11.6 : Autooxydation des éthers en peroxydes et hydroperoxydes

When working with low molecular weight ethers, such as diethyl ether,  two hazards must be avoided in the laboratory.

Firstly, given their low boiling points, ethers are highly flammable. Thus, care must be taken to keep them away from open flames or sources of electric sparks to prevent possible explosions, and they should be used inside a fume hood.

Secondly, ethers undergo slow oxidation in the presence of atmospheric oxygen to form peroxides and hydroperoxides, which are dangerous, as they can explode upon heating.

Autoxidation of ethers in the presence of molecular oxygen occurs through a free radical chain mechanism involving initiation, propagation, and termination steps.

Initiation is the first step, wherein an initiator abstracts a hydrogen atom from the carbon adjacent to the ether oxygen to form a carbon radical.

Next is propagation, which occurs in two stages. In the first propagation step, the carbon radical couples with molecular oxygen to form an oxygen radical.

In the second propagation step, the oxygen radical abstracts a hydrogen atom from the carbon of another ether molecule to form a hydroperoxide and a new carbon radical that can cycle through an additional round of propagation steps, making a chain reaction.

Finally, in the termination step, two carbon radicals couple together to form a non-radical adduct and terminate the reaction.

Overall, the net reaction for the autoxidation of ether is given by the sum of the two propagation steps.

Although autoxidation of ethers is a slow process, old ether bottles exposed to air can accumulate dangerous amounts of peroxides and hydroperoxides.

Hence, ether samples used in the laboratory should be tested for these dangerous compounds before use.

For instance, peroxides can be detected by mixing a portion of ether with an acidified 10% aqueous solution of KI. If present, peroxides liberate iodine, giving a yellow color to the solution.