Nucleophilicities of Primary and Secondary Amines in Water

Abstract

The kinetics of the reactions of 26 primary and secondary amines with benzhydrylium ions in water were investigated photometrically. Because the parallel reactions of the benzhydrylium ions with hydroxide and water are much slower, the second-order rate constants for the reactions of amines with benzhydrylium ions could be determined reliably. Reactivities of anilines were also studied in acetonitrile solution. Plots of log k2,N for these reactions vs the electrophilicity parameters E of the benzhydrylium ions were linear, which allowed us to derive the nucleophilicity parameters N and s for amines as defined by the equation log k(20 degrees C)=s(E+N). Because the slope parameters for the different amines are closely similar; the relative nucleophilicities are almost independent of the electrophiles and can be expressed by the nucleophilicity parameters N. The correlation between nucleophilicity N and pKaH values is poor, and it is found that secondary alkyl amines and anilines are considerably more nucleophilic, while ammonia is much less nucleophilic than expected on the basis of their pKaH values.

Leaving-Group Substituent Controls Reactivity and Reaction Mechanism in Aminolysis of Phenyl Y-Substituted-Phenyl Carbonates

“…(10) from pK a o results in Eq. (11). Since k 2 = k -1 at pK a o , the term (log k 2 /k -1 ) pKa o is zero.…”

Limitations of the s(E+N) and Related Equations: Solvent Dependence of Electrophilicity

“…[16] Preferably, [22] there should be a ratio of five data points per independent or explanatory variable, whereas in the multiparameter correlation (MPC1, Scheme 1) required for the initial values of E, 92 parameters are calculated from only 200 data points; a ratio of approximately 3:1 is typical for calculations of N and s N for each nucleophile from plots of log k versus E (N = s N N/s N or intercept/slope). [18][19][20][21]23] Extrapolations are often required to obtain intercepts, thus introducing extrapolation errors arising from the complex substituent effects in benzhydryl substrates [24] and from cross interactions between the electrophile and the incoming nucleophile. [25] The E scale currently spans a range from + 6 to À23, [15] and the lower region refers to neutral substrates.…”

“…[16,26] A key assumption in the design of Equation (1) is that solvent-independent values of E for benzhydrylium cations, reference electrophiles for Equations (1) and (2), can be obtained by a multiparameter correlation (MPC1, Scheme 1) of kinetic data for p C=C nucleophiles (e.g. alkenes) in dichloromethane; [17] it is considered [27,28] to be unlikely that the same E values would apply to the log k versus E plots [18][19][20][21] for other solvents such as water.…”

“…The limitations of Equations (1) and (2) will be illustrated below by correlations of kinetic data for reactions of benzhydrylium cations 1 and 2 [18][19][20][21] and for the methylvinylpyridinium cation 3.…”

“…(1)]. Consequently, published values of s N = 0.6 or 0.7 in water [18] and DMSO [23] are too low, and the substituent effect s N is not simply a nucleophile-specific parameter'. As illustrated below for hydrogen isotope effects, there is a distinction between substrate and solvent effects.…”