Qualitative Elucidation via TLC Staining

TLC (Thin Layer Chromatography) offers a simple approach to identifying a solvent(s) for separating out a mixture, monitoring a reaction to completion, etc. Furthermore, the TLC plates can be stained to identify the presence or absence of various functional groups such as amines, ketones, etc. This qualitative experiment does offer various drawbacks.

The process of staining typically requires the items shown in the illustration below.

 The following websites offer more insight into the process of staining.

http://webpub.allegheny.edu/employee/s/smurphre/research/visualization.pdf

Org Prep Daily: http://orgprepdaily.wordpress.com/2006/09/27/tlc-staining-solutions/

Curly Arrow Blog: http://curlyarrow.blogspot.com/2006/11/lets-talk-about-tlcs-part-1-vanillin.html

Qualitative Elucidation via TLC Staining

TLC (Thin Layer Chromatography) offers a simple approach to identifying a solvent(s) for separating out a mixture, monitoring a reaction to completion, etc. Furthermore, the TLC plates can be stained to identify the presence or absence of various functional groups such as amines, ketones, etc. This qualitative experiment does offer various drawbacks.

The process of staining typically requires the items shown in the illustration below.

 The following websites offer more insight into the process of staining.

http://webpub.allegheny.edu/employee/s/smurphre/research/visualization.pdf

Org Prep Daily: http://orgprepdaily.wordpress.com/2006/09/27/tlc-staining-solutions/

Curly Arrow Blog: http://curlyarrow.blogspot.com/2006/11/lets-talk-about-tlcs-part-1-vanillin.html

Allene versus Carbonyl

Functional groups such as allene (C=C=C) and cumulene (C=C=C=C) present an interesting challenge in an elucidation project, especially when an elucidator is not expecting them. The 13C chemical shifts for allene carbons are typically expected at 80, 200 and 100 ppm +/- ~20 ppm per shift. The confusion, or better described as bias, arises at treating a 13C chemical shift of 200 ppm as a carbonyl (C=O) rather than considering the possibility of an allene group.

In the example below, there are an odd number of sp2 carbon atoms along with 1 oxygen atom and 6 hydrogen atoms.

The example leads into two possible candidates: an allene in 1-(ethenyloxy)propadiene ethenyl propadienyl ether or an alkene plus a carbonyl group in penta-1,4-dien-3-one.

Allene versus Carbonyl

Functional groups such as allene (C=C=C) and cumulene (C=C=C=C) present an interesting challenge in an elucidation project, especially when an elucidator is not expecting them. The 13C chemical shifts for allene carbons are typically expected at 80, 200 and 100 ppm +/- ~20 ppm per shift. The confusion, or better described as bias, arises at treating a 13C chemical shift of 200 ppm as a carbonyl (C=O) rather than considering the possibility of an allene group.

In the example below, there are an odd number of sp2 carbon atoms along with 1 oxygen atom and 6 hydrogen atoms.

The example leads into two possible candidates: an allene in 1-(ethenyloxy)propadiene ethenyl propadienyl ether or an alkene plus a carbonyl group in penta-1,4-dien-3-one.

Evaluating a Peak’s Line Shape on a Chromatogram

The line shape of a peak on a chromatogram can offer some insight into the presence of a functional group. However, issues with the instrument are also known to affect the line shape for a peak.

The GC/MS EI+ Total Ion Chromatogram/Current (TIC) below shows five peaks. All of the peaks with the exception of the peak at 1.8 minutes exhibit a symmetrical line shape. The asymmetry of the peak at 1.8 minutes is typical for a structure containing an acidic group (i.e. -COOH).

The peak at 1.8 minutes is 2-ethylhexanoic acid.

Evaluating a Peak’s Line Shape on a Chromatogram

The line shape of a peak on a chromatogram can offer some insight into the presence of a functional group. However, issues with the instrument are also known to affect the line shape for a peak.

The GC/MS EI+ Total Ion Chromatogram/Current (TIC) below shows five peaks. All of the peaks with the exception of the peak at 1.8 minutes exhibit a symmetrical line shape. The asymmetry of the peak at 1.8 minutes is typical for a structure containing an acidic group (i.e. -COOH).

The peak at 1.8 minutes is 2-ethylhexanoic acid.

Blog on Hiatus from December 21 to January 2

As the holidays approach, the weblog Philosophy to Chemistry to Elucidation (P2C2E) will be taking a short break. Posts will resume in the New Year.

Happy Holidays everyone.