20.14
Radical Autoxidation
The oxidation of an organic compound in the presence of air or oxygen is called autoxidation. For example, cumene reacts with oxygen to form hydropero…
Oxidation of organic compounds in the presence of air or atmospheric oxygen is termed autoxidation.
It is a free radical chain reaction consisting of initiation, propagation, and termination steps.
In the initiation step, an initiator abstracts hydrogen from the organic compound to form a carbon-centered radical.
The radical then enters the first propagation step, where it couples with the molecular oxygen to form a peroxy radical.
In the second propagation step, the peroxy radical abstracts hydrogen from another organic compound molecule forming the product peroxide and regenerating the carbon-centered radical.
The regenerated radical further propagates the chain reaction.
Finally, in the termination step, two carbon-centered radicals couple together to form a non-radical product.
Generally, the autoxidation process is initiated by light. In the absence of light, the process is very slow. For this reason, most organic chemicals are stored in dark bottles.
In particular, compounds with allylic and benzylic hydrogens are more susceptible to autoxidation as the free radicals formed in the initiation step are resonance stabilized.
View the full transcript and gain access to JoVE Core videos
Q1: What are the three main steps in a radical autoxidation chain reaction?
Radical autoxidation consists of initiation, propagation, and termination steps. In initiation, an initiator abstracts hydrogen from an organic compound to form a carbon-centered radical. During propagation, the radical couples with molecular oxygen to form a peroxy radical, which then abstracts hydrogen from another molecule, regenerating the carbon-centered radical and forming a peroxide product. Finally, termination occurs when two carbon-centered radicals couple together to form a non-radical product.
Q2: Why are organic chemicals typically stored in dark bottles?
Light generally initiates the autoxidation process in organic compounds. In the absence of light, autoxidation proceeds very slowly. Therefore, most organic chemicals are stored in dark bottles to minimize light exposure and prevent unwanted oxidation reactions that could degrade the compounds during storage.
Q3: Which organic compounds are most susceptible to autoxidation and why?
Compounds with allylic and benzylic hydrogens are highly susceptible to autoxidation. The free radicals formed during the initiation step at these positions are resonance stabilized, making them more stable and reactive. This stabilization allows the chain reaction to proceed more readily, increasing the rate of autoxidation compared to other organic compounds.
Q4: What are antioxidants and how do they prevent autoxidation?
Antioxidants like BHT and BHA are radical inhibitors that scavenge and destroy radicals by reacting with them to form resonance-stabilized radicals. The tert-butyl groups of these compounds sterically hinder the radical center, decreasing reactivity. Natural antioxidants such as vitamin E and vitamin C work similarly, converting reactive radicals into less reactive, stabilized forms to prevent oxidation of foods and biological compounds.
Q5: Why can old ether bottles become dangerous during distillation?
Ethers are susceptible to autoxidation in the presence of oxygen, forming hydroperoxides over time. Even though this reaction is slow, old ether bottles accumulate small amounts of peroxide. During ether distillation, these peroxides can decompose explosively when heated, making old ether bottles hazardous to use in the laboratory.
Q6: How does autoxidation affect unsaturated oils and fats?
Naturally occurring fats and oils, like vegetable oil, contain triglycerides with multiple hydrocarbon chains and double bonds. Autoxidation occurs at the allylic positions of these triglycerides, forming hydroperoxides responsible for the foul smell of oxidized foods. Without radical inhibitors as food preservatives, foods containing unsaturated oils have a short lifetime due to this oxidative degradation.
Q7: What is the role of the peroxy radical in the autoxidation propagation cycle?
In the propagation phase of autoxidation, the peroxy radical is formed when a carbon-centered radical couples with molecular oxygen. The peroxy radical then abstracts hydrogen from another organic compound molecule, producing the peroxide product and regenerating a carbon-centered radical. This regenerated radical continues the chain reaction, allowing autoxidation to propagate efficiently.