AAMC 9: Gas Chromatography

[sudo]

Here's the Question/Answer:

Here's Table 1:

And here are the compounds:

As you can see, I mathematically figured out that C and D weren't going to be my answer, so I luckily guessed between A and B. Can someone explain to me the reasoning behind the connection between Peak 1 = Compound II and Peak 2 = Compound III?

I thought: increased polarity = decreased Rf. Compound III looks most polar, so I thought it would be associated with Peak 1, for having the lower retention time, but obviously this is the wrong reasoning..

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[N1st]

Here's the Question/Answer:

Here's Table 1:

And here are the compounds:

As you can see, I mathematically figured out that C and D weren't going to be my answer, so I luckily guessed between A and B. Can someone explain to me the reasoning behind the connection between Peak 1 = Compound II and Peak 2 = Compound III?

I thought: increased polarity = decreased Rf. Compound III looks most polar, so I thought it would be associated with Peak 1, for having the lower retention time, but obviously this is the wrong reasoning..

The better the affinity of the compound for the stationary (liquid) phase, the longer the retention time of the compound. Compound III indeed looks more polar, so if you used a polar stationary phase, its retention time would be longer.

You don't necessarily use a polar stationary phase. It would work just as well to use a nonpolar one, and then the order of elution would be the opposite. Perhaps there was a passage attached to the question and it mentioned whether the stationary phase was polar or nonpolar?

 

[co2013]

In this case, you didn't really really need to have outside knowledge to figure out the answer. The passage states that product II was formed initially followed by the formation of product III. Peak 1 had the shorter retention time so that corresponds to the first product, compound II. Peak 2 is for compound III which formed afterwards and had the longer retention time. Just another example where some close reading can earn some crucial points haha. Reread the passage and see if it makes sense OP

 

[Night Hawk]

Here's the Question/Answer:

Here's Table 1:

And here are the compounds:

As you can see, I mathematically figured out that C and D weren't going to be my answer, so I luckily guessed between A and B. Can someone explain to me the reasoning behind the connection between Peak 1 = Compound II and Peak 2 = Compound III?

I thought: increased polarity = decreased Rf. Compound III looks most polar, so I thought it would be associated with Peak 1, for having the lower retention time, but obviously this is the wrong reasoning..

The Br's in cpd 3 are mucho electronegative and remember that there is rotation around that central bond. So they are going to do everything in their to stay in an anti-staggared conformation, giving them a net dipole of zero, making them less polar than cpd 2.

 

[sudo]

In this case, you didn't really really need to have outside knowledge to figure out the answer. The passage states that product II was formed initially followed by the formation of product III. Peak 1 had the shorter retention time so that corresponds to the first product, compound II. Peak 2 is for compound III which formed afterwards and had the longer retention time. Just another example where some close reading can earn some crucial points haha. Reread the passage and see if it makes sense OP

I definitely see where you're coming from. I think, originally, the reason I chose the right answer was because I was vaguely making this connection but wasn't certain if it was the right approach.


Thank you all

 

[kpatel1123]

The polarity is a mute issue here because it depends on you absorbent being used. If absorbent is polar then polar gas molecules will elute more slowly due to the attraction with the absorbent or stationary phase. Since both of these compounds are polar they will both interact with the absorbent in pretty much the same way will little difference.
Now in terms of figuring out which compound is associated with which peak you need to recall that in gas chrome that samples are vaporized into gas for analysis. Therefore, the compounds move through the apparatus as gases. With all things being equal, since they are both polar, they will both interact with the absorbent to the same degree. Therefore, what else would effect the rate at which a gas diffuses/elutes? Well that would be molecular weight from Graham's law of diffusion. Form this we know that the heavier gas will move or elute more slowly in this situation.
This means compound III is associated with peak II. Another important thing to be aware of is that the areas under the peaks of gas chromatogram give are directly proportional to the amount eluted. Therefore, in this situation we have a total of 15cm^2, so 5/15=.33 for peak one. This means that 33% of the elutant will be the compound associated with peak I or compound II.