Esters chromic

Lucchi , who studied the oxidation of substituted benzaldehyde derivatives found that chlorine atoms in the meta and para position accelerate the reaction and alkyl groups retard the oxidation. A Hammett plot of Lucchi s data yields a good straight line with the slope p = 1.06. These data suggest that the reaction proceeds by way of the chromic ester of hydrated benzaldehyde as intermediate, viz. 

An interpretation of the oxidation of acetals by chromium trioxide is summarized in Scheme 1 the reagent could initially remove a proton from the acetalic carbon atom, to give a dioxolan-(or dioxan-)2-ylium ion, rapidly hydrolyzed to an ester (formyl, acetyl, or benzoyl) of an a-alcohol, which would be further oxidized to a ketone [see Scheme 1, path (a)] in the case of methylene acetals, in the presence of acetic anhydride (which acts as a water scavenger), the intermediate is further oxidized, probably through a chromic ester. 

An interesting consequence of the fast formation of the chromic ester is that, sometimes, chromium-based oxidants counter-intuitively are able to oxidize quicker alcohols possessing a greater steric hindrance, as the initially fonned chromic ester releases greater tension on evolving to a carbonyl. Thus, axial alcohols are oxidized quicker than equatorial ones with chromic acid.6 The reverse—a somehow expected behavior—is observed, for example in oxidations with activated DMSO.7... 

Oxides and oxyacids of Cr(VI) are powerful oxidants. The oxidative power of chromic acid is comparable to that of nitric acid. Chromic esters have been proposed as intermediates in these oxidations, thus isopropyl chromate has been isolated and converted into acetone by pyridine-catalyzed elimination in benzene solution [Eq. (2)]. Decomposition of the chromate ester involves removal of the proton attached to the... 

Chromic ester The cliromium analog to an organic ester and an intermediate in the Cr(VI) oxidation of alcohols. 

The proposed mechanism assumes the formation of a cyclic complex between the reactive protonated PDC and lactic acid which undergoes C-C fission. The similarly retarded reactions of benzaldehyde and 4-nitrobenzaldehyde are second order in H+ ion. The decrease in rate with increasing dielectric constant of the solvent suggests a cation-dipole interaction. The proposed mechanism assumes the formation of a cyclic chromic ester between the hydrated benzaldehyde and the protonated PDC species, followed by C-H fission in the rate-determining step which is believed to feature a transition state with an electron-deficient reaction centre. " ... 

Primary alcohols are oxidized in well-defined conditions by chromic anhydride CrOs, giving the corresponding carboxylic acids, while chromium reduces from state+VI to + III. In the same conditions, secondary alcohols give the corresponding ketones and tertiary alcohols give orange-colored chromic esters. This color contrasts with the green color of chromic salts obtained with both previous alcohols. 

What is the mechanism of the chromium(VI) oxidation of alcohols The first step is formation of an intermediate called a chromic ester the oxidation state of chromium stays unchanged in this process. 

Aldehyde Formation from a Chromic Ester an E2 Reaction... 

Chromic acid (H CrO ) reacts with alcohols to form a chromic ester in which the alcohol oxygen atom bridges the carbon and chromium atoms. The ester forms by nucleophilic attack of the alcohol oxygen atom on the chromium atom. This reaction is analogous to an Sj 2 reaction with a—OH group of chromic acid as the leaving group. 

In the second step of the reaction, water extracts a hydrogen atom from the alcohol carbon, the electron pair in the C—H bond acts as a nucleophile, and the O—Cr bond in the chromic ester breaks to form a carbon—carbon double bond. 

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