Corrrelation of the Specific Rates of Solvolysis of Ethyl Fluoroformate Using the Extended Grunwald-Winstein Equation

Abstract

Abstract:The specific rates of solvolysis of ethyl fluoroformate have been measured at 24.2 °C in 21 pure and binary solvents. These give a satisfactory correlation over the full range of solvents when the extended Grunwald-Winstein equation is applied. The sensitivities to changes in the N T solvent nucleophilicity scale and the Y Cl solvent ionizing power scale, and the k F /k Cl values are very similar to those for solvolyses of n-octyl fluoroformate, consistent with the addition step of an addition-elimination pathway being rate-determining. For methanolysis, a solvent deuterium isotope effect of 3.10 is compatible with the incorporation of general-base catalysis into the substitution process. For five representative solvents, studies were made at several temperatures and activation parameters determined. The results are also compared with those reported earlier for ethyl chloroformate and mechanistic conclusions are drawn.

Correlation of the Rates of Solvolysis of i-Butyl Fluoroformate and a Consideration of Leaving-Group Effects

“…, bimolecular pathway), values of close to unity, reflecting a large electron deficiency at the carbonyl carbon of the fluoroformate, would be frequently observed, and the bimolecular pathway through a tetrahedral intermediate formed by the rate-determining addition of solvent to the carbonyl carbon would be characterized by k F ≥ k Cl . A previous report concerning the solvolyses of 4 , 5 , 6 and 7 [ 17 – 20 ], which were believed to solvolyze by an addition-elimination mechanism, found a k F / k Cl ratio to be somewhat below unity in ethanol and methanol and slightly greater than unity for solvolyses in mixtures of water with ethanol, methanol, acetone, or TFE ( Table 5 ). For other haloformate esters, k F / k Cl ratios of 1.09 to 7.16 for solvolyses in 70% aqueous acetone at 30.1 °C have been reported [ 28 ].…”

“…We have also reported that the solvolyses of primary and secondary alkyl fluoroformates (e.g., methyl ( 3 ) [ 16 ], ethyl ( 4 ) [ 17 ], n -propyl ( 5 ) [ 18 ], n -octyl ( 6 ) [ 19 ], and isopropyl ( 7 ) [ 20 ], Figure 1 ) in a wide range of hydroxylic solvents follow an addition-elimination pathway. Solvolyses of t -butyl fluoroformate ( 8, Figure 1 ) [ 21 ] were found to proceed entirely by an ionization pathway ( Equation 4 ), which included the loss of carbon dioxide to give the relatively stable t -butyl cation.…”

Rate and Product Studies of 1-Adamantylmethyl Haloformates Under Solvolytic Conditions

“…As shown in Table 1, the k F /k Cl ratios for solvolyses of 1 and 2 are similar to those previously reported for all other the primary alkyl substrates. 4,[8][9][10] This is true in all of the solvents except for very low nucleophilic solvents and very high ionizing solvents (i.e., 90% HFIP, k F /k Cl = 3.2 × 10 −2 ), which have been reported to proceed through a bimolecular pathway. The very low value for 90% HFIP is expected because of the previously demonstrated dominance of an ionization pathway for chloride solvolysis in this solvent.…”

“…In methanol, the k MeOH /k MeOD ratio was in the range of 2.00 to 3.98 for solvolyses of alkyl and aryl haloformates, which have been reported to proceed through a bimolecular mechanism. [1][2][3][4][8][9][10][11]30,31 The k ROH /k ROD values for i-propyl chloroformate and t-butyl fluoroformate in the range of an ionization mechanism were 1.25 5 in pure water (S=H) and 1.26 12 in methanol (S=Me), respectively. Activation Parameters.…”

“…Higher m values for the addition-elimination solvolyses of 2 relative to 1 may reflect the need for increased solvation of the developing negative charge on the carbonyl oxygen in the presence of the fluorine attached at the carbonyl carbon. 4,[8][9][10][11]34,35 Product Studies. The percentage compositions and selectivity (S) values for reactions of 1 in ethanol and aqueous ethanol are reported in Table 5.…”