Kinetics and Mechanism of the Cerium(IV) Oxidation of Arnold's Base and the Protonation and Hydroxylation of Michler's Hydrol Blue

Abstract

AbstractIn aqueous H2SO4, Ce(IV) ion oxidizes rapidly Arnold's base((p‐Me2NC6H4)2CH2, Ar2CH2) to the protonated species of Michler's hydrol((p‐Me2NC6H4)2CHOH, Ar2CHOH) and Michler's hydrol blue((p‐Me2NC6H4)2CH+, Ar2CH+). With Ar2CH2 in excess, the rate law of the Ce(IV)‐Ar2CH2 reaction in 0.100 M H2SO4 is expressed ‐d[Ce(IV)]/dt = kapp[Ar2CH2]0[Ce(IV)] with kapp = 199 ± 8M−1s−1 at25°C. When the consumption of Ce(IV) ion is nearly complete, the characteristic blue color of Ar2CH+ ion starts to appear; later it fades relatively slowly. The electron transfer of this reaction takes place on the nitrogen atom rather than on the methylene carbon atom. The dissociation of the binuclear complex [Ce(III)ArCHAr‐Ce(III)] is responsible for the appearance of the Ar2CH+ dye whereas the protonation reaction causes the dye to fade. In highly acidic solution, the rate law of the protonation reaction of Michler's hydrol blue is ‐d[Ar2CH+]/dt = kobs[Ar2CH+] where Kobs = ((ac + 1)[H*] + bc[H+]2)/(a + b[H+]) (in HClO4) and kobs= ((ac + 1 + e[HSO4−])[H+] + bc[H+]2 + d[HSO4−] + q[HSO4−]2/[H+])/(a + b[H+] + f[HSO4−] + g[HSO4−]/[H+]) (in H2SO4), and at 25°C and μ = 0.1 M, a = 0.0870 M s, b = 0.655 s, c = 0.202 M−1s−1, d = 0.110, e = 0.0070 M−1, f = 0.156 s, g = 0.156 s, and q = 0.124. In highly basic solution, the rate law of the hydroxylation reaction of Michler's hydrol blue is ‐d[Ar2CH+]/dt = kOH[OH−]0[Ar2CH+] with kOH = 174 ± 1 M−1s−1 at 25°C and μ = 0.1 M. The protonation reaction of Michler's hydrol blue takes place predominantly via hydrolysis whereas its hydroxylation occurs predominantly via the path of direct OH attack.