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Effect of solvent on reactivity and basicity: aminolyses of p ‐nitrophenyl acetate in H 2 O and in DMSO
Author(s) -
Um IkHwan,
Lee EunJu,
Jeon SangEun
Publication year - 2002
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.483
Subject(s) - chemistry , piperidine , reactivity (psychology) , reaction rate constant , solvent , medicinal chemistry , alicyclic compound , dimethyl sulfoxide , solvent effects , stereochemistry , kinetics , organic chemistry , medicine , physics , alternative medicine , pathology , quantum mechanics
Second‐order rate constants ( k N ) have been measured spectrophotometrically for the reactions of p ‐nitrophenyl acetate with a series of alicyclic secondary amines in H 2 O and in DMSO at 25.0 ± 0.1 °C. The solvent change from H 2 O to DMSO resulted in rate enhancements. The effect of solvent on reactivity was found to be most significant for the reaction with piperazinium ion and least significant for the reaction with piperidine. The p K a values of the conjugate amines studied were also determined in DMSO. It was found that piperazinium ion is more basic in DMSO than in H 2 O by 1.04 p K a units while piperidine is less basic in DMSO by 0.50 p K a units. The Brønsted‐type plot was linear with a large slope (β nuc = 0.76–0.82) for both reactions run in H 2 O and in DMSO, indicating that the aminolyses proceed through rate‐determining breakdown of the addition intermediate. The difference in p K a values determined in DMSO and in H 2 O (Δp K a = p K a in DMSO − p K a in H 2 O) showed a linear correlation with the difference in the second‐order rate constants determined in DMSO and in H 2 O (Δlog k N = log k N in DMSO − log k N in H 2 O) with a slope close to unity, suggesting that Δp K a is fully responsible for the rate enhancement in DMSO. Copyright © 2002 John Wiley & Sons, Ltd.