Oxidation by oxygen. Whenever you are in need of oxidizing a compound, oxygen must be the obvious choice. It is not, explosion risk plays a role. Not for the first time now a France-based research group takes up the challenge. They demonstrate here how it could be done with substrate 2-ethylhexanal (available through the oxo process) which requires oxidation to 2-ethylhexanoic acid (used industrially as precursor to certain catalysts). Key features: a continuous flow reactor, control of oxygen bubble flow and some free radicals as in free beer.
The air droplets migrating in the continuous flow tube are big! See the movie in the supplementary info. Should resemble something like Taylor-Couette flow. The reaction does not require any catalyst which is odd for one described as a radical raction (autoxidation to the peracid, nucleoplic addition of a second aldehyde to a Criegee intermediate followed by rearrangement). The researchers went to some length to find an initiation mechanism and arrived at "a trace amount of over-oxidized aldehyde present at the ppm level in the starting materials"
Warning: the research group appears to be somewhat nomenclaturual (if that is a word) challenged: the supplementary info books 2-ethylhexanoic acid as 2-ethylhexylcarboxylic acid. Confusing.