Lab 707: Seperating a Mixture of Biphenyl, Benzhydrol, and Benzophenone by Thin- Layer Chromatography
Spotting/ Developing TLC Plates
I used a 400 mL beaker as the developing chamber which seemed too big for the TLC plate, since it slipped. Even so, it didn't appear to effect my results. My eluent, dichloromethane, went up the TLC plate until about .5 cm from the top and evaporated quickly from the plate once out of the chamber.
Identifying the Compounds in a Mixture
There was a discripancy between which solution was on the bottom and which was in th middle. One group had Benzhydrol as being in the middle and another had Benzophenone , and the same results for the bottom spot. It was determined that some of the other groups had mistaken one of the individual solutions for the stock solution containing all three compounds. The following week, one of the groups had ran a TLC plate of Benzphenone and determined it was the middle spot, so by process of elimination, the lowermost spot would be Benzhydrol.
Analyzing an Unknown Mixture
My unknown solution (#13) was comprised of Biphenyl and Benzhydrol.
Post Lab Questions:
1a) The eluents in order of increasing polarity are hexane, toluene, dichloromethane, ethyl acetate and methanol.
1b) Hexane is very non polar so it causes the Rf the stock solution to stay low, and doen't allow for seperation of the compounds in the solution. Toulene and dichloromethane allows the compounds in the solution to seperate because it has an intermediate polarity and would thus give a range of Rf values based on the polarity of the individual compounds. Ethyl acetate and methanol are both very polar so they don't allow the compounds that are more non-polar to seperate out and would carry them all together towards the eluent front. This would then give a higher Rf value.
1c) Compound polarity and the polarity of the eluent determine how big of a Rf value you will get, if the compound is non-polar but is put into a very polar eluent, your Rf value will be higher because the stronger attraction from the eluent.
2) We chose to use toluene as the eluent because it seperated the compounds well. Hexane, ethyl acetate and methanol didn't allow the compounds to seperate at all so it would have been foolish to chose them as an eluent. The only other option was dichloromethane, which also seperated the compounds from the solution but we didn't choose it for arbitrary reasons.
3a) If a sample that is too large is applied while spotting the plate it may lead to poor seperation .
3b) If a spotted plate is put into a developing chamber that has an eluent level above the origin the compounds that were spotted on the plate may be dissolved in the eluent, with none left on the TLC plate.
3c) If you allow the plate to remain in the developing chamber until the eluent reaches the top of the plate you will not be able to have a eluent front to compare to the spots to give Rf values.
3d) If you remove the plate from the chamber when the eluent front has moved only halfway up the plate , you will not allow the plate to fully develop and the compounds to reach their polarities.
3e) If you develop the plate in an uncovered chamber you could allow the eluent to evaporate, and not travel up the TLC plate.
4) You could use a TLC plate to tell when the reaction is complete when the solution shows a compound that has a low polarity as indicated by the TLC plate.