Reactions of hydrogen atoms in 6 mol dm–3 sulphuric acid. Part 2.—The transition from activation to diffusion control

Abstract

The relative rates of abstraction by hydrogen atoms of hydrogen and deuterium atoms from propan-2-ol and [²H₇]propan-2-ol have been studied, as a function of temperature, in 6 mol dm^–3 H₂SO₄, using both γ-radiation and photolysis of Fe²⁺ as the source of H. As the temperature is decreased the expected activation control, reflected in an Arrhenius temperature dependence, gives way to diffusion control. Similar behaviour is shown for the same reactions of deuterium atoms in 6 mol dm^–3 D₂SO₄, and for the competition between propan-2-ol and Fe²⁺ for hydrogen atoms. In all cases, the low temperature behaviour is inadequately described by the diffusional parameters derived in the preceding paper, and a low temperature diffusion process, with an Arrhenius temperature dependence, is proposed to explain the discrepancies. From the competition studies, absolute rate parameters are obtained for the following reactions: (i) H +[²H₇]propan-2-ol (abstraction of α-D)A=(6.3 ± 0.5)× 10¹⁰ dm³ mol^–1 s^–1, E= 20 ± 2.6 kJ mol^–1, (ii) H +[²H₇] propan-2-ol, (abstraction of hydroxylic H)A=(2.5 ± 0.6)× 10¹⁰ dm³ mol^–1 s^–1, E= 23 ± 3 kJ mol^–1, (iii) H + Fe²⁺, A=(1.4 ± 0.6)× 10¹⁰ dm³ mol^–1 s^–1, E= 14 ± 3 kJ mol^–1.