Late night homework help ;)

[sweetcalie08]

Hey guys,

I've been working on a lab report for some time now and it's due tomorrow. I'm getting confused about something and thought there'd be someone on here up late. Do any of you remember doing an analgesic separation in orgo lab? If so, maybe you can help.

We used a TLC coated with silica gel and used ethanol as our solvent. When we used nonpolar hexane solvent for a different lab, lower Rf values meant greater polarity (due to the affinity for the polar silica gel so less movement on the plate). When the ethanol solvent is used, do lower Rf values still mean greater polarity? Or, does the solvent make a difference. I am currently concluding that caffeine is more polar than aspirin and acetaminophen...and that starch/cellulose is the least polar of them all.

Now, trying to explain why Acetaminophen is insoluble in CH2Cl2 but soluble in ethanol has got me tripped up too.

Gosh, I think I'm making things more complicated than they should be! Maybe that's what 4 days of straight ochem studying does to you...even bringing homework to the fireworks. Any help would be appreciated...sorry I had to post this on SDN, but I'm running out of options!

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[MEG@COOL]

Hey guys,

I've been working on a lab report for some time now and it's due tomorrow. I'm getting confused about something and thought there'd be someone on here up late. Do any of you remember doing an analgesic separation in orgo lab? If so, maybe you can help.

We used a TLC coated with silica gel and used ethanol as our solvent. When we used nonpolar hexane solvent for a different lab, lower Rf values meant greater polarity (due to the affinity for the polar silica gel so less movement on the plate). When the ethanol solvent is used, do lower Rf values still mean greater polarity? Or, does the solvent make a difference. I am currently concluding that caffeine is more polar than aspirin and acetaminophen...and that starch/cellulose is the least polar of them all.

Now, trying to explain why Acetaminophen is insoluble in CH2Cl2 but soluble in ethanol has got me tripped up too.


For solulability you not only need to consider whether a solvent is polar or not but its dielectric constant or strength of polarity, (and I think ability to hydrogen bond).

As for your TLC I am guessing that the more polar substance would move farther up the strip in this case. A non polar substance should not move as I recall.


Anyone feel free to correct me if I am wrong.... This is all just from memory!!! MAy memory serve me well. lol.
Gosh, I think I'm making things more complicated than they should be! Maybe that's what 4 days of straight ochem studying does to you...even bringing homework to the fireworks. Any help would be appreciated...sorry I had to post this on SDN, but I'm running out of options!

For solulability you not only need to consider whether a solvent is polar or not but its dielectric constant or strength of polarity, (and I think ability to hydrogen bond).

As for your TLC I am guessing that the more polar substance would move farther up the strip in this case. A non polar substance should not move as I recall.


Anyone feel free to correct me if I am wrong.... This is all just from memory!!! MAy memory serve me well. lol.

 

[Medikit]

Are you sure the stationary phase was still silica on the second run?

Mega, if the pH was acidic it would explain the caffeine being more "polar" over the aspirin, but not the aspirin being more polar than the cellulose. I'm guessing that it's reverse phase (non-polar stationary phase with a polar solvent)

 

[peanut99]

i may be wrong too, but i thought that the more polar the substance the lower Rf value it would have (due to the the substance adhering to the silica plate). assuming the same silica plate, i think the more polar substance would still have the lowest Rf value.

as for polarity, definitely look at interactions (hydrogen bonds) between the solute and the solvent. more interactions=more soluble.

it has been a while for orgo, so i hope this is right.

good luck with the lab!

 

[MEG@COOL]

i may be wrong too, but i thought that the more polar the substance the lower Rf value it would have (due to the the substance adhering to the silica plate). assuming the same silica plate, i think the more polar substance would still have the lowest Rf value.

as for polarity, definitely look at interactions (hydrogen bonds) between the solute and the solvent. more interactions=more soluble.

it has been a while for orgo, so i hope this is right.

good luck with the lab!

But the solvent is also polar... So Was this misinformation that she gave or what would be the determining factor in movement? Polar substrate+polar solvent= ??.

 

[sweetcalie08]

i may be wrong too, but i thought that the more polar the substance the lower Rf value it would have (due to the the substance adhering to the silica plate). assuming the same silica plate, i think the more polar substance would still have the lowest Rf value.

as for polarity, definitely look at interactions (hydrogen bonds) between the solute and the solvent. more interactions=more soluble.

it has been a while for orgo, so i hope this is right.

good luck with the lab!

Thanks. And yep, I'm sure it was silica. When I was in the tutor room last week, I remember the TA saying it was silica. Well, a couple people have confirmed my original thought that more polar=lower Rf.

Medikit: I'm confused about this-- Mega, if the pH was acidic it would explain the caffeine being more "polar" over the aspirin, but not the aspirin being more polar than the cellulose. I'm guessing that it's reverse phase (non-polar stationary phase with a polar solvent). --- This doesn't make sense for polarity (?): cellulose<acetaminophen<aspirin<caffeine ... cellulose is not UV active so its Rf value could not be taken...that's why I'm trying to confirm that cellulose really is nonpolar.

 

[sweetcalie08]

But the solvent is also polar... So Was this misinformation that she gave or what would be the determining factor in movement? Polar substrate+polar solvent= ??.

Yeah, that's part of my confusion. I didn't know if silica would still be the most polar (over the polar ethanol solvent)...wasn't sure which would dictate polarity.

 

[peanut99]

But the solvent is also polar... So Was this misinformation that she gave or what would be the determining factor in movement? Polar substrate+polar solvent= ??.

again, working from total memory...
i thought Rf was referring to what was on the plate and how the substance (solute dissolved in solvent) adhered to the stationary phase.

i would think polar solvent + polar substrate would mean lower Rf than nonpolar substrate + polar solvent. i think it's important to know what the stationary phase is.

 

[sweetcalie08]

Thanks everyone for your replies. I decided that, since it's already after 2am, I'm just going to be a little less scientific in my explanations and hope I'll be okay. I'm probably just thinking too deep into this anyway. Gosh, I really regret wasting so long on this worksheet now. Oh well, sweet dreams for me for 4 hours! Haha. Again, thanks.

 

[Violinrose]

silica is always the most polar substance you have with TLC - it is more polar than any solvent including ethanol.

no matter what the identity of your compound, the more polar your mobile phase is (e.g. ethanol vs. hexane) the further the compound will travel because of competition with the silica. The polarity of each will only affect the amount of separation you get between structures of varying polarities.

Based on the structures of each compound, I am guessing that in terms of Rf values: cellulose (polymer) > caffeine (relatively nonpolar amide functionalities with no possibility of H-bonding)> aspirin (polar COOH group) > Acetaminophen (polar amide + OH groups).

good luck

 

[Medikit]

Thanks. And yep, I'm sure it was silica. When I was in the tutor room last week, I remember the TA saying it was silica. Well, a couple people have confirmed my original thought that more polar=lower Rf.

Medikit: I'm confused about this-- Mega, if the pH was acidic it would explain the caffeine being more "polar" over the aspirin, but not the aspirin being more polar than the cellulose. I'm guessing that it's reverse phase (non-polar stationary phase with a polar solvent). --- This doesn't make sense for polarity (?): cellulose<acetaminophen<aspirin<caffeine ... cellulose is not UV active so its Rf value could not be taken...that's why I'm trying to confirm that cellulose really is nonpolar.

In acid caffeine will be positively charged, but I don't know what the pka is.

As for the reverse phase, in a reverse phase non-polar will have a lower Rf.

Cellulose is not non-polar, it is your most polar molecule. I don't know which is most polar out of acetaminophen, aspirin, and caffeine, my guess would be caffeine, then acetaminophen, then aspirin, which is the result you got.

 

[TTSD]

What the hell are you nerds talking about? Mods! I demand an immediate closure to this offensive thread.. ochem, ugh! Worse than the KKK!

 

[sweetcalie08]

So cellulose and starch are actually polar? Hmm.

 

[i61164]

So cellulose and starch are actually polar? Hmm.

Cellulose and starch are polymers made up of glucose monomers. Glucose is a 6-carbon ring with several -OH groups attached to it. So, cellulose and starch have many -OH groups which makes them polar and allows them to H-bond with silica gel.