Phenolic antioxidation Go to Slide Show mode and click the big red button

Low energy, low reactivity High energy, high reactivity R-H Allylic hydrogen in triglyceride

Low energy, low reactivity High energy, high reactivity R. + H.

Low energy, low reactivity High energy, high reactivity R. O2O2

Low energy, low reactivity High energy, high reactivity RO2.O2. Hydroperoxyl radical

Low energy, low reactivity High energy, high reactivity RO2.O2. R-H Allylic hydrogen in triglyceride

Low energy, low reactivity High energy, high reactivity ROOH Hydroperoxide, decomposes to form smelly products R. Returns to propagation, continues cycle

But……. Look what happens if we have a phenolic antioxidant around at the point where the hydroperoxyl radical would normally react with another molecule of lipid, RH

Low energy, low reactivity High energy, high reactivity RO2.O2. OH HO Hydroperoxyl radical

Low energy, low reactivity High energy, high reactivity O. HO ROOH This radical is low energy, does not continue propagation reaction

Compare…... The situation with and without a phenolic antioxidant

Low energy, low reactivity High energy, high reactivity O. HO This radical is low energy, does not continue propagation reaction ROOH Decomposes to form smelly products R. Returns to propagation, continues cycle WithoutWith ROOH Decomposes to form smelly products

Why? The question still remains why the ROO. (or the R.) radical would react with the phenolic antioxidant rather than another molecule of RH The answer lies in the concept of reaction activation energies

ROO. or R. RH phenolic Increasing energy

So….. The reason phenolic compounds are antioxidants (that is, they induce early termination) is The activation energies (energy necessary to do the reaction) are lower, and The product (phenolics and quinones) energies are lower