DETERMINATION OF THE MEAN ENTHALPY OF DISSOCIATION OF ARSENIC-OXYGEN BONDS IN SOME ARSENIC(III) ALKOXIDES

The standard molar enthalpy of formation of arsenic alkoxides in the l iquid 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 reac tion-solution calorimetry in chloroform based on the reaction: As(NEt( 2))(3)(1) + 3ROH(1) = As(OR)(3)(1); Delta(sol)H(m) degrees at T = 298. 15 K. This series of alkoxides gave the following molar enthalpy of fo rmation Delta(f)H(m) degrees values: -(758.1 +/- 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(-1), and -(863.4 +/- 8.9) kJ . mol(-1), respectively. The standard molar enthalpies of vaporization Delta(vap)H(m) (47.9 +/- 1.1) kJ . mol(-1), (51.2 +/- 1.8) kJ . mol(-1), (80.1 +/- 0.9) kJ . m ol(-1), (64.0 +/- 1.8) kJ . mol(-1) and (75.7 +/- 1.2) kJ . mol(-1), w ere obtained for the same sequence of alkoxides, by means of different ial scanning calorimetry. By combining the preceding values with the s tandard molar enthalpy of formation of the alkoxide Delta(f)H(m) in th e liquid phase, the standard molar enthalpy of formation of these comp ounds 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 a lkoxides 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 enthalpi es of formation of the liquid alkoxides Delta(f)H(m){As(OR)(3),1} and the respective alcohol Delta(f)H(m)(ROH,1), suggests that these thermo chemical data can be assessed for a series of normal and iso chain alc ohols. (C) 1996 Academic Press Limited