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General‐acid, specific‐base and intramolecular catalysis of the dehydration step in nitrone formation from furfurals and phenylhydroxylamine
Author(s) -
Fett Roseane,
Simionatto Edésio L.,
Yunes Rosendo A.
Publication year - 1990
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.610030911
Subject(s) - chemistry , catalysis , nitrone , reaction rate constant , nucleophile , intramolecular force , acid catalysis , bifunctional , aldehyde , dehydration reaction , medicinal chemistry , rate determining step , organic chemistry , kinetics , cycloaddition , physics , quantum mechanics
Nitrone formation from furfural and 5‐nitrofurfural with phenylhydroxylamine was studied in water at 25°C and ionic strength 1·0 (KCl). The reaction exhibits general acid catalysis (α = 0·52), specific base catalysis and a pH‐independent process. The results are consistent with a mechanism where dehydration of an N , N ′‐dihydroxy addition intermediate, formed by a rapid pre‐equilibrium, is the only rate‐determining step over the pH range studied (1–11). In contrast, nitrone formation from p ‐chlorobenzaldehyde and N ‐methylhydroxylamine exhibits a clear change of rate‐determining step, from dehydration of the addition intermediate to attack of the nucleophile on the aldehyde, with increase in pH. The profile of log k 2 ( k 2 = second‐order rate constant) vs pH indicates, by comparison with the corresponding profile for similar reaction, that phenylhydroxylamine exhibits unexpected nucleophilic behaviour in this reaction. The pH‐indpendent dehydration process, which has a higher rate constant than that predicted from the Brønsted line for the water catalysis, appears to involve intramolecular catalysis rather than bifunctional catalysis by water.