11.6
Autoxidation of Ethers to Peroxides and Hydroperoxides
Ethers represent a class of chemical compounds that become more dangerous with prolonged storage because they tend to form explosive peroxides when st…
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.
View the full transcript and gain access to JoVE Core videos
Q1: Why do ethers form peroxides and hydroperoxides during storage?
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.
Q2: What are the steps in the free radical chain mechanism of ether autoxidation?
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.
Q3: How can you detect peroxides and hydroperoxides in ether samples?
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.
Q4: What makes peroxides and hydroperoxides dangerous in the laboratory?
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.
Q5: What is the net reaction for ether autoxidation?
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.
Q6: Why should ethers be obtained in small quantities and used promptly?
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.
Q7: What role do chain carriers play in ether autoxidation?
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.