Enthalpy-entropy correlations in reactions of 2,4-dinitrophenyl benzoate with phenols in the presence of potassium hydrogen carbonate and with potassium phenoxides in dimethylformamide

Abstract

Temperature dependences of the relative reactivity of substituted phenols RC 6 H 4 OH in the presence of potassium hydrogen carbonate and of potassium phenoxides RC 6 H 4 O -K + toward 2,4-dinitrophenyl benzoate in dimethylformamide were studied using the competitive reactions technique. Correlation analysis of the relative rate constants k R /k H and differences in the activation parameters (∆∆H ≠ and ∆∆S ≠ ) of competitive reactions revealed the existence of two isokinetic series for each type of nucleophiles. The mechanism of transesterification was interpreted in terms of an approach based on analysis of the effect of substituent in the nucleophile on the activation parameters.

Reactions of Aryl Benzoates with Potassium Aryloxides: Solvent Effects on Reaction Pathway and Kinetics

“…Earlier, we reported about the concave downward Hammett‐type plot with a break at σ − (Х) ∼ 0 ( ρ electron‐donating substituents (EDS) > 0 and ρ electron‐withdrawing substituents (EWS) < 0), which is the characteristic feature for the reactions of 3‐nitro‐, 4‐nitro‐, and 2,4‐dinitrophenyl benzoates with potassium aryloxides XC 6 H 4 O − K + in DMF proceeded through the four‐step mechanism (Fig. ) . To study the reactions of 2,4‐dinitrophenyl benzoate with potassium aryloxides in 50 mol% DMF − 50 mol% H 2 O mixture, we used the relative rate constants k X / k H .…”

“…Hammett‐type plots for the reactions of 3‐nitro‐, 4‐nitro‐, and 2,4‐dinitrophenyl benzoates with potassium aryloxides XC 6 H 4 O − K + in DMF at 25°C .…”

“…It was difficult to use this direct kinetic method for studying fast reactions. The relative rate constants k X / k H for the fast reactions of 2,4‐dinitrophenyl benzoate with potassium aryloxides in 50 mol% DMF − 50 mol% H 2 O mixture were determined using the competitive reactions technique . In a typical run, the solution of a pair of potassium aryloxides with the p K a values being larger than that of the leaving aryloxide (2.5 mmol + 2.5 mmol) in 50 mL of DMF was placed in a glass reactor equipped with a magnetic stirrer and heated to the desired temperature.…”

“…A new type of the stepwise mechanism has been proposed for the reactions of aryl benzoates PhCO 2 C 6 H 4 Y (Y = H, 4‐Cl, 3‐NO 2 , 4‐NO 2 , 2,4‐(NO 2 ) 2 ) with a series of potassium aryloxides XC 6 H 4 O − K + in dimethylformamide (DMF) . This mechanism has been supported by enthalpy–entropy correlations and involves four steps: (1) the nucleophilic attack of aryloxide at the carbonyl carbon generating the tetrahedral intermediate T1 , (2) the intramolecular nucleophilic attack of alkyloxide at the ipso position in the leaving group leading to the formation of the spiro ‐σ‐complex, (3) the spiro ring opening with yielding the zwitterionic intermediate T2 , and (4) the departure of the leaving group (Scheme ) .…”

“…The reactions of carboxylic esters with anionic nucleophiles in solution still remain the subject of extensive studies although it has already been obtained a considerable body of data on the kinetics and mechanisms of those processes . Two distinct mechanisms are mainly discussed for these reactions in water, alcohol, and aqueous mixtures of organic solvents: concerted A N D N (the nucleophilic attack at a carbonyl center occurs concertedly with the leaving group departure) and stepwise A N +D N (the reaction occurs through a tetrahedral intermediate) .…”