How is the transition from a carbonyl group to a carboxyl group an oxidation reaction?
Remember that oxidation is the loss of electrons, while reduction is the gain of electrons. In inorganic chemistry, this is typically determined using oxidation states and involves whole electron transfers. In organic chemistry, it is typically more qualitative and involves electron density, not discrete electrons, although you could do it the same way. When oxygen is bonded to carbon, it pulls electron density away from carbon because it is more electronegative. In this way, carbon is oxidized because it has less electron density around it than if it hadn’t formed a bond to oxygen. Adding more carbon-oxygen bonds means those electronegative oxygen atoms are pulling more and more electron density away from carbon, further oxidizing it. That is why going from a carbonyl group to a carboxyl group is oxidation. Carbon bonded to two oxygen atoms became carbon bonded to three oxygen atoms. More oxygen atoms means more electron withdrawal, and loss of electron density around an atom is oxida
