Homework Help: Organic Chemistry

Posted by James on Thursday, October 23, 2008 at 9:52pm.

--Why does caffeine have a larger Rf value than chlorophyll?

--If two compounds have the same Rf under identical conditions. Does this show that they have identical structures? Explain.

Organic Chemistry - DrBob222, Thursday, October 23, 2008 at 10:22pm
For the first question, look up the structures and compare the polarity of the two molecules. You would expect the component with the larger Rf value to be less polar because it isn't attracted to the adsorbent on the TLC plate as much as the more polar compound. This point is discussed in the site below.
http://orgchem.colorado.edu/hndbksupport/TLC/TLCrf.html

Also discussed at the same site is the identity question. Two compounds that have the same Rf value MAY be the same but the Rf value is not proof positive. If the Rf values are not the same you KNOW the two compounds are different. If the two Rf values are the same they MAY be the same compound.

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