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Abstraction Reaction Lab Report

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The addition of OH radical to β-carotene is slightly less thermodynamically favourable than the H-atom abstraction reactions. The free energy of the OH-radical addition reactions to the 15E and 15Z isomers is illustrated in Figure 5 and the thermochemistry of the reactions calculated at various levels are summarized in Table 4. The OH radical addition reactions are more entropy driven in the solvent medium than in the gas phase. The OH-radical adduct resulting from the 15Z isomer is more exergonic by ~ 15 kJ/mol than the OH-radical adduct at the 15E isomer in all the three phases studied. A previous study on the addition of NO2 radical to C5-position of β-carotene reported the reaction to be endergonic in the polar and non-polar solvents.13 The exergonicity of the OH-radical additions to β-carotene is attributed to the high reactivity of OH radical compared to that of NO2 radical. On comparing with the H-atom abstraction reactions, the OH radical addition reactions are less entropy driven owing to the fact that the addition reactions can occur spontaneously only at low temperatures. Similar to the H-atom abstraction reactions, the addition reaction to 15Z isomer is equally favourable as that of the all-trans isomer. It is interesting …show more content…

The addition of the OH radical to the C5-position of β-carotene has an effect on the orientation of the β-ionone rings on both the isomers. Further, the conjugated system along the polyene chain is modified, as evident from the C-C single and double bond distances shown in Figure 5. In both the isomers, the conformation of the hydroxyl group pointing away from the methyl group is found to be energetically more stable. The spin is delocalized over the polyene chain and also in the -CH2 groups. This result in a larger degree of conjugation in the central part of the polyene chain and also the spin in the central part of the chain remains no

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