The use of radical inhibitors or antioxidants to improve yields does not appear to have many precedents in organic synthesis. A keyword search in 2011 provided only two references- both related to the stabilization of m-chloroperbenzoic acid towards thermal degradation during the epoxidation of resistant olefins.
Y. Kishi, M. Aratani, H. Tanino, T. Fukuyama and T. Goto, J.C.S. Chem. Comm. 1972 64 and
D.M. Tal, Steroids (1989), 54(1), 113-22.
The best inhibitor found by Kishi for stabilizing m-chloroperbenzoic acid was 4,4’-thiobis-(6-t-butyl-3-methyl-phenol) that allowed 100% of an m-chloroperenzoic acid charge to be retained after 3 hours heating at 90 C in ethylene dichloride. Octene-1, dodecene-1 and methyl methacrylate were quantitatively epoxidized using such stabilized oxidant.
Synthetic chemists apparently assume that free radical reactions do not occur unless free radical initiators are present in the reaction mixture or unless the reaction mixture is irradiated. It might seem they think it can’t happen unless they are intending it to happen. Obviously, this is not true! Free radical reactions can take place not just during the contemplated reaction phase but during the work-up of the reaction when we might think that all the reacting is stopped. Actually, the opportunity is greater in the work-up phase; this phase usually takes more time, particularly when the process is being scaled up.
Are free-radical reactions inhibited by particular pH ranges of the solvent medium? No, they are not. The most frequent type of free radical reaction is oxidation and only the relative amounts of different species that can be oxidized are affected by pH not particularly the oxidation rates.
Oxidation often produces coloured products when it can introduce new unsaturation into molecules. The presence of unexpected colour in a reaction is suggestive of unanticipated oxidation. I recall that in the preparation of some aniline compounds the procedure teaches the addition of hydrogen sulfide to the aqueous phase during isolation to prevent colour development from exposure to air during workup and crystallization. The usual response to a colored product is to use charcoal in the recrystallization rather than trying to prevent colored by-products in the first place.
If you are performing a distillation and the contents of the still pot are darkening why wouldn't you add an antioxidant? Answer- I've never thought of it.
If you are performing a distillation and the contents of the still pot are darkening why wouldn't you add an antioxidant? Answer- I've never thought of it.
It is quite well known (and often critical) if you are working with monomers for radical polymerization (eg acrylates). If you are trying to minimize color, the catch is oxidation of phenolic inhibitors generates yellow impurities. But at least it isn't consuming your product and yellow is better than brown or black and may be more separable than oxidized product.
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