2 + 2

Acetaldehyde is oxidised to acetic acid with oxygen. Co and/or Mo acetate catalysts.

The reaction proceeds via a peracetic acid intermediate (and when the reaction is run under mild and non-catalytic conditions this can be the major product).

Industrial Organic Chemistry, 3rd. Ed. K. Weissermel, H.-J. Arpe, VCH (1997)

Reaction defined as:
Industrial Process: Organic
Oxidation: Of Substrate By Reagent