Question

6.)

a. Two compounds have the same R_f (0.74) under identical conditions. Does it mean that they have identical structures? Explain.

b. On the basis of chemical structures, briefly explain why xanthophylls are more polar than carotenes, and chlorophylls more polar than pheophytins.

c. Which compounds do you expect would have a larger R_f value, carotenes or xanthophylls?

d. Arange the following compunds in order of increasing polarity: lutein, neoxanthin, $\beta$ -carotene, violaxanthin?

e. Do you think that the hexanes extract wykd contain flavonoids? Briefly explain.

Answer 1

(a) No. Not necessarily. Under certain limited conditions where they are only a few possible compounds present, and each is known to have a different Rf facto, then it would reasonable to say that having the same Rf factor means that they are the same molecuels.

(b) Xanthophylls have -OH groups where as carotenes are unsaturated hydrocarbons. Hence xanthopyls are more polar than carotenes.

Pheophytins are similar in structue with chlorophylls except in the place of Mg, they have Hydrogen.(less polar) . Hence Pheophytins are less polar than chlorophylls.

(c) More is the polariy less is the Rf value. Hence Xanthopylls have larger Rf value than carotenes.

(d) More is the number of hydroxyl groups more is the polarity.

Hence the following order,

Beta carotene < Leutin < violaxanthin < neoxanthin