Mechanistic assessment of S(N)Ar displacement of halides from 1-halo-2,4-dinitrobenzenes by selected primary and secondary amines: Brønsted and Mayr analyses.

Pseudo-first-order rate constants (k(obsd)) have been measured spectrophotometrically for nucleophilic substitution reactions of 1-X-2,4-dinitrobenzenes (1a-d, X = F, Cl, Br, I) with various primary and secondary amines in MeCN and H(2)O at 25.0 ± 0.1 °C. The plots of k(obsd) vs [amine] curve upward for reactions of 1a (X = F) with secondary amines in MeCN. In contrast, the corresponding plots for the other reactions of 1b-d with primary and secondary amines in MeCN and H(2)O are linear. The Brønsted-type plots for reactions of 1a-d with a series of secondary amines are linear with ß(nuc) = 1.00 for the reaction of 1a and 0.52 ± 0.01 for those of 1b-d. Factors governing reaction mechanisms (e.g., solvent, halogen atoms, H-bonding interactions, amine types) have been discussed. Kinetic data were also analyzed in terms of the Mayr nucleophilicity parameter for the amines with each aromatic substrate. Provisional Mayr electrophilicity parameter (E) values for 1-X-2,4-dinitrobenzenes have been determined: E = -14.1 for X = F, E = -17.6 for X = Cl and Br, and E = -18.3 for X = I. These values are consistent with the range and order of E values for heteroaromatic superelectrophiles and normal 6-π aromatic electrophiles.

Pitfalls in assessing the α-effect: reactions of substituted phenyl methanesulfonates with HOO(-), OH(-), and substituted phenoxides in H(2)O.

Toward resolving the current controversy regarding the validity of the α-effect, we have examined the reactions of Y-substituted phenyl methanesulfonates 1a-1l with HOO(-), OH(-), and Z-substituted phenoxides in the gas phase versus solution (H(2)O). Criteria examined in this work are the following: (1) Brønsted-type and Hammett plots for reactions with HOO(-)and OH(-), (2) comparison of ß(lg) values reported previously for the reactions of Y-substituted phenyl benzenesulfonates 2a-2k with HOO(-) (ß(lg) = -0.73) and OH(-) (ß(lg) = -0.55), and for those of 1a-1l with HOO(-) (ß(lg) = -0.69) and OH(-) (ß(lg) = -1.35), and (3) Brønsted-type plot showing extreme deviation of OH(-) for reactions of 2,4-dintrophenyl methanesulfonate 1a with aryloxides, HOO(-), and OH(-), signifying extreme solvation vs different mechanisms. The results reveal significant pitfalls in assessing the validity of current interpretations of the α-effect. The extreme negative deviation by OH(-) must be due, in part, to the difference in their reaction mechanisms. Thus, the apparent dependence of the α-effect on leaving-group basicity found in this study has no significant meaning due to the difference in operating mechanisms. The current results argue in favor of a further criterion, i.e., a consistency in mechanism for the α-nucleophiles and normal nucleophiles.

Nonlinear Hammett plots in pyridinolysis of 2,4-dinitrophenyl X-substituted benzoates: change in RDS versus resonance contribution.

Second-order rate constants (k(OH)-) have been measured for nucleophilic substitution reactions of 2,4-dinitrophenyl X-substituted benzoates (1a-j) with Z-substituted pyridines in 80 mol% H(2)O/20 mol% DMSO at 25.0 +/- 0.1 degrees C. The Hammett plots for the reactions of 1a-j with pyridines consist of two intersecting straight lines, i.e., a large rho value for the reactions of substrates (1a-c) possessing an electron-donating group (EDG) in the benzoyl moiety and a small one for substrates (1e-j) bearing an electron-withdrawing group (EWG). The nonlinear Hammett plots have been attributed to stabilization of the ground state of substrates 1a-c through resonance interactions between the electron-donating substituent and the carbonyl functionality, since the corresponding Yukawa-Tsuno plots exhibit excellent linear correlations with large r values. It has been shown that substrates are not unusually more reactive than would be expected from the Hammett substituent constants, but rather, substrates 1a-c exhibit lower reactivity than would be predicted. The Brønsted-type plots for pyridinolysis of 1a-j are linear with beta(nuc) = 0.74-0.98, indicating that the reaction proceeds through a stepwise mechanism in which the second step is the RDS. It has been concluded that the electronic nature of the substituent X in the benzoyl moiety does not influence the RDS, but the degree of bond formation (or the effective charge on the nucleophilic site) in the transition state becomes more significant as the substituent X changes from a strong EDG to a strong EWG.