Determining keto/enol ratio with IR

[Horseb0x]

How can you determine the ratio of the keto tautomer of a compound to its enol form? I know which functional groups are associated with which but I have no idea how you'd use an IR spectrum for quantitative analysis like this.

 

[chemisttree]

Well... the enol form has that OH group and the Keto form has that carbonyl group. I'm not sayin' it'll be easy, just noting that there are those two groups to look at...

 

[DDTea]

Using IR for quantitative analysis of concentrations is finicky. Couple that with the fact the keto-enol equilibrium strongly favors the keto form (such that the enol form is absolutely miniscule in most cases), and this is going to be a difficult route to doing this.

 

[Horseb0x]

The IR for the compound in question shows no -OH peaks at all so I can probably assume that the vast majority of it is in the keto form but the question asked if the ratio can be determined with IR. I suppose the answers no.

 

[DDTea]

It's *possible* to design an experiment where you could, but as I said it's finicky. You would use Beer's Law, so you'd have to keep the pathlength constant. As usual for using Beer's law, you would construct a calibration curve. A more sophisticated analysis might use a multivariable calibration curve. In any case, the peaks that you are measuring, I presume O-H, C=O, and C=C, would have to be remote enough from adjacent peaks that they could be easily measured. Also, as you mentioned, a lot of things can be lost in the background noise especially if the concentration of the analyte is very low (as the enol form would be).