Correlation of the Rates of Solvolysis of Benzoyl Fluoride and a Consideration of Leaving-Group Effects

Abstract

The specific rates of solvolysis of benzoyl fluoride have been determined at 25.0 degrees C in 37 pure and binary solvents. Together with seven values from the literature, these give a satisfactory correlation over the full range of solvents when the extended Grunwald-Winstein equation is applied. The sensitivities to changes in solvent nucleophilicity and solvent ionizing power are very similar to those for octyl fluoroformate, suggesting that the addition step of an addition-elimination mechanism is rate determining. In the solvent-composition region where benzoyl chloride also shows bimolecular solvolysis, the appreciable k(Cl)/k(F) values are proposed as being primarily due to a more efficient ground-state stabilization for the fluoride.

Rate and Product Studies of Solvolyses of Benzyl Fluoroformate

“…Omission of these three points does not appreciably change the l and m values (Figure 2), but it does lead to a considerable improvement in the correlation coefficient, to 0.974 (F-test value of 90). The entries presented in Table 3 for comparison show that the l and m values are very similar to those previously observed for n-octyl fluoroformate [9] and benzoyl fluoride [15]. These three solvolyses have an l/m ratio which is very close to two in value.…”

“…These values reflect an appreciable ground-state stabilization for the fluoride [19] and the need to break a strong carbon-fluorine bond in the rate determining step. In contrast, if the addition step is rate-determining, values of close to unity (and frequently above it), reflecting a large electron deficiency at the carbonyl carbon of a haloformate incorporating fluorine [15], are frequently observed. This situation has recently been discussed in a consideration of n-octyl haloformate solvolyses [9], where k F /k Cl specific rate ratios of 0.6 to 15 were observed.…”

Correlation of the Rates of Solvolysis of Methyl Fluoroformate Using the Extended Grunwald-Winstein Equation

“…15 These values reflect an appreciable ground-state stabilization for the fluoride 17 and the need to break a strong carbon-fluorine bond in the rate determining step. In contrast, if the addition step is rate-determining, values of close to unity (and frequently above it), reflecting a large electron deficiency at the carbonyl carbon of a haloformate incorporating fluorine, 18 are frequently observed. This situation has recently been discussed in a consideration of n-octyl haloformate solvolyses, 10 where kF/kCl specific rate ratios of 0.6 to 15 were observed.…”

Application of the Extended Grunwald-Winstein Equation to Solvolyses of n-Propyl Chloroformate

“…Although both the N T and Y X scales are based on standard systems involving substitution reaction at an sp 3 -hybridized carbon [3,4], the scales have also been used with considerable success in the correlation analyses of solvolysis reactions of substrates which have attack at the sp 2 -hybridized carbon of acyl halides [5][6][7] and chloroformate esters [8][9][10][11][12][13][14][15], at the phosphorus atom of phosphorochloridate esters [16,17] or amides [18], and at the sulfur atom of arenesulfonyl chlorides [19].…”