|Abstract or Summary
- A spectrophotometric study has been made of catalysis of the decomposition of sulfuryl chloride in carbon tetrachloride solution to an equilibrium mixture of sulfuryl chloride, sulfur dioxide and chlorine. The catalysts were activated charcoal, acetonitrile and 18-crown-6 polyether. A weak acetonitrile-sulfur dioxide complex also was established; formation constants found at 10, 20, 30 and 40° were 3.32 M⁻¹, 2.76 M ⁻¹, 2.60 M⁻¹ and 0.99 M⁻¹. The absorbances measured were related to the concentrations of the principal absorbing species, sulfur dioxide and chlorine by the demonstration that Beer's law held and by the determination of extinction coefficients as a function of wavelength. Kinetics data for the approach to equilibrium as well as actual equilibrium data were gathered for acetonitrile and crown ether catalysis. These systems obeyed the integrated rate law for the reversible reaction scheme A B + C. First order rate constants (forward reaction) were obtained for both systems. The acetonitrile system showed an approximate eighth order dependence on catalyst (present in excess) giving the rate law: Rate = k [ SO₂C1₂] [CH₃CN] ⁸. Rate constants (x 10⁸) based on this expression at 10, 20 and 30° were 1.36 ± .18, 4.41 :f 1.32 and 5.86 ± 1.53 sec⁻¹ M⁻⁸. Activation parameters derived from the temperature dependence of these data were Ea = 11.7 k 3.9 Kcal/mole, log A = 1.26 ± 2.90 and ΔS = -54.7 ± 13.1 e. u. The crown ether system gave the approximate rate law: Rate = k [S0₂C1₂][crown ether] , with activation parameters Ea = 25.7 Kcal/mole, A = 3.2 x 10¹⁹ sec M⁻¹ and 6,S = -O. 4 e. u. ⁻¹ The dissociation constants observed in the presence of each of the catalysts were as follows (the acetonitrile values were corrected for sulfur dioxide-acetonitrile complex formation): o 10⁴ x K 25°C, Δ Ho, 6.G298, 5298 M Kcal /mole Kcal /mole e. u. Charcoal 9.6 16.0 4.09 40 Acetonitrile 1.30 11.6 5.31 21 Crown ether 11.3 9.1 4.04 17 Calculated 1.8 11.7 5.1 22 Comparison of these values suggests non-ideality in the carbon tetrachloride solutions (the charcoal and crown ether systems) as the major factor causing a difference from the calculated values based on ideal solution behavior. The additional possibility of a crown ether-sulfur dioxide complex might account for the deviations between the charcoal and crown ether results. Agreement between the acetonitrile results and the calculated ideal system is considered probably to be fortuitous. The differences between the acetonitrile and charcoal systems are considered to be due mainly to differences in solvation effects on going from pure carbon tetrachloride to the mixed solvent, acetonitrile-carbon tetrachloride. The proposed general mechanism is: Fast SO₂ C1₂ + Catalyst SO₂ C1₂ Catalyst Slow SO ₂ C1₂ Catalyst SO ₂ Catalyst + C1₂ Fast SO₂ Catalyst SO + Catalyst This mechanism is based on the conclusion that the crown ether and acetonitrile catalysis processes are similar, the relative effectiveness of the catalysts being determined by their relative basicities. The differences in the empirical rate laws observed are interpreted in terms of the acetonitrile requiring significant aid from solvation to provide a measurable rate of decomposition.