Comparison of the mechanisms of the thermal decompositions of 2,2′-azisobutane and azoisopropane

The gas phase pyrolyses of 2,2′-azoisobutane and azoisopropane have been investigated in the temperature range 471-573 K. The products of 2,2′-azoisobutane (AIB) decomposition result from the following elementary reactions. AIB ^k1→ 2t-C₄H₉ + N₂ (1) 2t-C₄H₉ ^kd→ iC₄H₁₀ + iC₄H₈ (2) 2t-C₄H₄ ^kc→ 2,2,3,3-tetramethylbutane (3) The disproportionation-combination rate constant ratio, k_d/k_c, for t-butyl radicals has been determined as 2.60 ± 0.04 in the temperature range 483 to 515 K. In addition to the corresponding reactions (1) to (3) involving isopropyl radicals, azoisopropane (AIP) decomposition involves the abstraction reaction, iC₃H₇ + AIP ^ka→ C₃H₈ + AIP·. (4) In the temperature range, 518-573 K, k_a is given by the expression, log₁₀ k_a/(k_c)^1/2/cm^3/2 mol^-1/2 s^-1/2 = 5.26 ± 0.22 - (41.64 ± 2.31) kJ mol^-1/2.303 RT and the disproportionation-combination rate constant ratio for isopropyl radicals has been found to fall from 0.52 at 518 K to 0.49 at 573 K.