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Offline wildbiologist2

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Adjacent GC signals?
« on: April 02, 2014, 01:08:16 PM »
Hello fellow H. sapiens!

I am analyzing a GC printout of an organic layer that contains several organic compounds which were byproducts of a reaction. It contains four distinct peaks, but here is the problem:

The GC instrument is pretty crappy, so certain chemicals, such as ortho-xylene, can produce two adjacent signals. I asked a friend of mine and he said that its because there are two resonance forms. Although it is just one chemical on average, the bonding between carbon 1 and carbon 2 is sometimes a single bond and sometimes a double bond. But I disagree to a certain extent...

Could resonance contributors of the same chemical yield two adjacent signals due to the two "distinct" resonance forms? My understanding of resonance is that a molecule does not exist is a certain resonance form at a given point in time, but rather, all exist to some degree simultaneously in an electron delocalization event.

So who is right? What could cause two adjacent GC signals of the same chemical?

Offline discodermolide

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Re: Adjacent GC signals?
« Reply #1 on: April 02, 2014, 01:22:34 PM »
Resonance forms will certainly not produce two peaks on a GC.
Have you thought about the possibility of xylene isomers?
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Offline wildbiologist2

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Re: Adjacent GC signals?
« Reply #2 on: April 02, 2014, 01:58:22 PM »
I was using ortho-xylene as an example.

A little background on the experiment:

It was a grignard synthesis of benzoic acid using CO2 and a synthesized phenylmagnesium bromide.  Made the grignard reagent by adding bromobenzene to Mg and waited for the synthesis to occur. Then I reacted it with dry ice to yield benzoic acid.

However, it occured to me that the there are several impurities. Firstly, benzene, (grignard reagent reacted with atmospheric water.) There could also be bromobenzene which was unreacted from the grignard synthesis. Also there was biphenyl and also trace amounts of benzoic acid in the ether layer when it was analyzed.

The signal which had two adjacent peaks is infact benzene. How could benzene produce two signals? There are no stereoisomers, so there is no possibility that an R stereoisomer could have a different GC signal than the S stereoisomer (as is true with several R/S stereoisomers.)

It still has me perplexed as to the EXACT reason why benzene could yield two adjacent signals...

Offline discodermolide

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Re: Adjacent GC signals?
« Reply #3 on: April 02, 2014, 02:16:03 PM »
Did you inject a sample of benzene from another source?
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Offline wildbiologist2

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Re: Adjacent GC signals?
« Reply #4 on: April 02, 2014, 02:45:12 PM »
Nope, didn't inject any other benzene from another source.

I'm really confused as to why this would occur given the circumstances...

Offline discodermolide

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Re: Adjacent GC signals?
« Reply #5 on: April 02, 2014, 06:18:38 PM »
Then you should try this and see if you get the same result.
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Offline wildbiologist2

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Re: Adjacent GC signals?
« Reply #6 on: April 03, 2014, 02:12:24 PM »
I really wish I could, but I don't have access to the GC instrument. A few other students got the same thing occur for the benzene retention time signal (adjacent peaks.) If I could run another GC, I would.

Offline kriggy

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Re: Adjacent GC signals?
« Reply #7 on: April 03, 2014, 02:14:12 PM »
how do you know that the two peaks are both benzene?

Offline wildbiologist2

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Re: Adjacent GC signals?
« Reply #8 on: April 03, 2014, 04:46:23 PM »
Given the reaction mechanics, compounds used to carry out the reaction, and controlled setting of the reaction when it was taken place, only benzene, biphenl, bromobenzene, and benzoic acid are possible in the ether layer...

Offline kriggy

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Re: Adjacent GC signals?
« Reply #9 on: April 04, 2014, 11:13:04 AM »
Wait. I just re-read it whole from the beggining. You have 4 peaks in your GC and you just listed 4 compounds which can be in the organic layer so where is the problem?
How far away are the peaks?

Offline wildbiologist2

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Re: Adjacent GC signals?
« Reply #10 on: April 08, 2014, 02:58:49 PM »
I have four compounds, I have five peaks.

Two of the peaks are very close to eachother, (within only 10 seconds or so.) Both of which are very close to the standard retention time of benzene.

Offline kriggy

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Re: Adjacent GC signals?
« Reply #11 on: April 09, 2014, 12:10:31 PM »
My experience with GC is very limited but it seems very unlikely that one coumpound could give two separate peaks differentiating by 10 seconds..
What about an impurity in starting material?

Offline voidSetup

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Re: Adjacent GC signals?
« Reply #12 on: April 09, 2014, 07:24:41 PM »
What's your diluent for this analysis? Did you run a blank to see what peaks your solvent contains?  Also, in my experience when I've run xylenes they typically contain bits of the other isomers as well (I.e. p-xylene might contain o-xylene as well)

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