Chromium oxide, isotopic oxygen

Winter 10) relates isotopic exchange of molecular oxygen with magnesium oxide, zinc oxide, chromium oxide, nickel oxide, and iron oxide. He also compares the rates of isotopic exchange of these oxides with oxygen and the rates of adsorption and catalytic activity relating to the oxidation of CO and the decomposition of NgO. 

For the convenience of comparison the data for the activity of all oxides are recalculated for 700° temperature. With the exception of chromium oxide this succession is very close to the experimental one for reactions of isotopic exchange in oxygen. 

At several points within the text we have referred to instances in which isotope studies have furnished information concerning the possible path of a reaction. For example, the oxidation of SOl with ClOj in water enriched with O18 (p. 366) yields SO " having practically no enrichment with O18, indicating that the reaction has taken place by direct oxygen transfer from CIO to SQ. Similarly, the oxidation of Or(II) with Co(NH3)5Cl2+ must proceed through transfer of a chlorine atom from the cobalt ion to the chromium ion, for it has been shown that one Cr—Cl bond is formed for each chromium(II) ion oxidized. Representing the reaction schematically ... 

Mechanism 15.3 outlines the mechanism of chromic acid oxidation of 2-propanol to acetone. The alcohol reacts with chromic acid in the first step to give a chromate ester. A carbon-oxygen double bond is formed in the second step when loss of a proton from carbon accompanies cleavage of the bond between oxygen and chromium. The second step is rate-determining as evidenced by the fact that (CH3)2CHOH reacts 6.7 times faster than (CH3)2CDOH. If the second step were faster than the first, no deuterium isotope effect (Section 5.17) would have been observed. 

Tertiary alcohols without protons on the carbon are not oxidized. It is tempting to consider that removal of the proton in the activated states frees a pair of electrons to be transferred through the oxygen bridge to the metal ion. In the case of chromium(VI) oxidation of secondary alcohols there is general acid catalysis and a large deuterium isotope effect. 

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