Determination of the mean enthalpy of dissociation of arsenic–oxygen bonds in some arsenic (III) alkoxides

Abstract

The standard molar enthalpy of formation of arsenic alkoxides in the liquid phase, As(OR) 3{R = ethyl (Et), n-propyl (Pr n), i-propyl (Pr i), n-butyl (Bu n) and i-butyl (Bu i)}, was determined through reaction-solution calorimetry in chloroform based on the reaction: As(NEt 2) 3(1) + 3ROH(1) = As(OR) 3(1); Δ sol H m oat T= 298.15 K. This series of alkoxides gave the following molar enthalpy of formation Δ f H m ovalues: −(758.1 ± 9.0) kJ·mol −1, −(774.3 ± 9.0) kJ·mol −1, (−774.3 ± 9.0) kJ·mol −1−(809.8 ± 9.0) kJ·mol −1, (−828.8 ± 8.9) kJ·mol −1and −(863.4 ± 8.9) kJ·mol −1, respectively. The standard molar enthalpies of vaporization Δ vap H m(47.9 ± 1.1) kJ·mol −1, (51.2 ± 1.8) kJ·mol −1, (80.1 ± 0.9) kJ·mol −1, (64.0 ± 1.8) kJ·mol −1and (75.7 ± 1.2) kJ·mol −1, were obtained for the same sequence of alkoxides, by means of differential scanning calorimetry. By combining the preceding values with the standard molar enthalpy of formation of the alkoxide Δ f H min the liquid phase, the standard molar enthalpy of formation of these compounds in the gas phase was obtained as −(710.2 ± 9.0) kJ·mol −1, −(723.1 ± 9.1) kJ·mol −1, −(729.3 ± 9.0) kJ·mol −1, −(764.8 ± 9.1) kJ·mol −1, and −(787.7 ± 9.0) kJ·mol −1, respectively. From these values, the mean enthalpy of dissociation of the arsenic-oxygen bond <D m>(As-O) was calculated for the sequence of the above alkoxides to give: (320 ± 5) kJ·mol −1, (301 ± 5) kJ·mol −1, (292 ± 5) kJ·mol −1, (293 ± 5) kJ·mol −1, and (294 ± 5) kJ·mol −1. The linear correlation between the standard molar enthalpies of formation of the liquid alkoxides Δ f H m{As(OR) 3(1)} and the respective alcohol Δ f H m(ROH, 1) suggests that these thermochemical data can be assessed for a series of normal and iso chain alcohols.