Chromic Acid Neutralization

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One obvious limitation of Jones oxidation is the use of acidic conditions that may cause interference with acid-sensitive functional groups. It must be mentioned that, due to the presence of separated organic and aqueous phases, containing respectively the organic substrate and sulphuric acid, such interferences are much less common than expected, and many protecting groups that can be deprotected using acid survive Jones oxidation. The concentration of sulfuric acid can be decreased in order to minimize interferences with acid-sensitive functionalities, although this causes a decrease on the oxidizing power of Jones reagent. The solution of the alcohol in acetone can be kept either over an ice-water bath or at room temperature during all the reaction. It is also possible to keep the reaction mixture over an ice-water bath during the addition of the chromic acid solution when the major exotherm is expected, and let it reach room temperature afterwards. For reactions on a multigram scale, cooling on an ice-water bath is particularly recommended. During the oxidation of very sensitive substrates, it may be advisable to perform the entire oxidation at a temperature as low as -20 C. The consumption of the alcohol can be signaled by the persistence of the red color of the chromium acid solution, which is being dropped into the reaction flask. As the red color of the solution being added is mixed with the green color of the reduced chromium species already present in the reaction flask, it may take some practice to appreciate the color changes. A sheet of white paper, placed bellow a reaction flask made of glass, substantially helps to distinguish these color changes. Although Jones oxidation is carried out in the presence of aqueoussulfuric acid, functionalities with a high sensitivity to acidic conditions canremain unchanged due to the segregation between the organic and aqueousphases. Only very acid-sensitive protecting groups are hydrolyzed under the conditions of the Jones oxidation. When free alcohols result from the hydrolysis of very acid-sensitive protecting groups, they are in situ oxidized to ketones or carboxylic acids. It must be mentioned that diverse acid strengths, temperatures and reaction times are used in Jones oxidation, which leads to uneven responses of the same protecting groups. Most silyl ethers, including the ubiquitous TBS ethers, Alcohols protected as esters, and diols protected as cyclic acetals resist Jones oxidation.

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