Chromatography Forum

do enol and keto forms of a same compound show two peaks in the HPLC measurement??? or only one???

In several paper they described they do have two at very low temperature (-60C)
some of them did the detection by UV spectrophotometer instead of HPLC

Could anyone give some hints>?????

If there is enough present of each isomer and both absorb within the range of your spectrometer than you must see them. Also, if the isomers are close enough in energy to have both present in reasonable amounts you will not seperate them by HPLC, . . . . the equilibrium is way too fast (I know of no exceptions).

If there is enough present of each isomer and both absorb within the range of your spectrometer than you must see them. Also, if the isomers are close enough in energy to have both present in reasonable amounts you will not seperate them by HPLC, . . . . the equilibrium is way too fast (I know of no exceptions).

Thank you very much Mueller, is there any ways I could distinguish if their isomers are energically close or not?

you need to google this if you want to know more, for instance,

http://oxalaceticacid.com/

Generally, as already mentioned one can check spectrometry to get the relative concentrations. I would not dare to guess where the equilibrium might lie. In the case where the enol forms an aromatic ring it is easy to guess, thus phenols ( the enol) are usually completely dominating the equilibrium.

Thank you very much Mueller, is there any ways I could distinguish if their isomers are energically close or not?

Yes, if you see only one peak!

Actually, the question is less one of the relative energy levels than of the activation energy (which will influence the rate of interconversion). If the rate is fast, such that equilibration occurs quickly relative to the residence time in the chromatography system, then you will only see one peak, representing the average composition. If the rate is very slow, then you may see two peaks (assuming, of course, that the chromatography system can separate them!). If the rate is comparable to the residence time on the column, then you may see a misshapen peak (flat-top, split, shoulder, or . . . ).

Well. if you see one peak it can mean that interconversion is fast relative to the chromatography, or that equilibrium lies all on one side. Thus phenol will give only one peak, because that is essentially the only species present (the energy difference between the enol and keto forms is quite large here).
Chemical kinetics and thermodynamics are not totally unrelated, that´s why I didn´t want to give any more details above.

Well. if you see one peak it can mean that interconversion is fast relative to the chromatography, or that equilibrium lies all on one side.

Agreed!