemical species might act either as a nucleophile or as a base. Scheme 5.4 lists some examples. It is therefore of great interest to be able to predict which chemical species Y " will act as nucleophiles and which will act as bases under a given set of circumstances. The definition of basicity is based on the ability of a substance to remove protons and refers to an equilibrium. 

Thus, there has been interest concerning whether it is the former or the latter species actually affecting the phenol oxygenation. And in fact. Stack, Solomon, and co-workers reported on a system where they could definitively show that phenol o-hydroxylation occurs from a bis-(/a-oxo)-dicopper(III) complex via an electrophilic mechanism (Fig. 3) (60). Their chemical-spectroscopic and recent theoretical (63) studies show that the binding of a phenolate substrate to a copper ion leads to 0—0 bond cleavage in an initially formed f-peroxo-dicopper(II) complex, generating the active Cu(lll)2-(0)2 moiety (Fig. 3) (60). Casella (64) also more recently reported on a similar finding. 

While the Bronsted acid/base terms specifically refer to proton donors and acceptors, respectively, the Lewis approach (named after G. N. Lewis, who introduced the idea in 1923) greatly broadens the definitions of what is an acid and what is a base. Recall that a Lewis acid is an electron pair acceptor and a Lewis base is an electron pair donor. All common organic reactions that do not involve radicals or concerted pericyclic processes can in some manner be discussed as Lewis acid-base reactions. Similarly, all these reactions can be considered to be occurring between electrophiles and nucleophiles. Recall that an electrophile is any species seeking electrons and a nucleophile is any species seeking a nucleus (or positive charge) toward which it can donate its electrons. In this context, a Lewis base is synonymous with a nucleophile, and a Lewis acid is synonymous with an electrophile it just de-... 

In the brief guidelines given above for what makes a good nucleophile and electrophile, we touched on the energy and accessibility of the electrophilic and nucleophilic orbitals. This brings us to another related concept, that of "hard" and "soft" acids and bases. In this definition, the acids and bases are best viewed as being of the Lewis type. Here we examine the "hardness" and "softness" of the acid and base to predict reactivity. In this analysis, the character of a nucleophile or electrophile is most often correlated with the polarizability of the species hard reactants are non-polarizable, whereas soft reactants are polarizable. The... 

The chemical potential, chemical hardness and softness and reactivity indices have been used by a number of workers to assess a priori the reactivity of chemical species from their intrinsic electronic properties. The concept of electrophilicity has been known for several decades, although there has not been a rigorous definition of it until recently, Parr et al. [39] proposed a definition did they inspired by the experimental findings of Maynard et al. [40]. The revolution begins, with this simple index which has the ability to connect the major facets of chemical sciences. 

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