Radiation-induced C—C bond cleavage in 1,2-diarylethanes as model compounds of coal. Part 1.—Pulse and steady-state radiolysis of 1,2-di(1-naphthyl)ethane

Abstract

1,2-Di(1-naphthyl)ethane (1,2-DNE) has been used to study the radiation-induced C—C bond cleavage of the ethane linkage. Pulse radiolysis was applied both in the absence and presence of NaAlH₄ and NaAlH₂(OR)₂ used as scavengers for solvent cations. In the presence of the sodium aluminium hydrides the radical anions of 1,2-DNE are stabilized by the metal cation, resulting in the formation of radical anion/sodium cation pairs (Na⁺, 1,2-DNE^˙–); these species have a rather long lifetime (τ > 2 ms) in DME as well as in THF, and show three absorption bands at 330, 370 and 750 nm. In THF ε₃₇₀= 14000 dm³ mol^–1 cm^–1 and ε₇₅₀= 5000 dm³ mol^–1 cm^–1 were established. Cleavage of the ethano bridge of 1,2-DNE was achieved by means of steady-state as well as multi-pulse radiolysis in the presence of alkali-metal aluminium hydrides. The efficiency of this process was studied by using NaAlH₄, NaAlH₂(OR)₂, NaAlH₂Et₂, NaAlEt₄ and LiAlH₄ in THF as well as in DME. The most efficient C—C bond cleavage was found to occur with NaAlH₄ in DME. Based on all the experimental results it is concluded that the C—C bond cleavage results from the successive attack of two sodium cation/electron ion pairs (Na⁺, e^–), forming the unstable dianions (2 Na⁺, 1,2-DNE^2–). These decompose under scission of the ethano bridge, producing naphthylmethyl carbanionic fragments C₁₀H₇CH^–₂Na⁺ which supposedly form aluminate salts, e.g. C₁₀H₇CH₂AlH^–₃Na⁺, with AlH₃ generated in the scavenging processes. From these aluminate salts 1-methylnaphthalene (1-MN) is obtained through hydrolysis.