Chiral Intermediates and the Oscillatory Effect of Circular Dichroism in the Belousov - Zhabotinskii Type Reaction of L-Ascorbic Acid

Investigating the Belousov-Zhabotinskii (BZ) type reaction of an acidic L-ascorbic acid (AA) / potassium bromate / cerous sulfate system, an oscillatory effect of circular dichroism is detectable at A = 300 nm. HPLC analysis of the oscillatory mixture and spectroscopic experi­ ments indicate that this effect is caused by 3,4,5-trihydroxy-2-oxo-L-valeraldehyde (TVA) a C5 oxidation fragment of AA. Because of the bromide ion production occurring before the metal catalyst addition the AA system shows no preoscillatory phase and a rather short entire length of oscillation up to a maximum of 20 min. Since AA is not brominated but oxidized by bromine which is formed by the Landolt typ “clock reaction” of AA with acidic bromate, par­ tially bromine-hydrolysis-controlled (BHC) oscillations are discussed for the overall mecha­ nism of this BZ system.