Ochem Help

[phosphorylation]

Does anybody know how I can approach these two questions?

The vapor pressures of 1,2-diphenylethane, p-dichlorobenzene, and 1,3,5-trichlorobenzene are 0.06, 11.2, and 1.4 torr respectively, at their melting point (52-54 C). Which compound is likely to be sublimed most rapidly at a reduced pressure of 15 torr and a temperature of 40 C?

Caffeine is soluble in ethyl acetate. Do you think that the purity of your product could be checked by TLC using ethyl acetate as an elution solvent? Explain

I thank you all in advance...

george

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[iamlonely]

damn, are those classes for the mcat, or for just your regular lab part of the course??

i have no idea about the 1st one, thought it was from gen chem?

but the 2nd one we did in the lab and:
if caffeine is not pure, then the tlc will show several different compounds(spots) on the plate, so it can be used... if its pure, it will show only 1 compound(spot).

 

[QofQuimica]

The vapor pressures of 1,2-diphenylethane, p-dichlorobenzene, and 1,3,5-trichlorobenzene are 0.06, 11.2, and 1.4 torr respectively, at their melting point (52-54 C). Which compound is likely to be sublimed most rapidly at a reduced pressure of 15 torr and a temperature of 40 C?

I agree, this IS an odd question for an organic class! The substance with the highest vapor pressure is the one that will boil (or sublime) off first. Remember that boiling (or sublimation) occurs when the vapor pressure is equal to the atmospheric pressure. So if you decrease atmospheric pressure to 15 torr, then you will also decrease the temp. needed to boil or sublime your substance. But the one that is most volatile will still have the greatest vapor pressure, and therefore it will sublime off fastest.

Caffeine is soluble in ethyl acetate. Do you think that the purity of your product could be checked by TLC using ethyl acetate as an elution solvent? Explain

Probably not, because the caffeine will have too high of an Rf. You could certainly load your sample in EtOAc, because it will dry off anyway before you run the TLC. But you would want to pick a less polar system (maybe 3:1 hexane/EtOAc would be a good first one to try) for your eluting solvent, because otherwise, that EtOAc will carry your caffeine all the way to the top of the plate with the solvent front, and you won't see the spots separate. Ideally you should shoot for an Rf somewhere in the middle of the plate, like 0.3-0.7, if you want to see the spots separate.

 

[gotgame83]

Id rather use HPLC...

and dichlorobenzene would probably sublime most rapidly

 

[phosphorylation]

thank you so much guys, I really appreciate your help...

its an organic lab I class, it really sucks, because i totally forgot general chem, and i find myself having to go online and google some old concepts. I did all the homework questions, but I just couldn't figure these ones out, even with with my old gen chem book...

thank you so much though, i greatly appreciate your help!

george

 

[gujuDoc]

Actually, these kind of things are the kind of things they can ask on an organic portion of the mcat because you can put them in the context of a passage and incorporate a few different ideas.

 

[iwutitan]

Does anybody know how I can approach these two questions?

The vapor pressures of 1,2-diphenylethane, p-dichlorobenzene, and 1,3,5-trichlorobenzene are 0.06, 11.2, and 1.4 torr respectively, at their melting point (52-54 C). Which compound is likely to be sublimed most rapidly at a reduced pressure of 15 torr and a temperature of 40 C?

Sounds like one of the MCAT questions that I guessed on last month.

Caffeine is soluble in ethyl acetate. Do you think that the purity of your product could be checked by TLC using ethyl acetate as an elution solvent? Explain

<-- Yes, one spot --> pure, more than one spot --> impure

 

[iamlonely]

interesting, i havent started reviewing this section for the mcat... but are there any practice questions on such topics in the prepbooks, like ek-orgchem or kaplan orgchem? thanks