| Jones oxidation | |
|---|---|
| Named after | Ewart Jones |
| Reaction type | Organic redox reaction |
| Identifiers | |
| Organic Chemistry Portal | jones-oxidation |
| RSC ontology ID | RXNO:0000356 |
TheJones oxidation is anorganic reaction for theoxidation of primary and secondaryalcohols tocarboxylic acids andketones, respectively. It is named after its discoverer,Sir Ewart Jones. The reaction was an early method for the oxidation of alcohols. Its use in organic chemistry has been displaced by related reagents such as thepyridine complex of chromium trioxide ("Collins reagent").[1]
Jones reagent is a solution prepared by dissolvingchromium trioxide in aqueoussulfuric acid. To effect a Jones oxidation, this acidic mixture is then added to anacetone solution of the substrate. Alternatively,potassium dichromate can be used in place of chromium trioxide. The oxidation is very rapid and quiteexothermic. Yields are typically high. The reagent is convenient and cheap. However, Cr(VI) compounds are carcinogenic, which deters the use of this methodology.
Jones reagent will convert primary and secondary alcohols toaldehydes and ketones, respectively. Depending on the reaction conditions, the aldehydes may then be converted to carboxylic acids. For oxidations to the aldehydes and ketones, two equivalents of chromic acid oxidize three equivalents of the alcohol:
For oxidation of primary alcohols to carboxylic acids, 4 equivalents of chromic acid oxidize 3 equivalents of the alcohol. The aldehyde is an intermediate.
The inorganic products are green, characteristic ofchromium(III) aquo complexes.[2]
Like many other oxidations of alcohols by metal oxides, the reaction proceeds via the formation of a mixedchromate ester:[3][4] Theseesters have the formula CrO3(OCH2R)−
Like conventional esters, the formation of this chromate ester is accelerated by the acid. These esters can be isolated when the alcohol is tertiary because these lack theα hydrogen that would be lost to form the carbonyl. For example, usingtert-butyl alcohol, one can isolatetert-butyl chromate ((CH3)3CO)2CrO2), which is itself a good oxidant.[5]
For those structures with hydrogen alpha to the oxygen, the chromate esters degrade, releasing the carbonyl product and an ill-defined Cr(IV) product:
The deuterated alcohols HOCD2R oxidize about six times slower than the undeuterated derivatives. This largekinetic isotope effect shows that the C–H (or C–D) bond breaks in therate-determining step.
The reaction stoichiometry implicates the Cr(IV) species "CrO2OH−", whichcomproportionates with the chromic acid to give a Cr(V) oxide, which also functions as an oxidant for the alcohol.[6]
The oxidation of the aldehydes is proposed to proceed via the formation ofhemiacetal-like intermediates, which arise from the addition of the O3CrO-H− bond across the C=O bond.
The reagent rarely oxidizes unsaturated bonds. In certain cases, depending on veryexact stereoelectronic factors, production of epoxides may occur.
It remains useful inorganic synthesis.[2][7] A variety of spectroscopic techniques, includinginfrared spectroscopy, can be used to monitor the progress of a Jones oxidation reaction. At one time the Jones oxidation was used inbreathalyzers.
The other principalalcohol oxidation processes utilize Collins reagent,Cornforth reagent, andPCC. Many of these reagents represent improvements over inorganic chromium(VI) reagents such as Jones reagent with respect toselectivity, specifically in increased favorablility of oxidizing primary alcohols to aldehydes over carboxylic acids.[8]