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I successfully synthesized ethyl gallate from gallic acid and ethanol (of course there are other reagents too, like DCC and Pyridine). Before conducting FTIR analysis, I performed column chromatography and verified the purity of my product through TLC.

Here are the FTIR results obtained for both my product and the gallic acid precursor: Ethyl Gallate FTIR

Gallic Acid Ftir

My questions/problems:

  1. Personally, I'm fairly confident that my product is ethyl gallate. However, my Professor said that my ethyl gallate IR spectra closely resemble my gallic acid IR spectra, especially in the fingerprint region. So, I'm wondering if my product is truly ethyl gallate or not.
  2. If it's truly ethyl gallate, could you help with some convincing arguments as to why it is genuinely ethyl gallate?
  3. Lastly, are there any weird or unusual peaks present in my ethyl gallate IR spectra?

Please feel free to share as many opinions as you'd like.

Note:

  1. Both the gallic acid and ethyl gallate samples utilized for FTIR analysis were in a solid state.
  2. I used a KBr pellet for this FTIR analysis
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    $\begingroup$ Can you get a 1H NMR of the product? $\endgroup$
    – Waylander
    Apr 5 at 10:51
  • $\begingroup$ @Waylander I actually want to get a 1H NMR of my product but sadly I can't because of various reasons. $\endgroup$
    – Jonathan
    Apr 5 at 12:16

2 Answers 2

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The short answer: consider a technique complementary to IR spectroscopy, for instance $\ce{^1H}$-NMR spectroscopy.

Your have good luck, as the public AIST spectral database holds records of IR spectra recorded under the same conditions as yours (a pellet of $\ce{KBr}$) both of gallic acid monohydrate (entry 806) and its ethyl ester (entry 6371):

enter image description here

and

enter image description here

The differences in the position of the absorption bands, and their relative intensity ratios are small.* But as the spectra are plot to same dimensions, one can easily superimpose the two. The freely available program imagemagick offers a comparison, basically running the command

compare image_1.gif image_2.gif difference.png

to compare two images image_1.gif and image_2.gif of same dimension to write a new file (difference.png) to highlight in red color areas where the two former differ. The three individual illustrations can be joined in an animated gif:

enter image description here

Do you see the variation for instance in the region around 500 to $\pu{800 cm-1}$, and around $\pu{1300 cm-1}$? It appears as subtle, but more importantly is difficult to quantify if you want to determine the yield of isolated ethyl ester. You are better off with NMR spectroscopy here, if available to you.

* The concentration of your analyte in a disc of $\ce{KBr}$ can vary from experiment to experiment. Hence absolute intensities (similar to UV-Vis spectroscopy) are less easy to compare between samples recorded under identical conditions, than e.g. by ATR IR spectroscopy.


Side note on the representation of the IR spectra: it is better to split the energy axis and to expand the fingerprint region instead of one axis running continuously from $4000\ldots\pu{500 cm-1}$ . In the case of Omnic, the software by Nicolet / ThermoScientific to record and process IR spectra, this option is available in the menu view -> display -> set x axis with either level default, split at 2000 cm-1, or split at 2000 and 1000 cm-1. This often is a more (visually) accessible presentation of the data, and offers more space to annotate the peaks, especially if applied consistently. Below the example of the company's reference spectrum of acetophenone (fine dashed lines mark the breaks):

enter image description here

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In determining whether you successfully synthesized ethyl gallate from your gallic acid precursor, let's go over what differences between the two you should expect in your spectra.

Ethyl gallate (image source: NIST) Ethyl gallate (image source: NIST)

Gallic acid (image source: NIST) Gallic acid (image source: NIST)

In your gallic acid, you should expect a nice, broad O-H stretch from your carboxylic acid group (which we see.) However, because your product should have gone through esterification, that peak shouldn't be there. We see a nice dip at 2921 which seems to indicate the compound did undergo esterification.

Now let's look for the ester. This should stretch at around 1740. I'm not seeing it in your spectra, but it could be masked by the C=O stretch (which you'll notice on the spectra for both compounds.) You should also look at around ~1200 for your C-O stretch on your ester. I'm not seeing it (that doesn't mean it isn't there.)

Lastly, let's go over the fingerprint region. I had to look up ethyl gallate and gallic acid spectra in literature. There appear to be minor differences in the fingerprint region of each, but it's not discernable to me whether I could truly compare your spectra with their spectra (and indicate whether you have ethyl gallate or not.) The fingerprint regions between the two in literature are too similar, in my eyes, for me to apply them to your experimental results.

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