The leaving abilities of pyridines and phenolates from a tetrahedral intermediate formed in the reactions of 2,4-dinitrophenyl methyl carbonate with 3-and 4-substituted pyridines are quantitatively compared through a curved Brqnsted plot obtained from these nucleophilic reactions. Curvature is explained through a tetrahedral intermediate and a change in ratedetermining step from its breakdown to its formation as the basicity of the nucleophile increases; a semiempirical equation reported based on these assumptions gives account of the experimental results. Curvature, centered at pK, -7.8, shows that 2,4-dinitrophenolate (pK, -4) has the same leaving ability as a pyridine of pK, -7.8, i t . , pyridines seem to be ca. lo4 times better leaving groups from a tetrahedral intermediate than phenolates of the same basicity.Leaving abilities of nitrogen and oxygen bases from tetrahedral intermediates can be evaluated from curved Brfnsted plots obtained in nucleophilic reactions, but quantification has been difficult owing to scatter of points in the plots arising from dissimilar nucleophiles employed.' 3-and 4-substituted pyridines offer the advantage of a homogeneous series with a wide range of pK, values, which has proved useful in quantitative interpretation of curved Brfnsted T h e present paper reports a quantitative evaluation of the relative leaving abilities between pyridines and 2,4-dinitrophenolate from the tetrahedral intermediate formed in the nucleophilic reactions of 2,4-dinitrophenyl methyl carbonate with a series of 3-and 4-substituted pyridines (eq 1).
OCH,
Experimental Section2,4-Dinitrophenyl methyl carbonate was prepared according to Pianka,4 mp 87 "C (lit.4 87-88 "C), and identified by N M R spectroscopy. Nicotinamide, p.a., Merck, was used directly. The rest of the pyridines and reagents were purified as previously de~cribed.~ Reactions were followed measuring the increase in 2,4-dinitrophenolate or the corresponding phenol absorbance by means of a Pye-Unicam S P 1800 spectrophotometer. Two to four pH values and several concentrations of total base were used (see Table I). Temperature was kept at 25 f 0.1 "C and the ionic strength adjusted to 0.2 M with NaC104. Good pseudo-first-order rate constants ( k o b s d ) were obtained by the "infinity" method (at least 20-fold excess of total base over substrate was used). Reactions were followed for at least 3 half-lives and infinity absorbance values taken after 8-10 halflives.2,4-Dinitrophenolate was identified as a stable product of the reactions by comparison of its UV spectra with those of authentic samples. The existence of a methoxycarbonylpyridinium intermediate was inferred from the observation of a decrease in absorbance of the spectra obtained by repetitive scanning between 270 and 400 nm, after completion of the reactions of 2,4-dinitrophenyl methyl carbonate with 4-dimethylamino-and 4-methylpyridines under conditions such that the formation of the intermediate was faster than its hydrolysis. The rate constants for hydrolysis of these substitute...
