Journal of the Chemical Society, Dalton Transactions p. 1805 - 1812 (1994)
Update date:2022-08-04
Topics:
Bellicini, Marcello
Cattalini, Lucio
Marangoni, Giampaolo
Pitteri, Bruno
The kinetics of the forward and reverse steps of the process a number of pyridines and other heterocyclic nitrogen bases covering a wide range of basicity) has been studied in methanol at 25 deg C.Both forward and reverse reactions obey the usual two-term rate law observed in square-planar substitution.The second-order rate constants for the forward reactions, k2f, show only a slight dependence upon the nature of the entering pyridine, and steric hindrance due to the presence of one or two methyl groups in α position to the nitrogen markedly decreases the reactivity.The first- and second-order rate constants for the reverse reaction are very sensitive to the basicity of the leaving group and a plot of log k2r against the pKa of the conjugate acids of unhindered pyridines is linear with a slope of -0.56.Steric retardation for monosubstituted α-methylpyridines is relatively small.The equilibrium constants for these reactions have been determined from the ratio of the rate constants and a plot of log K against the pKa of the unhindered pyridines is linear with a slope of 0.58.The results are compared with data from the literature and discussed in terms of the reaction profile.
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