Phenolic antioxidation

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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.

O2

Low energy, low reactivity

High energy,high reactivity R O2

.

Hydroperoxylradical

Low energy, low reactivity

High energy,high reactivity R O2

.

R-HAllylic hydrogen in triglyceride

Low energy, low reactivity

High energy,high reactivity

ROOH

Hydroperoxide,decomposes to formsmelly 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 R O2

.

OHHO

Hydroperoxylradical

Low energy, low reactivity

High energy,high reactivity

O.HO

ROOH

This radical is low energy,does not continuepropagation 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 continuepropagation reaction

ROOHDecomposes to formsmelly products

R.Returns to propagation,continues cycle

Without With

ROOHDecomposes to formsmelly 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. orR.

RH

phenolic

Incr

easi

ng

ener

gy

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