Nucleophilic constants

Empirical measures of nucleophilicity may be obtained by comparing relative rates of reaction of a standard reactant with various nucleophiles. One measure of nucleophilicity is the nucleophilic constant ( ), defined originally by Swain and Scott. Taking methanolysis of methyl iodide as the standard reaction, n was defined as... 

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... 

Table 5.7. Nucleophilic Constants of Various Nuclei hiles" ...

Turning to cation-anion recombination reactions we find that most of the quantitative studies have been by Ritchie,who defined a nucleophilic constant by Eq. (7-71),... 

Electronic absorption studies were performed on compounds 90 and 91 in dimethylformamide (DMF) the longest wavelength maxima at 503-567 nm arising from intramolecular charge transfer (ITC) are shifted bathochromically with an increase in the sum of the nucleophilic constant btt-attached to the fluorene ring. An equation to enable calculation of this effect was devised <2001MM2232>. 

Esters are formed in nucleophilic substitution reactions in which the nucleophile is a carboxylate anion. The anions of carboxylic acids are relatively weak nucleophiles towards sp3-hybridized carbon. Swain s nucleophilic constant, n, for acetate ion is 2.7183, slightly smaller than that for chloride. Thus acetate is selectively alkylated by alkyl halides in aqueous solution, e.g. 

Swain and Scott s (19S3) nucleophilic constants from Hine (1962). Methanolysis ... 

The present procedure illustrates the use of added iodide anion to promote the Mannich cyclization of an alkyne to afford 3-alkylidenepiperidines. As illustrated in Table I a variety of nonbasic nucleophiles with nucleophilic constants T1-CH3I >5.8 are useful promoters of formaldiminium ion-alkyne cyclizations. Piperidines containing both endocyclic and exocyclic allylic unsaturation can be efficiently assembled in this way from readily available alkynol precursors (see Table 1). To the limits of f H NMR detection at 500 MHz all nucleophile-promoted cyclizations that form... 

Successful cyclization reactions of alkynes with weakly electrophilic iminium ions requires the presence of strong external nucleophiles. The only study to date which addresses the nature of useful cyclization promoters indicates that nonbasic nucleophiles with nucleophilic constants T)-Mel > 5.8 are required. ... 

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

A plot of log k lk ) against the nucleophilic constants of Swain and Scott gives a straight line. In calculating the values of kjk (Table 8) from k/k by means of eqn. (26), kjkv, = 229 was chosen in accord with the nucleophilic constant of hydroxide ion. The linear relationship for the various halide ions shows that the relative amounts by which these ions decrease the rate of the basic hydrolysis of chloroform are proportional to their relative nucleophili-cities. 

Table 2.7 lists nucleophilicity constants in water for a number of naturally occurring nucleophiles, values of rate constants for nucleophilic substitution (ky) relative to H2O, and the calculated concentrations, [NucJso, at which the rate constant for the Sn2 displacement by the nucleophile of interest is equal to the rate of 8 2 displacement by H2O (kNuc[Nuc]5oivo = kn2o[H20]). It is readily apparent that as the magnitude for the value of n increases, the concentration of nucleophile needed to compete with hydrolysis decreases. 

The relationship of to the nucleophilic constant n of the Swain-Scott equation appears to be linear. 

The ease with which a given 8 2 displacement occurs depends on multiple factors, such as the nucleophilicity of the incoming nucleophile (which depends on both its electronic and steric character), steric hindrance at the electrophilic carbon center, the effectiveness of the leaving group, and the solvent and other environmental effects. By defining a standard substrate and standard reaction conditions, the reactivity of different nucleophiles may be quantified. One such measure of nucleophilicity is the Swain-Scott nucleophilicity constant n, for which methyl iodide is chosen as the standard substrate and reaction rates are measured in methanol at 25 °C ... 

TABLE 1.1 Swain-Scott Nucleophilicity Constants and Conjugate Acid pK Values of Some Common Nucleophiles ... 

TABLE 1.2 Qualitative Classification of Nucleophiles, Based on the Swaln-Scott Nucleophilicity Constants... 

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