Chromium compounds Jones’ reagent

A variety of chromium (VI) oxidizing systems have been developed which allow for the oxidation of a wide range of sensitive compounds. One of the most widely used chromium (VI) reagents is the Jones reagent (/), whose use is detailed in the procedure. A related system employs acetic acid as the solvent, and an example of this reagent is also given. 

The main applications of oxidation with chromium trioxide are transformations of primary alcohols into aldehydes [184, 537, 538, 543, 570, 571, 572, 573] or, rarely, into carboxylic acids [184, 574], and of secondary alcohols into ketones [406, 536, 542, 543, 575, 576, 577, 578, 579, 580, 581, 582, 583, 584]. Jones reagent is especially successful for such oxidations. It is prepared by diluting with water a solution of 267 g of chromium trioxide in a mixture of 230 mL of concentrated sulfuric acid and 400 mL of water to 1 L to form an 8 N CrOj solution [565, 572, 579, 581, 585, 556]. Other oxidations with chromic oxide include the cleavage of carbon-carbon bonds to give carbonyl compounds or carboxylic acids [482, 566, 567, 569, 580, 587, 555], the conversion of sulfides into sulfoxides [541] and sulfones [559], and the transformation of alkyl silyl ethers into ketones or carboxylic acids [590]. 

A variety of reaction conditions have been used for effecting oxidations of alcohols by Cr(VI) on a synthetic scale. The generally preferred method involves addition of an acidic aqueous solution containing chromic acid (known as Jones reagent) to an acetone solution of the compound to be oxidized. Oxidation normally occurs quite rapidly, and overoxidation can be minimized. Often, the reduced chromium salts precipitate, and the acetone solution can be decanted, thus facilitating workup. Entries 2 and 5 in Scheme 9.1 are examples of this technique. 

Due to the toxic nature of chromium compounds, alternatives to the Jones reagent and PCC have been developed. These more modem methods are gradually replacing HjCrO j and PCC. 

Jones Reagent (test is positive for 1° and 2° alcohols, aldehydes, mercaptans, stdfides and sulfoxides). To 1 mL of 5% (mass/ mass) sodium (or potassium) dichromate add 10 drops of the unknown compound. The formation of a green color (sometimes appearing as brown due to the mixture of the green chromium (III) and the unreacted orange chromium (VI) colors) indicates a positive test. 

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