CO₂ production in the bromate‐1,4‐cyclohexanedione oscillatory reaction

Abstract

NMR and GC/MS spectroscopy of the organic extracts of the oscillatory bromate-1,4-cyclohexanedione reaction illustrate the presence of ring-opening products 5-(dibromomethylene)-2(5H)-furanone, (E)-5,5,5-tribromo-4-oxo-2-pentenoic acid, and dibromoacetic acid, particularly at elevated temperatures. The loss of a carbon atom from the six-membered ring after ring opening led to gas formation and such a process became more vigorous at >60 -C, with the direct observation of bubbles in a stirred batch reactor. Gravimetric experiments confirm that the amount of carbon dioxide gas produced increases rapidly with reaction temperature. Parallel experiments suggest that the ring-opening process involves the oxidation of brominated benzoquinones by bromate.

Are Uncatalyzed Bromate Oscillators Truly Gas-Free?

“…As far as only the total amount of gas produced is considered, under the conditions employed, 1,4-cyclohexadiene confirmed its prime as a substrate for bubble-free bromate oscillators, with only 4.0À4.2 mL of gas at standard conditions produced during 21600 s, and as little as 0.25À0.26 mL of gas produced to the point when oscillations disappear. This corresponds fairly well with the recent results, 47 where approximately half of the amount was determined at 0.60Â the initial concentration of 1,4-cyclohexanedione, even though a much greater extent of degradation is suggested at higher temperatures. Both pyrocatechol and pyrogallol were demonstrated to produce significantly larger volumes in 21600 s of measurement at given conditions representing as much as 26.0À26.5 and 39.3À40.1 mL of gas at standard conditions or 0.71À0.72 and 1.07À1.09 equiv, respectively.…”

“…The products of the reaction were isolated and investigated by NMR to prove that decarboxylation takes place and gas must be produced . Moreover, similar strategy, with added support of gravimetric data, was also employed with the reaction of bromate with 1,4-cyclohexanedione, and the results suggest that even this reaction, conventionally considered truly gas-free, does evolve some gaseous products, particularly at elevated temperatures . However, the information available still remained to contain only little time-resolved detail, especially for closed-system measurements without any additional flows of auxiliary gases, not mentioning the scarcity of measurements with common temperature and concentration conditions across several representative UBOs.…”

Unusual Chemistry in an Uncatalyzed Bromate–Aniline Oscillator: Ring-Contraction Oxidation of Aniline with Pulsative CO2 Production

“…Also, the Wang group has shown clear evidence of the production of a brominated protoanemonin, 5-(dibromomethylene)-2­(5 H )-furanone, in a BZ oscillator with pyrocatechol in both catalyzed and uncatalyzed systems . Such observations cast doubt − on the previous perception that UBOs represent gas-free systems. We examined the studied reaction for the possibility of CO 2 /CO production (see the Experimental Section for the methodology) and found pulsative evolution of CO 2 that follows the oscillatory dynamics described above.…”

“…The UBO substrates are oxidizable organic compounds, such as derivatives of phenol or AN or their precursors, e.g., cyclohexane-1,4-dione. , The BZ system in both standard and modified versions has served for decades as an experimental and theoretical model in the study of nonlinear phenomena such as chemical oscillations, chaos, and traveling waves . In particular, the UBOs and catalyzed BZ reactions with aromatic substrates remain the focus of current research in the field of nonlinear dynamics − as their mechanisms are still generally not known. Such research could open new insights into the oxidation pathways of organic substrates, help to clarify the role of free radicals in such reactions, and explore interactions between the organic and inorganic counterparts of oscillators from both the chemical and nonlinear dynamics points of view.…”

Oxidation and Reduction

“…178 The oscillatory oxidation of 1,4-cyclohexanedione by BrO 3 − ion resulted in the formation of CO 2 and the ring-opening products 5-(dibromomethylene)-2(5H)-furanone, (E)-5,5,5-tribromo-4-oxo-2-pentenoic acid, and dibromoacetic acid. 179 The tetrabutylammonium tribromide (TBATB) oxidation of p-substituted acetophenones to α-bromoacetophenone in AcOH-H 2 O (50% vol/vol) was first order in TBATB and the rate increased with the substrate concentration. The reactive TBATB species was Br 3 − ion, and its electrophilic attack on acetophenone gave a transition state which decomposed in the rate-determining step.…”