Late-night homework help ;)

[sweetcalie08]

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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! 😕

 

[pharmd-08]

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! 😕

Hi, since I just took the very same lab last semester, I hope I can be a help here.
1. Yes, you are right, lower Rf value= more polar, because the compound would stick to the silica gel more often while being carried on by the solvent.
The less polar substance doesn't have high affinity to the silica gel, thus it will just go straight to the top along with the solvent.

2. This rule above applies to all solvents. When you use a more polar solvent, (such as ethanol vs ethylene chloride) it will have a better ability to 'dissolve' the substance, even those nonpolar ones. Therefore acetaminophen has a better solubility in ethanol rather than in CH2Cl2


I hope this helps, I gave my files to my friend and I already sold my Pavia. So I don't have any concrete reference to back this up.

 

[sweetcalie08]

Thanks for your help pharmd-08! Now let's just hope my picky TA doesn't find every little chance to dock points this time. 🙄 (I did my last lab report very well in comparison to other students' IMHO but their TA's hardly docked any points while mine took a ton off--but that's another story) Now, why did you give all your files away? They could've sold for mega-bucks on SDN. JK 😛 Oh, that good 'ol Pavia text too. Aren't you glad to be done with this class?! I am jealous of you right now, haha. Anyway, better finish up this worksheet so I can get some sleep tonight. Have a lecture exam Wednesday and then a lab quiz Thursday and lab report due Thursday as well. Fun fun.

 

[pharmd-08]

Anytime!! I know Chem2311 is a pain, especially if you have a nasty TA. I was lucky I finally found my TA's style in grading, so I could score pretty high in the lab reports and worksheets.
I didn't give my files away, just lend it to my friend/neighbor who is taking the class right now.
Do you have anyone you could borrow an old file from? This could be a huge help you know.
Good luck on your report! It's better to be busy during the summer rather than not doing anything at all.