Extended Grunwald-Winstein Analysis - LFER Used to Gauge Solvent Effects in p-Nitrophenyl Chloroformate Solvolysis

Abstract

Specific rates of solvolysis at 25o C for p-nitrophenyl chloroformate (1) are analyzed using the extended (two-term) Grunwald-Winstein equation. For 39 solvents, the sensitivities (l = 1.68±0.06 and m = 0.46±0.04) towards changes in solvent nucleophilicity (l) and solvent ionizing power (m) obtained, are similar to those previously observed for phenyl chloroformate (2) and p-methoxyphenyl chloroformate (3). The observations incorporating new kinetic data in several fluoroalcohol-containing mixtures, are rationalized in terms of the reaction being sensitive to substituent effects and the mechanism of reaction involving the addition (association) step of an additionelimination (association-dissociation) pathway being rate-determining. The l/m ratios obtained for 1, 2, and 3, are also compared to the previously published l/m ratios for benzyl chloroformate (4) and p-nitrobenzyl chloroformate (5).

Kinetic Studies that Evaluate the Solvolytic Mechanisms of Allyl and Vinyl Chloroformate Esters

“…The large KSIE value observed for 3, together with the large l parameters in the more nucleophilic solvents calculated for 3 and 4 , also indicate the strong possibility of general-base catalysis [21,26,30,32,33] to the nucleophilic attack in both 3 and 4 .…”

“…The kinetic solvent isotope effects (KSIEs) in methanol and methanol-d ( k MeOH / k MeOD ) for the substituted phenyl chloroformates indicate that general-base catalysis is operating, and its importance decreases with an increase in electron-donating ability of the substituent [21,26,30,32,33]. For the parent phenyl chloroformate ( 1 ), over a full-range of 49 pure and binary solvents of widely varying nucleophilicity and ionizing power values, the G-W [Equation (2)] analyses resulted in an l value of 1.66 and an m value of 0.56 [27].…”

“…For the parent phenyl chloroformate ( 1 ), over a full-range of 49 pure and binary solvents of widely varying nucleophilicity and ionizing power values, the G-W [Equation (2)] analyses resulted in an l value of 1.66 and an m value of 0.56 [27]. It was suggested [10,23,27,30,31] that such l (bond making) and m (bond breaking) values be taken as typical values that are to be expected for substrates that proceed with a rate-determining addition step in a stepwise carbonyl addition-elimination process. Other research groups also proposed homogenous stepwise mechanisms for 1 , where the formation of a zwitterionic tetrahedral intermediate is the rate-determining step [34–38].…”

Use of Linear Free Energy Relationships (LFERs) to Elucidate the Mechanisms of Reaction of a γ-Methyl-β-alkynyl and an ortho-Substituted Aryl Chloroformate Ester

“…We also list in Table 3 the previously published l / m ratios of 2.96 for phenyl chloroformate ( 3 ) [50,54], 2.81 for p -methoxyphenyl chloroformate ( 5 ) [55], and 3.65 for p -nitrophenyl chloroformate ( 6 ) [24], in 49, 44, and 39 solvents respectively.…”

On the Importance of the Aromatic Ring Parameter in Studies of the Solvolyses of Cinnamyl and Cinnamoyl Halides

“…The efficiency of transmission of the destabilizing electronic effects in 5 is made possible due to the complete coplanarity (as shown in the 3-D image, 5’ ) between the nitro group, the aromatic ring, the vinylic double bond and the carbonyl group. At the other end of the spectrum, it was shown [49] that the observed specific rate order of k p -nitrophenyl chloroformate > k p -nitrobenzyl chloroformate is due in part to the p -nitrobenzyl group twisting out of the plane with its ether oxygen and therefore being able to exert only a fraction of its inductive ability.…”