Solvent steric effects. II. Free-radical chemistry of azobisisobutyronitrile and azobis-3-cyano-3-pentane in viscous and crystalline media

Abstract

Product distributions have been studied for photolysis of azobisisobutyronitrile (AIBN) in fluid solution, in glassy benzyl benzoate, in frozen mixtures, and in two pure crystalline modifications. Azobis-3-cyano-3pentane (AGP) was similarly studied in benzene solution and in the pure crystal. For AGP the fluid and crystalline products were similar with a ketenimine predominating and somewhat less tetraethylsuccinodinitrile, but for AIBN the crystalline products (95% disproportionation, 5% tetramethylsuccinodinitrile (TMSN)) differed dramatically from those in solution (5% disproportionation, 40% TMSN, 55% ketenimine). A number of alternatives to explaining the product distribution for AIBN crystals in terms of a very specific influence of the ordered environment on the behavior of the intermediate radical pair are considered and excluded. The transition between the crystalline phases of AIBN was studied thermochemically (AH monoclinic -triclinic, 0.32 ± 0.04 and 0.26 ± 0.04 kcal/mol for AIBN and AIBN-i/^, respectively). The cage effect in crystalline AIBN is greater than 91 %.

Solubility in Different Solvents, Crystal Polymorph and Morphology, and Optimization of Crystallization Process of AIBN

“…The phase transition point (Tp) of AIBN form I was found to be 343.86 K, and the uncertainty u was u(Tp )=0.3 K. In other words, the AIBN that crystallized in the monoclinic (form I) form transformed into the more stable triclinic form (form II) 1,4,16 . The solid-solid transformation did not significantly influence the much larger exothermic peak related to the AIBN decomposition.…”

Photochemical Mechanism in Single Crystals: FTIR Studies of Diacyl Peroxides

“…Sometimes solid-state product distributions seem not to be very sensitive to lattice destruction [20,15]. In other cases the sample may anneal during reaction, preserving the integrity of the reactant solid by segregating decomposed molecules into a separate phase [21].…”

Photolysis of an asymmetrically substituted diazene in solution and in the crystalline state

“…As this result is opposite to that reported by McBride with crystalline AIBN 9 one must recognize the importance of specific molecular and packing effects on the reactivity of different crystalline diazenes. 10 We can now compare the fate of the BME ∑ radical in the photolysis of the diazene and of the ketone. In the former case, a singlet geminate radical pair is formed, while the dissociation of the ketone yields a triplet radical pair.…”

“…Crystal data. C 19 H 24 N 2 , M = 280.40, orthorhombic, a = 5.9800 (7), b = 8.0417 (10), c = 33.922(4) A ˚, U = 1631.3(3) A ˚3, T = 120(2) K, space group P2 1 2 1 2 1 (no. 19), Z = 4, 14 614 reflections measured, 3902 unique (R int = 0.0705) which were used in all calculations.…”

“…While radical pairs derived from ketones tend to give combination products, McBride and collaborators observed a large amount of disproportionation products (50-95%) in the photolysis of several crystalline diazenes. 10,11 In the photolysis of solid 2,2¢-azobis(2methylpropionitrile) (AIBN) and 2,2¢-azobis(2-cyanopropanol) (ACP) they observed no more than 5% of disproportionation products in solution and about 95% in the solid state. They reported melting of the crystal upon completion of photolysis.…”