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Proton exchange and protonation of substituted oxadiazoles in trifluoroacetic acid
Author(s) -
Puar Mohindar S.,
Cohen Allen I.
Publication year - 1972
Publication title -
organic magnetic resonance
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0030-4921
DOI - 10.1002/mrc.1270040613
Subject(s) - trifluoroacetic acid , protonation , chemistry , proton , reaction rate constant , medicinal chemistry , proton nmr , organic chemistry , kinetics , physics , ion , quantum mechanics
The rates of NHCOOH proton exchange between 5‐amino‐( 1a ) and 5‐ N ‐methylamino‐( 1b )3‐[2‐(5′‐nitro‐2′‐furyl)vinyl]‐1,2,4‐oxadiazoles and trifluoroacetic acid (TFA) have been measured by NMR spectroscopy. The values of the first‐order rate constant and thermodynamic parameters for 1a and 1b , respectively, are: k app (sec −1 ) = 820 and 40 (50°C), ΔF 323 #(kcal/mole) = 14·7 and 16·5, ΔH # (kcal/mole) = 17·3 and 24·3 and ΔS # (e.u.) = 17 and 34. The comparison of rate constants indicates that after correction for proton equivalency proton exchange in 1a is faster than in 1b by a factor of ten. The presence of an NH 2 proton resonance ( 1a ) and an N ‐methyl doublet ( J = 5·0 Hz) between 0 and 30° ( 1b ) suggests that 1a and 1b are present as amines and not as imines in TFA.