Nucleophilic strength table

As we have seen the nucleophile attacks the substrate m the rate determining step of the Sn2 mechanism it therefore follows that the rate of substitution may vary from nucleophile to nucleophile Just as some alkyl halides are more reactive than others some nucleophiles are more reactive than others Nucleophilic strength or nucleophilicity, is a measure of how fast a Lewis base displaces a leaving group from a suitable substrate By measuring the rate at which various Lewis bases react with methyl iodide m methanol a list of then nucleophihcities relative to methanol as the standard nucleophile has been compiled It is presented m Table 8 4... 

Nucleophile strength increases down a column of the periodic table (in solvents that can hydrogen bond, such as water and alcohols). For example,... 

The nucleophilic strength of anions, within the same row of the periodic table, follows the same order as basicity the more electronegative... 

The nucleophilic strength of anions and neutral atoms increases on going down a group of the periodic table. The electrons are held less tightly to the nucleus as the atom size increases, and hence they are more available for forming bonds. Larger atoms, with more loosely held electrons (than smaller atoms), are said to have a higher polarisability. 

Table 2.12. Nucleophilic Strength with Bromoethane as Ordered by the Swain-Scott Equation...

Nucleophilic strength for a given substituent can be measured in terms of the rate of the Sn2 reaction or in reactions with carbonyl derivatives. The relative rates of several nucleophiles were determined by reaction with iodomethane and are shown in Table 2.13. As mentioned previously, several factors contribute to nucleophilic strength. Electronic effects are important, as illustrated by the electron releasing methyl group, which should make methoxide more nucleophilic than hydroxide. The rate of the Sn2 reaction of sodium hydroxide with iodomethane is 1.3 x lO" M s whereas the rate with sodium methoxide with iodomethane is 2.51 x 1Q2 M- s-1.98... 

In platinum(II) substitution reactions, however, the order of nucleophilic strength is I" > Br > Cl and is independent of solvent. This is clearly shown by the results in Table 6.3.1 which compares values of ky for the reaction ... 

Except for F, leaving group trends in Table 13.1 exactly follow predictions based on pKa data. Nucleophile strength also follows pKa, but there are some important exceptions. Cite at least four species in Table 13.2 that are expected to be poor nucleophiles based on pKa data, but instead are good or very good nucleophiles. 

Table 2.7.2. Nucleophilic strength and polarizability of some entering ligands " ...

There is also evidence for stable 3,4-adducts from the X-ray analysis of 2-amino-4-ethoxy-3,4-dihydropteridinium bromide, the nucleophilic addition product of 2-aminopteridine hydrobromide and ethanol (69JCS(B)489). The pH values obtained by potentiometric titration of (16) with acid and back-titration with alkali produces a hysteresis loop, indicating an equilibrium between various molecular species such as the anhydrous neutral form and the predominantly hydrated cation. Table 1 illustrates more aspects of this anomaly. 2-Aminop-teridine, paradoxically, is a stronger base than any of its methyl derivatives each dimethyl derivative is a weaker base than either of its parent monomethyl derivatives. Thus the base strengths decrease in the order in which they are expected to increase, with only the 2-amino-4,6,7-trimethylpteridine out of order, being more basic than the 4,7-dimethyl derivative. 

In this section three main aspects will be considered. Firstly, the basic strengths of the principal heterocyclic systems under review and the effects of structural modification on this parameter will be discussed. For reference some pK values are collected in Table 3. Secondly, the position of protonation in these carbon-protonating systems will be considered. Thirdly, the reactivity aspects of protonation are mentioned. Protonation yields in most cases highly reactive electrophilic species. Under conditions in which both protonated and non-protonated base co-exist, polymerization frequently occurs. Further ipso protonation of substituted derivatives may induce rearrangement, and also the protonated heterocycles are found to be subject to ring-opening attack by nucleophilic reagents. 

The tendency for N-nitrosamides to undergo hydrolysis by a nucleophilic catalysed pathway has been confirmed by studies of N-alkylnitroso acetamides (22) Results summarised in Table I for N -n-butyl-JJ -nitroso acetamide show that its decomposition is also subject to steric constraints (2,6-lutidinestrong nucleophiles (eg. imidazole, thiols) irrespective of their base strength (pK ). Further, the second order dependence on [Imidazole] is more clearly defined for the decomposit-... 

However, the behavior of the local descriptors of reachvity is somewhat different. The local quantities are calculated using the Lowdin-based method of population analysis [47], It was observed that when the electric held is applied toward the more electronegative atom of the systems HF and HCN, the value of its nucleophilicity (CFF for electrophilic attack) decreases with increasing held strength (Figure 25.2c). The variation of FF can be well supported with the variation in the induced dipole moment, given in Table 25.1, of the species in... 

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