SINGLET OXYGEN MECHANISM AUTOXIDATION VIA SINGLET OXYGEN MECHANISM Group : 8 OLEOCHEMISTRY NO. NAME MATRIC NO 1. WONG HON REN 151597 2. ERIKA NABILA BT AHMAD 154676 3. NOR FAEZAH BT ZAITONG 153344 4. NUR AMIERA BT AZMAN 155629

Photosensitizes-Oxidation Question: Photosensitizes-Oxidation Of Oils and Fats

PHOTOSENSITIZIED-OXIDATION INTRODUCTION: Known as Photooxidation or Light-promoted Oxidation. Free radical reaction. Require a sensitizer (excited by light to triplet state) Two types of reaction Type 1 photooxidation Type 2 photooxidation

Mechanism of Type 1 Photooxidation Initiation Step: Chain reaction initiated by the abstraction of an allylic hydrogen from the unsaturated oil by triplet sensitizer. sensitizer 3sensitizer 3sensitizer + RH R• + H• Radicals (R• ) that initiate the propagation step is produced. light

Mechanism of Type 2 Photooxidation Initiation Step: Energy from triplet sensitizer is transferred to O2. O2 is converted to excited singlet state. 3sensitizer + 3O2 sensitizer + 1O2 Singlet oxygen reacts with unsaturated double bonds of oil to form allylic hydroperoxide. 1O2 + RH ROOH Allylic hydroperoxide formed further decomposes and produces radicals. ROOH RO• + •OH Radicals (RO• ) that initiate the propagation step is produced. decompose

Question: How singlet oxygen is produced in oils and fats

Oil oxidation is accelerated by light, especially in the presence of sensitizers such as chlorophylls. Sensitizers in singlet state absorb light energy very rapidly, in picoseconds, and become excited. Excited singlet sensitizers can return to their ground state via emission of light, internal conversion, or intersystem crossing

Fluorescence and heat are produced by emission of light and internal conversion, respectively. Intersystem crossing results in excited triplet state of sensitizers.

Excited triplet sensitizers may accept hydrogen or an electron from the substrate and produce radicals (type I) as shown in Figure 6. Excited triplet sensitizers react with 3O2 and produce superoxide anion by electron transfer.

Superoxide anion produces hydrogen peroxide, one of the reactive oxygen species by spontaneous dismutation, and the reaction of hydrogen peroxide with superoxides results in singlet oxygen formation by Haber–Weiss reaction in the presence of transition metals such as iron or copper

The excitation energy of triplet sensitizers can be transferred onto adjacent 3O2 to form 1O2 by triplet–triplet annihilation, and the sensitizers return to their ground singlet state (type II). Photosensitized oxidation of edible oil follows the singlet oxygen oxidation pathway. 1O2 was suggested to be involved in the initiation of the oil oxidation. 1O2 either reacts chemically with other molecules or transfers its energy to them. When 1O2 reacts with unsaturated fatty acids, mostly allyl hydroperoxides are formed by ene reaction.

Electrophilic 1O2 can directly react with high-electron-density double bonds without the formation of alkyl radical, and form hydroperoxides at the double bonds. When hydroperoxide is formed, double bond migration and trans fatty acid occur, producing both conjugated and nonconjugated hydroperoxides. Hydroperoxides formed by 1O2 oxidation are decomposed by the same mechanisms for the hydroperoxides formed by 3O2 in autoxidation. Heptenal was formed only in 1O2 -oxidized soybean oil in the presence of chlorophyll and light .

THE END