NMR Spectroscopic Study of the Aldoxane Formation in Aqueous Acetaldehyde Solutions

Crotonaldehyde is an interesting intermediate in the chemical industry. It is usually produced from aqueous acetaldehyde in a two step process in which the first step is carried out under basic and the second step under acidic conditions. It is commonly assumed that acetaldehyde is converted in the first step to acetaldol and that acetaldol is subsequently dehydrated in the second step to crotonaldehyde. We demonstrate by ¹H and ¹³C NMR spectroscopic studies that acetaldol is hardly present in the reacting solutions at lower temperatures and that the key intermediate is aldoxane (2,6-dimethyl-1,3-dioxane-4-ol). For the first time, data on the chemical equilibrium of the aldoxane formation in aqueous acetaldehyde solutions is provided. Furthermore, preliminary information on the kinetics of that reaction is presented

Online ¹H NMR Spectroscopic Study of the Reaction Kinetics in Mixtures of Acetaldehyde and Water Using a New Microreactor Probe Head

Mixtures of acetaldehyde and water are reactive multicomponent systems because poly­(oxymethylmethylene) glycols are formed. A study on the kinetics of the formation of these oligomers was carried out using a new microreactor NMR probe head that combines online flow ¹H NMR spectroscopy with microreaction technology. The study covers temperatures between 278 and 298 K and pH values between 3.5 and 10.3. From the peak areas in the ¹H NMR spectra, quantitative results for the conversion of acetaldehyde were obtained. On the basis of the new data, a reaction kinetic model was developed and numbers for the kinetic constants of poly­(oxymethylmethylene) glycol formation were determined together with a correlation that describes their dependence on the temperature and pH value