The reactions of methyl 4-nitrophenyl carbonate (MNPC) with a series of secondary alicyclic amines (SAA) and quinuclidines (QUIN), methyl 2,4-dinitrophenyl carbonate (MDNPC) with QUIN and 1-(2-hydroxyethyl)piperazinium ion (HPA), and phenyl 2,4-dinitrophenyl carbonate (PDNPC) with SAA are subjected to a kinetic investigation in aqueous solution, at 25.0 degrees C and an ionic strength of 0.2 M. By following spectrophotometrically the nucleofuge release (330-400 nm) under amine excess, pseudo-first-order rate coefficients (k(obsd)) are obtained. Plots of k(obsd) vs [amine] at constant pH are linear, with the slope (k(N)) being pH independent. The Brönsted-type plot (log k(N) vs amine pK(a)) for the reactions of SAA with MNPC is biphasic with slopes beta(1) = 0.3 (high pK(a) region) and beta(2) = 1.0 (low pK(a) region) and a curvature center at pK(a)(0) = 9.3. This plot is consistent with a stepwise mechanism through a zwitterionic tetrahedral intermediate (T(+/-)) and a change in the rate-determining step with SAA basicity. The Brönsted plot for the quinuclidinolysis of MNPC is linear with slope beta(N) = 0.86, in line with a stepwise process where breakdown of T(+/-) to products is rate limiting. A previous work on the reactions of SAA with MDNPC was revised by including the reaction of HPA. The Brönsted plots for the reactions of QUIN and SAA with MDNPC and SAA with PDNPC are linear with slopes beta = 0.51, 0.48, and 0.39, respectively, consistent with concerted mechanisms. Since quinuclidines are better leaving groups from T(+/-) than isobasic SAA, yielding a less stable T(+/-), it seems doubtful that the quinuclidinolysis of PDNPC is stepwise, as reported.

In this paper we describe a method to obtain estimates of the relative nucleophilicity for a series of neutral and charged electron donors from their solution phase ionization potential (I s ). The relationship between nucleophilicity and the solution phase ionization potentials is first tested for experimental I s values in aqueous solution. On the basis of the meaningful relationship found, the method is then applied to the theoretical solution phase I s obtained at the IPCM-MP2/6-311G(2d,p) level of theory. The comparison between the experimental nucleophilicity as given by Ritchie's N+ scale and the solution phase ionization energy for a series electron donors split out into two families: a first group of marginal and moderate nucleophiles that mainly contains atoms of the first row (H 2 O, NH 2 CONHNH 2 , CF 3 CH 2 NH 2 , NH 3 , CH 3 ONH 2 , NH 2 OH and CH 3 O -), with nucleophilicity number N+ < 6.0; a second group of strong nucleophiles, mainly including second-row sulfur atom (CH 3 CH 2 S -, CH 3 CH 2 CH 2 S -, OHCH 2 CH 2 S -, C 6 H 5 S -) and the first-row electron donors piperidine and morpholine, with nucleophilicity number N+ > 7.0. An approximate expression for a local nucleophilicity index is proposed. The results show that the nucleophilicity power of the electron donors is consistently shown at the expected nucleophilic sites in these molecules. The solvent effect on the predicted nucleophilicity is also discussed.

The reactions of 4-methylphenyl 4-nitrophenyl carbonate (MPNPC) and 4-methylphenyl 2,4-dinitrophenyl carbonate (MPDNPC) with a series of secondary alicyclic amines are subjected to a kinetic investigation in 44 wt% ethanol-water, at 25.0 • C, ionic strength 0.2 M (KCl). Under amine excess over the substrate, pseudo-first-order rate coefficients (k obs ) are obtained. Plots of k obs against [amine] are linear, with k N as slopes. A biphasic Brönsted-type plot for k N is obtained for the aminolysis of MPNPC, with slopes β 1 = 0.2 (high pK a ) and β 2 = 0.9 (low pK a ). This is in accordance with a stepwise mechanism, through a zwitterionic tetrahedral intermediate (T ± ), and a change in the rate-determining step, from formation to breakdown of T ± as the amine pK a decreases. For the aminolysis of MPDNPC, a slightly curved Brönsted-type plot for k N is obtained, with β 1 = 0.1 (low pK a ) and β 2 = 0.55 (high pK a ). This is consistent with a concerted mechanism.