The Reaction of 1, 4-Naphthoquinones with Sodium Thiosulfate

Abstract

The reaction mechanism and rate-determing step (r. d. s. ) in the 1, 4-naphthoquinones (NQ) -Na₂S₂O₃ reaction systems have been examined by comparison of the oxidation-reduction potentials of NQ (ENQ) with the oxidation potential of Na₂S₂O₃ (E_ST). The addition of Na₂S₂O₃ to NQ proceeds as follows:
NQ + Na₂S₂O₃ + H ⁺ →1.2-Addition[I]→Prototropy[II]
The detection of stable intermediates [I] indicates the r. d. s. to be step ( 2 ) rather than step ( 1 ). This result differs from that in the p-benzoquinone (PQ)-Na₂S₂O₃ reaction system where the r. d. s. was step (1). This difference may be explained by the fact that the oxidation-reduction potential of PQ (E_PQ) is more positive than those of E_NQ. In the 2-methyl, 4-naphthoquinone (MNQ)-Na₂S₂O₃ reaction system the 3-position of the quinone ring is attacked, whereas in the MNQ-Na₂SO₃ reaction system the 2-position is attacked by Na₂SO3. This difference is caused by the fact that E?, NQ-EST is positive and E_XNQ-E_ss is negative.