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Benzocaine has the following IR spectrum:

IR spectrum of Benzocaine and here is it's structure:

Structure of Benzocaine

From what I understand: The two leftmost peaks are the antisymmetrical and symmetrical stretchs of the amine group. Just before the fingerprint region at about 1690 is the carbonyl ester peak and to the right of it (also before the fingerprint region) is the benzene C-C bonding peaks. The messy broad peaks just before 3000 are the C-H bonds of the non aryl C. The tiny peak at ~3040 is probably the aryl C-H bonds.

However I can't assign the peak at ~3200. It could be water of crystillisation in the sample (anywhere from 3100-3600cm^1) but surely there wouldn't be any in a proffesional sample? I also considered whether it could be due to -NH3^+ but the sample is made in a procedure that ensures the solution is alkaline before final purification...

Please could someone point me to a resource that will allow me to assign this peak or explain what it is to me?

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  • $\begingroup$ 3200-3600 peak is usually for O-H bond, (2200-3600, if this bond belongs to carboxylic acid) so it's better to check for possible O-H bonds (water, alcohol|acid). It can be even air moisture in the worst case. $\endgroup$ – Kelly Shepphard Dec 3 '18 at 17:56
  • $\begingroup$ @KellyShepphard Thanks. This peak is on all the IR spectrums of benzocaine I have looked at. I looked at quite a few because origionally I thought that the peak was maybe a mistake in my spectrum. So would even the professional IR spectrums have this peak due to water? $\endgroup$ – Mirte Dec 3 '18 at 18:10
  • $\begingroup$ Air moisture is omnipresent, but for pro-spectra, sample preparation is way too good to have "water" peak (usually). You have a good spectrum, though! No need to worry (but if you want, you can master sample preparation to make it the best possible in your lab conditions). $\endgroup$ – Kelly Shepphard Dec 3 '18 at 18:24
  • $\begingroup$ I'm betting it's a common primary amine shoulder band. See here: "A shoulder band usually appears on the lower wavenumber side in primary and secondary liquid amines arising from the overtone of the N–H bending band: this can confuse interpretation." $\endgroup$ – SendersReagent Dec 4 '18 at 4:32
  • $\begingroup$ @SendersReagent, I have had a look at the weblink page, however it does not say what causes the shoulder band... I would like to understand what is causing it, not just accept it. Do you know? $\endgroup$ – Mirte Dec 4 '18 at 18:53

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