Chromatography: Polarity Of Compounds
Chromatography
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Separation based on polarity of compounds
Two potential phases for a compound to exist in: mobile and stationary
Partitioning of compounds between mobile phase and stationary phase occurs: o Compounds that are less polar move more in the mobile phase, those that are more polar “stick” more on the stationary phase o These polarity differences cause compounds move at different rates and therefore can be separated
1. Mobile Phase: the phase the moves; can be gas or liquid (solvent; eluent)
• For liquids, the polarity of a solvent is defined as its ability to dissolve polar organic compounds.
• Compounds dissolved into the solvent spend more time moving in the mobile phase • Non-polar solvents can only dissolve …show more content…
• Very polar solvents will be able to dissolve ALL compounds.
For Chromatography: “Polar Dissolves More” not “like dissolves like”
Some Common Solvents: (listed by increasing polarity)
• Petroleum ether (C5 isomer compounds, like pentane; not a true “ether”)
• Ligroin (C6 isomer compounds, such as hexane)
• Diethyl Ether (CH3CH2OCH2CH3)
• Dichloromethane (CH2Cl2)
• Ethyl Acetate (ester: CH3CH2OC(O)CH3)
• Methanol (alcohol: CH3OH)
• Acetic Acid (carboxylic acid: CH3CO2H)
And of course – mixtures of the above solvents can be used.
Consider the following example: Compound A is a non-polar compound and compound
B is a polar compound.
Add Petroleum ether (non-polar!) – Which will dissolve?
Only Non-Polar A
Add Diethyl Ether (more polar!) – Which will dissolve?
Both!
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In today’s lab, we’ll begin the separation for column chromatography with petroleum ether and only remove the non-polar compound. We will then switch to a more polar mixture of petroleum ether and diethyl ether to remove a more polar …show more content…
same solvent system). To identify an unknown compound, spot known compounds onto the same plate as your unknown compound and run the compounds side-by-side to compare.
• The spots for the same compounds will not only have the same Rf value but also have the same appearance.
Same Compound or Not?
TLC can also be used to check the purity of a compound. A pure compound will always appear as a single spot (because it is a single compound) regardless of what solvent system is used for developing the plate. The solvent system should be neither very polar nor very non-polar when checking this. This should result in an
Rf value in the range of 0.2-0.8, ideally. Non-polar solvents would not move the spot enough and polar solvents would move them too much to be able to evaluate.
Pure or Not?
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non-polar solvent polar solvent Using TLC to follow the progress of a reaction:
In the following reaction, the alcohol product is more polar than the starting ketone. With regard to the TLC, you would expect that the ketone appear higher on the TLC plate (higher Rf value) while the alcohol would appear lower on the plate
(lower Rf
•
•
•
Separation based on polarity of compounds
Two potential phases for a compound to exist in: mobile and stationary
Partitioning of compounds between mobile phase and stationary phase occurs: o Compounds that are less polar move more in the mobile phase, those that are more polar “stick” more on the stationary phase o These polarity differences cause compounds move at different rates and therefore can be separated
1. Mobile Phase: the phase the moves; can be gas or liquid (solvent; eluent)
• For liquids, the polarity of a solvent is defined as its ability to dissolve polar organic compounds.
• Compounds dissolved into the solvent spend more time moving in the mobile phase • Non-polar solvents can only dissolve …show more content…
• Very polar solvents will be able to dissolve ALL compounds.
For Chromatography: “Polar Dissolves More” not “like dissolves like”
Some Common Solvents: (listed by increasing polarity)
• Petroleum ether (C5 isomer compounds, like pentane; not a true “ether”)
• Ligroin (C6 isomer compounds, such as hexane)
• Diethyl Ether (CH3CH2OCH2CH3)
• Dichloromethane (CH2Cl2)
• Ethyl Acetate (ester: CH3CH2OC(O)CH3)
• Methanol (alcohol: CH3OH)
• Acetic Acid (carboxylic acid: CH3CO2H)
And of course – mixtures of the above solvents can be used.
Consider the following example: Compound A is a non-polar compound and compound
B is a polar compound.
Add Petroleum ether (non-polar!) – Which will dissolve?
Only Non-Polar A
Add Diethyl Ether (more polar!) – Which will dissolve?
Both!
1
In today’s lab, we’ll begin the separation for column chromatography with petroleum ether and only remove the non-polar compound. We will then switch to a more polar mixture of petroleum ether and diethyl ether to remove a more polar …show more content…
same solvent system). To identify an unknown compound, spot known compounds onto the same plate as your unknown compound and run the compounds side-by-side to compare.
• The spots for the same compounds will not only have the same Rf value but also have the same appearance.
Same Compound or Not?
TLC can also be used to check the purity of a compound. A pure compound will always appear as a single spot (because it is a single compound) regardless of what solvent system is used for developing the plate. The solvent system should be neither very polar nor very non-polar when checking this. This should result in an
Rf value in the range of 0.2-0.8, ideally. Non-polar solvents would not move the spot enough and polar solvents would move them too much to be able to evaluate.
Pure or Not?
6
non-polar solvent polar solvent Using TLC to follow the progress of a reaction:
In the following reaction, the alcohol product is more polar than the starting ketone. With regard to the TLC, you would expect that the ketone appear higher on the TLC plate (higher Rf value) while the alcohol would appear lower on the plate
(lower Rf