Gas Chromatography

Leeg17

Was pointed here by a mod, as he couldn't answer my question..

I did a GC this week in Organic Chemistry lab, throwing up 4 main peaks, 3 were given to us and as we were the only ones with a 4th peak we were told to find it out for ourself.

I've been googling for the answer but to no avail. The peak was small (~5%) and had a retention time of 3.37 minutes.

Does anyone know of a database where I could find the answer?

All help is appreciated

Baralis1

Assuming that you are using a Flame Ionization Detector, there is only one way to identify the peak using the GC. That is to inject a standard injection of a substance that you suspect might be the peak using the exact same GC set-up as your previous test and if the retention times are the same, they are likely to be the same substance but not definately.
An online database of peaks identified by retention times would not be possible as the retention time will depend on the system, column, system set up, gases, sample prep among other variables.
I imagine though that you are meant to identify the peak from your knowledge of the chemistry of whatever you injected. The GC you ran is only telling you that there is a fourth peak and what concentration/percentage it is compared to one or more of the other peaks.

SOL

Presumeably this was to analyse the products of an organic reaction? Maybe you just need to figure out the other products you might have?

concussion

What were the other three analytes? Did you prepare them as a group or did the whole class share a common set of standards?

Leeg17

It was the synthesis of Cyclohexanone from Cyclohexene.

We got 3 standards that nearly everyone else got,
A - Acetone
B - Cyclohexanone
C - Cyclohexanol

and D, the one I'm trying to figure out.

Unfortunately there's no chance of testing stuff we might suspect it to be.

We prepared the synthesis in groups of 4, but we all happened to share peaks A B C, D is what he got that nobody else did

concussion

What sequence did they elute? Is it as above - acetone, cyclohexanone, cyclohexanol then unknown?

What type of column was use?

It's most likely some of the unreacted start material or the product of a side-reaction. It's a bit strange that you didn't inject a sample of cyclohexene, that could have cleared things up right away.

Leeg17

As above yea.

The column was an AT wax (carbowax) column.

We were given the gas chromatographs of acetone, cyclohexanone and cycloexanol basically told these are ABC, find D.

concussion

Hmm, I don't think that D is some unreacted start material (cyclohexene). The separation is based on polar interactions between the PEG -OH groups and the ketone and hydroxyl groups of the analytes. Cyclohexene doesn't have any of these groups and should elute first. It could be a di-hydroxy product of the synthesis.

Leeg17

Talking to the lecturer today for the tutorial, and after a few mins she said that it's most likely ethanoic acid that we used at the start.

Thanks for your help

concussion

Two H-bonding sites? I'll take that