curcumin ques ans

Answers and Translations 03 f 3 3.2 Curcumin Answers Scheme 3.2-7 A. Two effects stabi lize the enol form. First, in the enol form a fully conjugated system exists which is accompanied by the release of energy of delocalization. Second, an intramolecular hydrogen bonding is established which again releases bond energy. Neither effect exists at the 1,3-diketo unit. B. Regioisomerism (constitut ional isomerism) . In the case that both tautomers exist in distinct amounts, a simple standard TLC on silica gel plates often shows a pair of spots with the enol spot in front, i.e. with the larger R value. This indicates that the enol form is the less polar one, w hich can be understood in terms of an internal “saturation of polar groups ” by the hydrogen bonding. C. The enol form has an extended chromophore with 20 -electrons in conjugation. The chromo phore contains phenyl rings and alkene units as weak chromophores and one keto group is included as a strong chromophore. A bathochromic shift of the absorption maximum is caused by OH and OCH groups as auxochromic substituents and also by the enolic OH group. It is not to assumed that the pure diketo form is coloured because the conjugation is interrupted there. If one cuts it into two units, a substituted cinnamic acid remains (ferulic acid) which has too small a conjugation to be as intensively coloured as the enol form. D. At higher pH, two processes can occur: rst the acidic enolic proton and then the phenolic hydrogen atoms dissociate. The observed red shift in the UV spectrum indicates a lower energy barrier of the charged -system between the ground and rst excited states. E. Free rotation of the phenyl rings around the C-1 C-1' bonds with both conformers popul ated.

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curcumin ques ans