Mechanisms chromic acid oxidation

FIGURE 15.4 A mechanism for chromic acid oxidation of an alcohol. 

Westheimer ° has reviewed other inductions of the chromic acid oxidation of iodide, indicating how these reactions afford insight into the mechanism of the simple oxidation. 

The apparent first-order rate coefficient obtained using excess oxidant increased exponentially with increase in acidity in the range 5 N < [H30" ] < 12 N. The reaction is first-order with respect to added manganous ions (k increasing sharply), but the activation energy (11.0 kcal.mole ) remains unchanged. At appreciable catalyst concentrations the reaction becomes almost zero-order with respect to bromide ion. The mechanism appears to be a slow oxidation of Mn(II) to Mn(III) followed by a rapid reduction of the latter by bromide. This reaction is considered further in the section on Mn(II)-catalysis of chromic acid oxidations (p. 327). 

In chromic acid oxidation of isomeric cyclohexanols, it is usually found that axial hydroxyl groups react more rapidly than equatorial groups. For example, trans-4-t-butylcyclohexanol is less reactive (by a factor of 3.2) than the cis isomer. An even larger difference is noted with cis- and trans-3,3,5- trimethylcyclohexanol. The trans alcohol is more than 35 times more reactive than the cis. Are these data compatible with the mechanism given on p. 748 What additional detail do these data provide about the reaction mechanism Explain. 

Mechanism. Chromic acid reacts with isopropanol to produce a chromate ester intermediate. An elimination reaction occurs by removal of a hydrogen atom from the alcohol carbon, and departure of the chromium group with a pair of electrons. The Cr is reduced from Cr (VI) to Cr (IV), and the alcohol is oxidized. 

Aldehydes are oxidized easily by moist silver oxide or by potassium permanganate solution to the corresponding acids. The mechanism of the permanganate oxidation has some resemblance to the chromic acid oxidation of alcohols (Section 15-6B) ... 

Exercise 16-38 Write mechanisms for the oxidative cleavage of 1,2-diols by lead tetraethanoate and sodium periodate based on consideration of the mechanism of chromic acid oxidation (Section 15-6B). ... 

The orders of reaction in the oxidation of /V,/V-dimcthylanilinc by chromic acid are one and zero, respectively.15 The reaction is catalysed by metal ions such as Cu2+ and Ag+, but retarded by Mn2+ a mechanism is proposed. In contrast, in the chromic acid oxidation of o-toluidine, the reaction is first order in both oxidant and substrate.16... 

A Cr(VI)-catalyst complex has been proposed as the reactive oxidizing species in the oxidation of frans-stibene with chromic acid, catalysed separately by 1,10-phenanthroline (PHEN), oxalic acid, and picolinic acid (PA). The oxidation process is believed to involve a nucleophilic attack of the olefinic bond on the Cr(VI)-catalyst complex to generate a ternary complex.31 PA- and PHEN-catalysed chromic acid oxidation of primary alcohols also is proposed to proceed through a similar ternary complex. Methanol- reacted nearly six times slower than methanol, supporting a hydride transfer mechanism in this oxidation.32 Kinetics of chromic acid oxidation of dimethyl and diethyl malonates, in the presence and absence of oxalic acid, have been obtained and the activation parameters have been calculated.33 Reactivity in the chromic acid oxidation of three alicyclic ketoximes has been rationalized on the basis of I-strain. Kinetic and activation parameters have been determined and a mechanism... 

M. Rhaman and J. Rocek, Chromium(IV) oxidation of primary and secondary alcohols, J. Am. Chem. Soc., 93 (1971) 5455-5462 Mechanism of chromic acid oxidation of isopropyl alcohol. Evidence for oxidation, ibid., 93 (1971) 5462-5464. 

The mechanism of chromic acid oxidation probably involves the formation of a chromate ester. Elimination of the chromate ester gives the ketone. In the elimination, the carbinol carbon retains its oxygen atom but loses its hydrogen and gains the second bond to oxygen. 

An alternative mechanism has also been proposed in which oxidation at the double bond leads to a ketol derivative, elimination of water from which then gives the unsaturated ketone (Scheme 18a)." Limited kinetic data are available and suggest diat Scheme 17 is obtained for chromic acid oxidations. 

Chromic acid oxidation of olefins can rarely be used for the preparation of oxiranes because they occur as intermediates that rapidly undergo further transformation. From an investigation of the mechanism of oxidation of triaryl-substituted olefins, it was concluded that a carbonium ion or cyclic chromate ester is a possible intermediate. Selective epoxidation of compounds containing conjugated double bonds is attainable by means of chromic-acid oxidation (Eq. 48) 535 Exclusively cis product was obtained from a highly substituted octalin with Na2Cr04, KMn04, or ozone (Eq. 49). ... 

The E2-like process depicted for the general oxidation mechanism in Table 4.1 is supported by the observation that deuterium substitution of the a-H in isopropanol slows the rate of chromic acid oxidation by sevenfold. Deuterium replacement at the methyl positions does not diminish the oxidation rate. Since C-D bonds are broken more slowly than C-H bonds, these results suggest that the a-H is removed in a slow step. 

Pfltzner and Moffatt report the interesting finding that an equatorial 11 a-hydroxy-steroid is oxidized readily by the DMSO-DCC combination whereas the 11 /3-epimer is inert under the same conditions. The situation is thus the reverse of that found in chromic acid oxidation (Eschenmoser). The authors offer an explanation based on a suggested mechanism for the oxidation. 

The mechanisms by which transition-metal oxidizing agents convert alcohols to aldehydes and ketones are rather complicated and will not be dealt with in detail here. In broad ontline, chromic acid oxidation involves initial formation of an alkyl chromate ... 

Propose a mechanism for the chromic acid oxidation of 1-propanol to propanal. 

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. 

The oxidation of cyclohexanol by concentrated nitric acid is mechanistically complex. A reasonable mechanistic route to the dicarboxyUc acid is given here. The first stage of the oxidation is considered to proceed by a mechanism similar to that found in chromic acid oxidations of alcohols (see Experiment [33]).The reaction here involves the initial formation of a nitrate ester intermediate, which, under the reaction conditions, cleaves by proton abstraction to form the ketone. This reaction is accompanied by reduction of the nitrate to nitrite. The proton transfer may involve a cyclic intramolecular rearrangement during the oxidation-reduction cleavage step. A likely mechanism is outlined below ... 

The mechanism of chromic acid oxidation is complicated, but can be summarized as a combination of two stages. In the first, the alcohol and chromic acid react to give a chromate ester. 

The mechanism of chromic acid oxidations of alcohols has been investigated thoroughly. It is interesting because it shows how changes in oxidation states occur in a reaction between an organic and an inorganic compound. The first step is the formation of a chromate ester of the alcohol. Here we show this step using a 2° alcohol. 

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