Nucleophilic species definition

In summary we think that, on a superficial basis, a comparison of the effects of different nucleophilic species added covalently at the (3-nitrogen atom of an arenedi-azonium ion yields results that are almost trivial. Of more interest are unexpected results such as those of Exner and Lakomy for the substituent -N = CHC6H5. A possible explanation for the latter results emerged when the twisted structure of the substituent became known. We emphasize, however, that definitive explanations on the basis of Hammett or related substituent constants are not found very frequently. 

Since nucleophiles, by definition, are species attracted to positive charges and since, by definition, protons are positively charged, nucleophiles are bases. The extent of nucleophilicity associated with a given nucleophile largely depends on the degree of its basicity. Thus, in general terms, the more nucleophilic a given nucleophile, the more basic it is. 

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) ... 

Table 5.7 lists the nucleophilic constants for a number of species according to this definition. It is apparent from Table 5.7 that nucleophilicity toward methyl iodide does not correlate directly with basicity. Azide ion, phenoxide ion, and bromide are all equivalent in nucleophilicity but differ greatly in basicity. Conversely, azide ion and acetate ion are... 

However, the temptation to link glutathione (GSH) adducts to MBI must be avoided since the identification of metabolite adducts with GSH does not confirm MBI, albeit it may be an indicator of other adverse drug reactions (Walgren et al., 2005). By definition, exogenous nucleophiles such as GSH should offer no protection from MBI [161], and in principle, even when GSH adducts are formed from the same chemical entity responsible for MBI, the exogenous adducts themselves may not be related to the enzyme-inactivating species [174]. 

S, I) are more polarizable (they are further away from the nucleus), and since larger soft species have, in general, lower solvation energies, they are better nucleophiles as compared to smaller hard species. (For definition of the term hard and soft nucleophiles, see Box 13.1.) Thus, we can qualitatively understand why, for example, nucleophilicity increases from F to Cl to Br to V (Table 13.3), and why HS- is a stronger nucleophile than OH". 

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... 

Table 4.3 lists the nucleophilic constants for a number of species according to this definition. 

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. 

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