Kinetics and Mechanism of the Oxidation of 10-Methyl-9,10-dihydroacridine by Chromium(VI,V,IV):  Electron vs Hydrogen Atom vs Hydride Transfer

The oxidation of AcrH 2 to AcrH + by hydrogen chromate ions in H 2 O/MeCN (4:1, v/v) is a chain reaction that is strongly inhibited by oxygen. The initiation reaction between AcrH 2 (or AcrD 2 ) and H 2 CrO 4 forms AcrH 2 • + (or AcrD 2 • + ) and occurs by a one-electron mechanism, k H = k D = 4.6 × 10 2 L mol - 1 s - 1 (25 °C). The Cr(V) produced along with AcrH • (from the acid ionization of AcrH 2 • + ) are chain-carrying intermediates. The propagating reaction between AcrH 2 and Cr(V), k = 1 × 10 8 L mol - 1 s - 1, is of key importance since it is a branching reaction that yields two chain carriers, AcrH • and CrO 2+, by hydrogen atom abstraction. The same partners react competitively by hydride ion abstraction, to yield Cr 3+ and AcrH +, k = 1.2 × 10 7 L mol - 1 s - 1, in the principal termination step. The reaction of CrO 2+ and AcrH 2, k H = 1.0 × 10 4 and k D = 4.8 × 10 3 L mol - 1 s - 1, proceeds by hydride ion transfer. The Cr 2+ so produced could be trapped as CrOO 2+ when O 2 was present, thereupon terminating the chain. AcrH 2 itself reacts with Cr 2 O 7 2 - + H +, k = 5.6 × 10 3 L 2 mol - 2 s - 1, but this step is not an initiating reaction. Two successive electron-transfer steps are believed to occur between Cr 2 O 7 2 - and AcrH 2, yielding Cr IV + Cr VI + AcrH + .