[7789-00-6] · CrK2O4 · Potassium Chromate · (MW 194.20)
(reagent for converting primary allylic and benzylic halides to aldehydes)
Physical Data: mp 968.3 °C; d 2.732 g cm-3.
Solubility: H2O (62.9 g/100 mL at 20 °C).
Form Supplied in: yellow crystals.
Preparative Methods: the reagent is prepared in Hexamethylphosphoric Triamide (HMPA) in the presence of Dibenzo-18-crown-6 or Dicyclohexano-18-crown-6.1
Handling, Storage, and Precautions: potassium chromate is a highly toxic cancer suspect agent. All chromium(VI) reagents must be handled with care. The mutagenicity of CrVI compounds is well documented.2 HMPA is also a highly toxic cancer suspect agent. This reagent should be handled in a fume hood.
Potassium chromate in HMPA in the presence of dibenzo-18-crown-6 or dicyclohexano-18-crown-6 at 100 °C converts primary allylic (eq 1) and benzylic (eq 2) halides to aldehydes.1 The yields are lower with alkyl halides. The yields of all primary halides are improved by use of polymer-supported chromic acid in benzene3 or Bis(tetrabutylammonium) Dichromate in CHCl3.4
Primary and secondary alcohols are oxidized to aldehydes and ketones, respectively, by potassium chromate and sulfuric acid in a two-phase system (water-CHCl3) in the presence of tetrabutylammonium hydrogen sulfate catalyst.5 Allylic alcohols and acid-sensitive alcohols are oxidized to carbonyl compounds in good yields.
In addition to polymer supported Chromic Acid in benzene3,6 and Bis(tetrabutylammonium) Dichromate in CHCl3,4 other reagents oxidizing halides to carbonyl compounds include Chromium(VI) Oxide in aqueous acetic acid, which oxidizes a-chlorohydrindene to a-hydrindanone,7 and chromic acid in aqueous acetone in the presence of silver chromate, which oxidizes a steroidal vicinal dibromide to an a-bromo ketone in the synthesis of corticoids.8 In aqueous systems it is possible that the halide is hydrolyzed to an alcohol which is oxidized to the carbonyl compound.
University of California, Irvine, CA, USA