Autoxidation of Ethers to Peroxides and Hydroperoxides

Ethers undergo autoxidation, a spontaneous oxidation process in air where molecular oxygen slowly reacts with ether molecules. This occurs through a free radical chain mechanism involving initiation, propagation, and termination steps. Over time, old ether bottles exposed to air accumulate dangerous amounts of peroxides and hydroperoxides, making them explosive hazards if concentrated or heated.

Autoxidation proceeds through three steps: initiation, where an initiator abstracts a hydrogen atom from the carbon adjacent to the ether oxygen to form a carbon radical; propagation, where the carbon radical couples with oxygen to form an oxygen radical, which then abstracts hydrogen from another ether molecule to form a hydroperoxide and regenerate the carbon radical; and termination, where two carbon radicals couple to form a non-radical adduct.

Peroxides and hydroperoxides can be detected by shaking an ether sample with an acidified aqueous 10% potassium iodide solution. If peroxides are present, they liberate iodine, which gives the solution a distinctive yellow color. This test should be performed on laboratory ether samples before use to ensure safety.

Peroxides and hydroperoxides formed from ether autoxidation are explosive compounds that can detonate upon heating or concentration. Even small amounts accumulated in old ether bottles pose serious explosion hazards. This danger is compounded by ethers' low boiling points and high flammability, requiring careful handling and storage in tightly sealed containers away from heat and air.

The net reaction for ether autoxidation is given by the sum of the two propagation steps in the free radical chain mechanism. In the first propagation step, a carbon radical couples with molecular oxygen to form an oxygen radical. In the second step, the oxygen radical abstracts hydrogen from another ether molecule, producing a hydroperoxide and regenerating the carbon radical for chain continuation.

Ethers should be obtained in small quantities and used promptly because they spontaneously form explosive peroxides and hydroperoxides when exposed to air over time. Storing large quantities increases the risk of dangerous peroxide accumulation. Keeping ethers in tightly sealed containers and using them quickly minimizes exposure to atmospheric oxygen and reduces the likelihood of hazardous peroxide formation.

Chain carriers are intermediate products formed during each step of the free radical chain reaction that regenerate in subsequent steps. In ether autoxidation, carbon radicals and oxygen radicals serve as chain carriers. The reaction continues as long as these chain carriers persist, allowing the propagation steps to repeat and produce more peroxides and hydroperoxides until termination occurs.