Electrophilic aromatic substitution. Part 34. Partial rate factors for detritiation of dithieno[1,2-b:4,3-b′]benzene, dithieno[1,2-b:3,4-b′]benzene, and dithieno[2,1-b:3,4-b′]benzene

Abstract

Dithieno[1,2-b:4,3-b′]benzene (I), dithieno[1,2-b:3,4-b′]benzene (II), and dithieno[2,1-b:3,4-b′]benzene (III), specifically labelled with tritium in each position, have been synthesized. Their rates of protiodetritiation, in either anhydrous trifluoroacetic acid or mixtures of acetic acid and trifluoroacetic acid, have been measured at 70 °C. The rate–acidity profiles for each compound show that there is weak hydrogen bonding between sulphur and trifluoroacetic acid, and the extent of this is closely similar to that previously found for thienothiophens thereby confirming the general trend observed with sulphur-containing heterocycles viz. that the extent of hydrogen bonding is proportional to the number of sulphur atoms in the aromatic compound. Partial rate factors for detritiation, free from the effects of hydrogen bonding, are calculated as follows (position and compound in parentheses) : 3.42 × 10⁶(1)-(I), 1.65 × 10⁷(2)-(I), 5.35 × 10⁵(4)-(I), 1.525 × 10⁷(2)-(II), 4.98 × 10⁶(3)-(II), 1.32 × 10⁶(4)-(II), 4.59 × 10⁵(5)-(II), 2.35 × 10⁷(7)-(II), 4.01 × 10⁶(8)-(II), 1.96 × 10⁷(2)-(III), 3.77 × 10⁶(3)-(III), 3.99 × 10⁵(4)-(III); the corresponding σ⁺ values are –0.745, –0.825, –0.655, –0.82, –0.765, –0.70, –0.645, –0.84, –0.755, –0.835, –0.75, and –0.64. The positional reactivity orders both within a given molecule and between molecules are satisfactorily predicted by localization energies calculated by the simple Hückel method using the parameters α_s 1.0, β_cs 0.6, these having been found previously to predict correctly the positional reactivities in other sulphur-containing heterocycles.