The ester forms by nucleophilic attack of the alcohol oxygen atom on the chromium atom. To the funnel is added 100 ml of saturated sodium chloride solution, and the mixture is extracted with two 50-ml portions of ether. : After exhaustion, treatment with NaOH will convert hydrogen dichromates to chromates and, subsequently, yield half of the chromate loading on elution. Towards this aim, chromium(III)-impregnated Nafion 511 (NAFK) has been used as a catalytic oxidant in the presence of t-butyl hydroperoxide.176 This reagent gives good yields of ketones (80–100%), but unfortunately oxidation of primary alcohols leads to the formation of complex mixtures. Chromic acid, H 2 C r O 4, is a strong acid and is a reagent for oxidizing alcohols to ketones and carboxylic acids. Unlike chromic acid, PCC will not oxidize aldehydes to carboxylic acids. Annamalai, in Reference Module in Materials Science and Materials Engineering, 2016. Above pH 6.5 mostly CrO42− is found. Unfortunately, optimum conditions require the use of very nonpolar solvents (best is cyclohexane) at 80 °C. The resulting compound retains the hydroxyl group of the original chromic half-ester and is a carboxylic acid. The speciation mainly depends on the pH value. Poly(vinylpyridinium chlorochromate) (PVPCC)174 is a mild oxidant for primary, secondary, allylic and benzylic alcohols. Anion exchange offers an ideal opportunity for the removal and recovery of chromates. One of its hydroxyl groups is oxidized by way of a chromic half-ester in the same manner as alcohols. Schematic of a chromic acid regeneration cell [86]. Potassium permanganate (KMnO4) oxidizes alcohols, but is a less selective reagent than chromium(VI) reagents. Some comments about the reactivity of these supported oxidants were made: (i) quaternary ammonium salts are more reactive than the tertiary pyridinium chromates; (ii) the reactivity of a complex chromate XCrO3− was found to increase as the basicity of X− decreases. Electrochemical regeneration is an attractive alternative. Reactions performed in chlorobenzene at 85 °C for 6–8 h using 4 equiv. ScienceDirect ® is a registered trademark of Elsevier B.V. ScienceDirect ® is a registered trademark of Elsevier B.V. URL: https://www.sciencedirect.com/science/article/pii/B9780128007808000152, URL: https://www.sciencedirect.com/science/article/pii/B9780128128381500165, URL: https://www.sciencedirect.com/science/article/pii/B9780080523491001906, URL: https://www.sciencedirect.com/science/article/pii/B0122267702042411, URL: https://www.sciencedirect.com/science/article/pii/B9780080977423007102, URL: https://www.sciencedirect.com/science/article/pii/B9780125049504500046, URL: https://www.sciencedirect.com/science/article/pii/B9780128035818092201, URL: https://www.sciencedirect.com/science/article/pii/B9781437744576100056, Comprehensive Organic Synthesis II (Second Edition), 2014. The gem-diol is an alcohol. W.H. Figure 4. Robert J. Ouellette, J. David Rawn, in Organic Chemistry (Second Edition), 2018. Removal of the remainder of the chromate from the strongly basic exchanger requires concentrated sodium sulfate solutions: In contrast to strongly basic exchangers, weakly basic ones can be completely regenerated by means of NaOH. In the acidic region (1
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