Chromate reactions with alcohol

The mechanism starts with the formation of HCr04 ions, that is, Cr(VI), from dichromate ion in solution. In acid, these form chromate esters with alcohols. The esters (boxed in black) decompose by elimination of the Cr(IV) HCrO, which subsequently reacts with a Cr(VI) species to yield 2 x Cr(V). These Cr(V) species can oxidize alcohols in the same way, and are thereby reduced to Cr(III) (the final metal-containing by-product). Cr(VI) is orange and Cr(III) is green, so the progress of the reaction is easy to follow by colour change. 

Alternatively, when the counterion of the initial adduct 18 is exchanged with tetramethyl anunonium, the mixed anhydride can be prepared by reaction with pivaloyl chloride. Further reaction with alcohols or amines then yields chromate esters and amides, respectively. Functional groups can be introduced into the heteroatom-linked chains that are not compatible with alkyl and aryl lithium species. The pimary alkyl iodide 19 was prepared by this method. ... 

HCrO ] has been shown to be the photo-active species in the photochemical reaction of dilute solutions of chromium(vi) oxyacids with alcoholic reducing agents. The association of chromate with Np , Th, and Fe" in perchlorate solution has been studied spectrophotometrically and the respective formation constants, 63.6,4.70, and 1.93, have been determined. The greater stability of the Np complex has been interpreted in terms of the limited donation of 5/-electron density from Np to the d-orbitals of chromium in the chromate ion. ... 

A naive look at the product suggests an oxidation to a ketone followed by a Baeyer-Villiger like reaction. The product is best explained by a fragmentation from an intermediate chromate ester, resulting on an aldehyde and a stabilized tertiary carbocation that is transformed into a tertiary alcohol by reaction with water. The hydroxyaldehyde so obtained may evolve to the final lactone either via a lactol or a hydroxyacid. 

The oxidation of a secondary alcohol to a ketone with chromium (VI) is a complex reaction. With unhindered alcohols, oxidation proceeds via initial rapid formation of the chromate ester followed by a rate-determining E2-type elimination of HCr03 as the leaving group. 

Cr produced by reaction of lower-valence states with hydroperoxides, can react with alcohols to produce chromate esters. These esters will, of course, decompose heterolytically, in the manner described above, to produce carbonyls and, for example, H2C1O3 [79]. A similar but somewhat altered mechanism of het-erolytic chromate ester decomposition has been proposed to explain differences noted among several alcohols in the oxidation of alcohols with Cr [81]. The use of chromium-containing catalysts to decompose hydroperoxides to carbonyls in reactions conducted without autoxidation has been frequently noted [82-85]. 

In either mechanism, a low value for the KIE is expected for non-linear hydrogen transfer, with non-linear transfer associated with similar enthalpies and diiferent energies of activation for the two isotopomers. The oxidation of benzyl alcohol by quinoxalinium dichromate is acid catalysed, being first order in dichromate, substrate and added / -toluenesulfonic acid, and the KIE for PhCD20H oxidation is 6.78. The latter is the product of a primary effect and a secondary a-deuterium effect if we assume the latter is around 1.2 (see Section 3.3.1), the reaction is a typical acid chromate oxidation with a linear... 

AMMONIUM CHROMATE (7788-98-9) Not combustible but will enhance an existing fire. A powerful oxidizer. A heat- and shock-sensitive explosive. Contact with strong reducing agents such as hydrazine, alcohols, or ethers can cause explosion. Contact with water produces an alkaline solution, with evolution of free ammonia. Violent reaction with combustible materials, finely divided metals, organic substances. Aqueous solution is incompatible with organic anhydrides, acrylates, alcohols, aldehydes, alkylene oxides, substituted allyls, cellulose nitrate, cresols, caprolactam solution, epichlorohydrin, ethylene dichloride, isocyanates, ketones, glycols, nitrates, nitromethane, phenols, vinyl acetate. Exothermic decomposition with maleic anhydride. 

BIS(lert-BUTYL)CHROMATE (1189-85-1) A strong oxidizer. Violent reaction with reducing agents, alcohols, combustible materials, ethers, fluorine, hydrazine, powdered metals including aluminum, magnesium, zirconium, potassium iodide, sodium tetraborate, sodium tetraborate decahydrate, sodium borohydride. Incompatible with water, steam. 

Alcohols also undergo a timilar hydrogen-exchange reaction with ethylene as the acceptor to give aldehydes or ketones in yields of 40-75 per cent. The reaction takes place at atmospheric (nessure, at a temperature of about 280°C, while using a copfter-zinc-mickel-barium chromate as the catalyst. 

The combination of reactions 4 and 3 has proved to be a sensitive and imerring test for the chromyl. We have used it to confirm the formation of Cr02+ in the controversial reaction of chromate with alcohols. 

Oxidation of a Primary Alcohol to a Carboxylic Acid (Section 10.8A) A primary alcohol is oxidized to a carboxylic acid by chromic acid. The mechanism involves initial formation of an alkyl chromate intermediate, followed by reaction with base to remove a proton, generating the carbonyl group of an aldehyde and simultaneously reducing the chromium(VI) to chromium(IV). An initially formed aldehyde adds water, generating an aldehyde hydrate, which is oxidized according to the same mechanism to give the carboxylic acid. 

PCC oxidation conditions are often also used with secondary alcohols, because the relatively nonacidic reaction conditions minimize side reactions (e.g., carbocation formation Sections 7-2, 7-3, and 9-3) and often give better yields than does the aqueous chromate method. Tertiary alcohols are unreactive toward oxidation by Cr(VI) because they do not carry hydrogens next to the OH function and therefore cannot readily form a carbon-oxygen double bond. 

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