Nucleophile Substitutionen an Kohlensäurederivaten. V. Kinetik und Mechanismus der Äthanolyse von Kohlensäure‐arylester‐chloriden

Abstract

Es wurden die Geschwindigkeitskonstanten pseudo‐erster Ordnung für die Äthanolyse von Kohlensäure‐arylester‐chloriden in Äthanol‐Aceton (35:65, V/V) und in reinem Äthanol bestimmt. Es konnte ein Additions‐Eliminierungsmechanismus ähnlich dem bei der Hydrolyse in Wasser‐Aceton (35 Vol.‐% H2O) festgestellt werden.

Correlation of the rates of solvolysis of phenyl chloroformate

“…Noting that similar behaviour in related solvolyses of diaryl carbonates had been corrected 32 by use of the 'normal' σ 0 values of Taft 33 (established using substrates with isolated substituted phenyl groups, with no resonance interaction 33-35 ), they also substituted σ 0 values for the original σ values and found that a good linear relationship (ρ = 1.03) then resulted. Parallel behaviour was found in 65% acetone 6 (ρ = 1.59) and ethanol 18 (ρ = 1.73). Therefore, it appears that, with use of the appropriate substituent constants, one good linear relationship is obtained for solvolyses over the full range of substituted phenyl chloroformates (substituents ranging from p-benzyloxy to p-nitro), consistent with an essentially constant reaction mechanism not only over a wide range of solvents but also over a wide range of ring substituents.…”

“…The most thorough study is presented in two papers by Bacaloglu and co-workers. 6, 18 For the parent phenyl chloroformate and eleven ring-substituted derivatives, they obtained a good linear plot against the traditional Hammett σ values for solvolyses in 10% aqueous acetone, 6 except that the data points for the p-methoxy and, especially, the p-benzyloxy substituents lay above the plot. Noting that similar behaviour in related solvolyses of diaryl carbonates had been corrected 32 by use of the 'normal' σ 0 values of Taft 33 (established using substrates with isolated substituted phenyl groups, with no resonance interaction 33-35 ), they also substituted σ 0 values for the original σ values and found that a good linear relationship (ρ = 1.03) then resulted.…”

“…Queen 7 considered the hydrolysis of phenyl chloroformate to proceed by the addition-elimination bimolecular pathway, and this viewpoint was subsequently supported for solvolyses in both aqueous acetone 6 and ethanol. 18 Recently, a direct displacement bimolecular pathway has been suggested for solvolyses in methanol-acetonitrile mixtures; 3 this pathway differs, however, from the Kivinen concept of an S N 2-type mechanism 2,9a,15 in that the initial attack is believed to resemble that leading to the addition-elimination mechanism. However, the reaction was considered to be concerted, due to the belief that the tetrahedral intermediate had an insignificant lifetime.…”

Dual Pathways in the Solvolyses of Phenyl Chlorothioformate

“…Several other techniques had previously been applied to studies of the solvolyses of phenyl chloroformate, including F/Cl leaving group effects, , Hammett treatments of substituent effects, − solvent isotope effects, − and consideration of the activation parameters. − These techniques have all indicated a bimolecular mechanism, almost certainly of the addition−elimination (tetrahedral intermediate) type (eq 2) but possibly with the first-formed intermediate so unstable that the mechanism could be considered 8 as an enforced concerted variant …”