Aminolyses of 4-nitrophenyl phenyl carbonate and thionocarbonate: effect of modification of electrophilic center from C=O to C=S on reactivity and mechanism.
A kinetic study is reported for nucleophilic substitution reactions of 4-nitrophenyl phenyl carbonate (5) and 4-nitrophenyl phenyl thionocarbonate (6) with a series of primary amines. The thiono compound 6 is less reactive than its oxygen analogue 5 toward strongly basic amines but is more reactive toward weakly basic CF3CH2NH2. The Brønsted-type plots obtained from the aminolyses of 5 and 6 are curved downwardly. The reactions are proposed to proceed through a stepwise mechanism with a change in the RDS on the basis of the curved Brønsted-type plots. The microscopic rate constants (k(1) and k(2)/k(-1) ratio) associated with the current aminolyses are consistent with the proposed reaction mechanism. The replacement of the C=O bond in 5 by a polarizable C=S group results in a decrease in the k(1) value but an increase in the k(2)/k(-1) ratio. Besides, such a modification of the electrophilic center causes a decrease in pKa degrees , defined as the pK(a) at the curvature center of curved Brønsted-type plots, but does not alter the reaction mechanism. The larger k(2)/k(-1) ratio for the reactions of 6 compared to those of 5 is proposed to be responsible for the decreased pK(a) degrees value.
Um
Ik-Hwan
IH
Department of Chemistry, Ewha Womans University, Seoul 120-750, Korea. ihum@mm.ewha.ac.kr
Kim
Eun Young
EY
Park
Hye-Ran
HR
Jeon
Sang-Eun
SE
D002254:Carbonates / Q000138:chemical synthesis* / Q000737:chemistry; D015394:Molecular Structure; D009574:Nitro Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D013237:Stereoisomerism; D013438:Sulfhydryl Compounds / Q000737:chemistry*
C516354:4-nitrophenyl phenyl carbonate; D002254:Carbonates; D009574:Nitro Compounds; D013438:Sulfhydryl Compounds