Kinetics and Mechanism of the Pyridinolysis of Phenyl Chloroformates in Acetonitrile

Abstract

Kinetic studies on the reactions of Y-phenyl chloroformates with X-pyridines in acetonitrile are carried out at 25.0 °C. Both the Hammett and Brönsted plots are linear with enhanced substituent constants, σp -, and basicities, pK a -, for strong para π-acceptor X-substituents, p-CN and p-CH3CO. This indicates that the electron-rich formate (O−C−O) moiety overlaps with the pyridine ring π-system enabling, through conjugation with the para π-acceptors, the rate-limiting formation of a tetrahedral intermediate. The difference in the aminolysis mechanism between methyl, II, and phenyl chloroformate, III, is attributed to the much stronger electron-donating polarizability effect of C6H5 than of CH3. The proposed mechanism is supported by a relatively small βX (≅0.3) and by the lower ΔH ⧧ (6.7 kcal mol-1) and ΔS ⧧ (−44 eu) values for a stronger donor Y (Y = p-CH3O) coupled with a stronger para π-acceptor (X = p-CN) in pyridine.

Kinetics and Mechanism of Nucleophilic Substitution Reaction of 4-Substituted-2,6-dinitrochlorobenzene with Benzylamines in MeOH-MeCN Mixtures

“…Rates were measured conductimetrically at least in duplicate, which is similar to previous studies. [21][22][23] Product analysis. Benzyl ammonium salts were liberated quantitatively and identified as one of the reaction products by comparing the UV-vis spectra with those of authentic samples under the same reaction conditions after the reactions were completed.…”

“…3,5 The reaction mechanism, reactivity, and solvents effects on these systems have been examined extensively. [6][7][8] We also have recently reported on the displacement reaction of strongly activated aromatic compounds by anilines and pyridines of chloride in MeOH-MeCN solvents. 9,10 These studies showed that the N-C bond forming step gave a great contribution to the overall second order rate constant.…”

Kinetic Studies on the Aminolysis of 2-(p-Substitutedbenzoyl)-4,5-dichloropyridazin-3-ones

“…The large positive ρY and βX values and the large negative ρX values indicate that the large negative charge is developed in the substrate carbonyl carbon at the TS, and the positive charge is developed in the nucleophile nitrogen atom at the TS. [10][11][12][13] This result indicates that the rates are faster for the reaction with a stronger nucleophile and/or the substrates 1 with a stronger electron-withdrawing group in phenyl ring as found in typical nucleophilic substitution reactions. Table 1 shows that the ρY values are larger than 2.5|ρX|.…”

Single Electron Transfer (SET) Pathway: Nucleophilic Substitution Reaction of 4-Chloro-7-nitrobenzofurazan with Anilines in MeOH-MeCN Mixtures

“…Rates were measured conductimetrically at least in duplicate as in previous work. 21,22,24 Product analysis. Anilinium salts were liberated quantitatively and identified as one of the reaction products by comparison of the UV-vis spectra after the completion of the reaction with those of authentic samples under same reaction conditions.…”

“…16,17 However, when specific solvation occurs between them, the rate is remarkably changed in isodielectric solvents such as methanol-acetonitrile mixtures. 16,18 The reaction mechanism, reactivity, and solvents effects in these systems have been examined extensively, [19][20][21] and we have recently studied the nucleophilic displacement reaction of strongly activated chlorinated aromatic compounds in methanol-acetonitrile solvents. 22 It was assumed that electrophilic catalysis by methanol occurred, likely as a result of hydrogen bonding between alcoholic hydrogen, leaving chloride in the TS.…”