autoxidation
/ aw-toh-ox-ih-DAY-shun /
Leave butter or cooking oil out too long and it goes rancid; cut an apple and it browns; old varnish and rubber crack and harden. A surprising amount of everyday decay is one slow chemical reaction: autoxidation, the spontaneous reaction of organic molecules with the oxygen in ordinary air, driven by a radical chain.
Oxygen (O2) is special: it is itself a kind of biradical, with two unpaired electrons, so it reacts readily with carbon radicals. Autoxidation runs as a radical chain. A weak C-H bond (especially an allylic or doubly-allylic C-H in an unsaturated fat) is abstracted, giving a carbon radical; O2 adds to it to make a peroxyl radical (R-O-O-dot); that peroxyl radical abstracts a hydrogen from another molecule, giving a hydroperoxide (R-O-O-H) and a new carbon radical to carry the chain. The hydroperoxides later fall apart into the small, smelly aldehydes and acids we recognize as 'rancid.' Because polyunsaturated fats have especially weak, resonance-stabilized doubly-allylic C-H bonds, they autoxidize fastest, which is why fish oil and vegetable oils spoil more readily than saturated fats.
Autoxidation is why food spoils, why we add antioxidants (radical inhibitors like vitamin E or BHT) to keep oils and plastics fresh, and why our own bodies fight 'oxidative stress.' The same chemistry has a dangerous edge: ethers left on the shelf slowly autoxidize into explosive peroxides. The defense is always the same, an inhibitor that traps the chain-carrying radical before it can propagate.
An open bottle of linoleic-acid-rich vegetable oil slowly turns rancid: O2 attacks the weak doubly-allylic C-H, building hydroperoxides that break down into pungent short-chain aldehydes, the smell of 'off' oil.
Air slowly oxidizes the weakest C-H of an unsaturated fat, and rancidity follows.
Autoxidation is a radical chain with ordinary O2, not the same as deliberate lab oxidation by a reagent; and antioxidants do not 'undo' it, they pre-empt it by intercepting radicals before the chain takes hold.