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

Abstract In aqueous H 2 SO 4 , Ce(IV) ion oxidizes rapidly Arnold's base(( p ‐Me 2 NC 6 H 4 ) 2 CH 2 , Ar 2 CH 2 ) to the protonated species of Michler's hydrol(( p ‐Me 2 NC 6 H 4 ) 2 CHOH, Ar 2 CHOH) and Michler's hydrol blue(( p‐ Me 2 NC 6 H 4 ) 2 CH + , Ar 2 CH + ). With Ar 2 CH 2 in excess, the rate law of the Ce(IV)‐Ar 2 CH 2 reaction in 0.100 M H 2 SO 4 is expressed ‐d[Ce(IV)]/dt = k app [Ar 2 CH 2 ] 0 [Ce(IV)] with k app = 199 ± 8M −1 s −1 at25°C. When the consumption of Ce(IV) ion is nearly complete, the characteristic blue color of Ar 2 CH + 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 Ar 2 CH + 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[Ar 2 CH + ]/dt = k obs [Ar 2 CH + ] where K obs = ((ac + 1)[H*] + bc[H + ] 2 )/(a + b[H + ]) (in HClO 4 ) and k obs = ((ac + 1 + e[HSO 4 − ])[H + ] + bc[H + ] 2 + d[HSO 4 − ] + q[HSO 4 − ] 2 /[H + ])/(a + b[H + ] + f[HSO 4 − ] + g[HSO 4 − ]/[H + ]) (in H 2 SO 4 ), and at 25°C and μ = 0.1 M, a = 0.0870 M s, b = 0.655 s, c = 0.202 M −1 s −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[Ar 2 CH + ]/dt = k OH [OH − ] 0 [Ar 2 CH + ] with k OH = 174 ± 1 M −1 s −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.