Oxidation by Cr

Steps 1 and 2 constitute an oxidation by the ionic pathway by Cr and steps 6 and 7 a similar oxidation by Cr which is produced by an electron-transfer process. Either (step 3) or (step 4) is produced in step 2] may abstract a... 

COVALENT COMPOUNDS, METAL IONS OXIDATION-REDUCTION 2. Oxidation by Cr(VI) and Mn(VII)... 

The oxidation by Cr(VI) of aliphatic hydrocarbons containing a tertiary carbon atom has been studied by several groups of workers. Sager and Bradley showed that oxidation of triethylmethane yields triethylcarbinol as the primary product with a primary kinetic isotope effect of about 1.6 (later corrected by Wiberg and Foster to 3.1) for deuterium substitution at the tertiary C-H bond. Oxidations... 

Although the fate of Cr(IV) is uncertain, (cf. the alcohol oxidation), some characteristics of the intermediate chromium species have been obtained by Wiberg and Richardson from a study of competitions between benzaldehyde and each of several substituted benzaldehydes. The competition between the two aldehydes for Cr(VI) is measured simply by their separate reactivities that for the Cr(V) or Cr(IV) is obtained from estimation of residual aldehyde by a C-labelling technique. If Cr(V) is involved then p values for oxidation by Cr(VI) and Cr(V) are 0.77 and 0.45, respectively. An isotope effect of 4.1 for oxidation of benzaldehyde by Cr(V) was obtained likewise. 

The kinetics of the oxidation of phenylphosphonous acid (132) to phenylphosphonic acid by chromic oxide have been investigated." The reaction, which is first order in Cr, is catalysed by both acid and pyridine and the mechanism suggested involves the initial formation of a complex between chromic acid and the tervalent form of the starting acid. Ratedetermining decomposition of this complex is followed by rapid oxidation by Cr. ... 

An important piece of evidence for this mechanism is the fact that a primary isotope effect is observed when the a-hydrogen is replaced by deuterium.2 The Cr(IV) that is produced in the initial step is not stable and is capable of a further oxidation. It is believed that Cr(IV) is reduced to Cr(II), which is then oxidized by Cr(VI) generating Cr(V). This mechanism accounts for the overall stoichiometry of the reaction.3... 

Chromium produces some of the most interesting and varied chemistry of the transition elements. Chromium(O) and chromium(I) are stabilized in organometallics (Prob. 8). There have been extensive studies of the redox chemistry of Cr(II), Cr(III) and Cr(VI). Generally the Cr(IV) and Cr(V) oxidation states are unstable in solution (see below, however). These species play an important role in the mechanism of oxidation by Cr(VI) of inorganic and organic substrates and in certain oxidation reactions of Cr(II) and Cr(III). Examination of the substitution reactions of Cr(III) has provided important information on octahedral substitution (Chap. 4). 

Chromium(IV) and (V) are important intermediates in oxidation by Cr(VI), see next section. Chromium(V) is generally more reactive than Cr(VI). It is believed that Cr(V) is most effective for C —H rupture whereas Cr(IV) best breaks C —C bonds. 

Since the stable product of oxidation by Cr(VI) is Cr(III) we are necessarily involved with three-electron reactions, with their attendant interests and eomplications. For inorganic reductants, R oxidized to O, the reaction scheme (1.118) holds... 

Acetamido-2-deoxy-D-glucose (27) is oxidized by Cr(VI) to the aldonic acid, and the reaction is proposed to occur by a reaction path involving Cr(V). However, Cr(V) intermediates could not be detected by EPR in the 0.15-1.0 M [H+] range used in the kinetic measurements.86... 

Although the alcoholysis of Cr(OBu )4 with several primary and secondary alcohols leads to oxidation by Cr to the aldehyde or ketone,1316 alcoholysis by the secondary alcohol 3,3-dimethyl-2-butanol affords a Cr alkoxide (equation 84)1318 which is sensitive to oxygen and moisture, but otherwise stable. Presumably the bulky f-butyl groups prevent the molecule from achieving the conformation required for hydrogen transfer in the oxidation step. 

---