Post-Lab
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.
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.
OR
1. a) The five eluents could be arranged as follows in order of increasing polarity.
Hexane < Toluene < Dichloromethane < Ethyl acetate < Methanol
b) The effects of eluent polarity on the Rf value of each stock solution mixture compound.
Based on the experiment I could say that two of the compounds were either too polar or too non polar which means that either did not move from the origin at all (hexane- too non polar) or they went very close to the top of the plate (methanol – too polar).
The rest three compounds (dichloromethane, ethyl acetate and toluene) gave us pretty good separation by leaving two to three spots on the plate.
c) As stated in general chemistry II “Like dissolves like” means that polar compounds will interact will polar and non polar with non polar. Therefore, the Rf value will depend on plarity.
- We selected Toulene as our eluent because it gave us the best separation by leaving three spots on the plate. Dichloromethane gave us also three spots but in the middle of the experiment something did not work out very well and we decided to take it out of the list. The other compounds either did not separate at all or gave us one or two spots.
- a.) If we apply too much of the sample on the plate, we are risking of getting not very good results because we might not be able to distinguish the spots and they might get mixed up if the distance between them is very small. Moreover, we will get poor separation.
b.) If we put the plate in the chamber with eluent level above the level of the plate, we risk the sample to dissolve from the plate into the eluent layer.
c.) If we forget and leave the plate in the chamber until the eluent front reaches the top of the plate, then the mixture compound may keep moving along the plate which will lead us to not valid Rf measurements.
d.) If we remove the plate from the chamber before it reaches the eluent front, then we might not be able to see the full process of separation. This will also lead us to not valid Rf values.
e.) If we leave the chamber open and keep developing the plate, then the eluent will evaporate in the air and will not allow proper separation of the plate which again will lead to wrong Rf values if any at all.
4. If you prepared benzhydrol by the reduction of benzophenone with sodium borohydride, we could use TLC to decide when the reaction was completed by performing the same experiment with compounds that will give separation and mark the spots. Then do the experiment again with sodium borohydride and spot it. The result will be on the same spot as benzhydrol.
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