I think it is not possible because enantiomers can be separated in chiral environment only (eg by using a chiral catalyst or solvent etc.). You can't separate enantiomers on C-18 coulmn, for that you have to use a chiral coulmn (eg Chiralcel OD-H, OJ-H etc.)

If the compound is pure, then the only three reasons I can think of that would cause it to give multiple peaks on a column are:

1) The compound is decomposing during the chromatography; or

2) The compound is converting into other forms (atropisomers, tautomers, epimers etc.); or

3) The compound is forming long-lived associations with other substances in the eluent (formic acid and trifluoroacetic acid are common additives to LC-MS mobile phases--there could be two semi-stable protonation states of your compound...)

If it's possible for your molecule to position itself in different (and stable) conformations, for example by intramolecular interactions, then it's possible to observe more than the expected pick. In proteins for instance, more than one configurationcan be consider, each one having different physical properties.

No. Of course it is possible.

If this compound has more than one basic center, it can have multiple protonation states, and this can show up on C-18 chromatograms as more than one peak. This happens a lot with polyamines, and it is more than a rule rather than an exception (although this not always happens). It will depend largely on the pH of your solvent mixture vs. the various pKa's of your compound. Also sample concentration and the type of your column can affect this to a degree.

Single, pure enantiomer will never give you two peaks in chromatography, irrespectively it is achiral C-18 or chiral column. So, in this case this could be some impurity or decomposition product or other diasteroisomers (if the molecule has 5 stereogenic centers there is a lot of possibilities).

OK....

Say, for example, you have a molecule that has three basicity centers with three different pKa values (as often happens). You run it on an analytical C-18 column using a typical gradient of mobile phase at, say, pH 2.5. It so happens that, at this pH, one of the three basic centers is only partially protonated. If it is hindered, equilibration can be slow. The result is that you get two peaks for this compound, one can actually get three or four.

This has been observed by many professional practitioners of polyamine chemistries. Admittedly, such is not your typical, routine situation, but it happens and is well known to people working with such compounds. I witness these phenomena literary every day.

I would respectfully recommend reconsidering any opinions to the contrary :-).

BTW, my sincere regards to my former mentors, teachers, colleagues and friends at the Adam Mickiewicz University Chemistry Department! These were the good times I will never forget.

With kind regards,

Jacek

Lots of very logical answers and comments provided, but I think the problem here is the original question that Paresh asked. "Can one enantiomer give  two peaks in C-18 column ?? " This question makes no sense as asked and I hate to speculate what the original poster is asking.... but, I will give it a shot.

If asking if the sample is a pure enantiomer, then can it provide two peaks on an achiral column?  *Purity being a very important assumption here, as we are discussing only one compound. A proper HPLC method must be developed to retain the single enantiomer on a column. If this is the case, then ONE pure enantiomer will always show one peak on either a chiral or achiral column. Done, easy answer. If the sample is impure, then everyone will agree there is no need to go on with the discussion as it should provide more than one peak when resolved achirally.

Speculating on what the real question might be?:

If only a single "pure" enantiomer (not a racemate or multiple enantiomers or racemates) is injected on an achiral column and two peaks are seen, then the second peak is not the related enantiomer. Achiral columns can not show the related enantiomer. It could be another enantiomer from another chiral group. It could be a salt form of the original (Which I do see on occasion). It could be an impurity or degradation product. Perhaps a poorly ionized form or maybe a diasteriomer. Any of these are possible explainations (and I may have not listed all of them).

If a single "pure" enantiomer is injected on a CHIRAL column which can resolve the enantiomers apart (from the actual racemate) and two peaks are seen, the second peak could be the related enantiomer (from racemization or an impurity). It could also be many of the same items listed earlier too. Additionally analysis is needed of the actual peaks for determination. *I start with comparisons of their UV/VIS spectra. Chiral rotation could be measured online. You could also collect them and run them on an achiral column to see if they elute at the same Rt (they should).

I should point out that the chromatography technique used (HPLC method parameters and column choice) will effect the observed peaks. Real world poorly constructed HPLC methods can and may show extra peaks due to many of the problems noted above (i.e. conformation, ionization, racemization...). I see this on a regular basis at major companies. Chromatography done poorly can generate all kinds of erroneous and confusing data. A good chromatography method which stabilizes the compound in solution, uses the best detection techniques and properly retains, then elutes the compound off a well chosen column with a good K prime should not exhibit these problems.

Just to add to Bill's post, if there are 5 stereocenters, then epimerization of one of those centers would result in diastereomeric compounds, which can be separated by an achiral column. The OP specified "enantiomers" but it is possible that "stereoisomers" was intended. Can OP clarify this point as well as Bill's question?

Exactly Aaron, this is why language is so important. How can we answer a question if we can not understand it? Best to have the original poster respond first and clarify the actual question.

BTW: Every week I am asked to confirm that someone has resolved a racemate on a C18 column without the use of any chiral selector. In all cases, the "other" peak turns out to be a diastereomer, impurity, salt or unretained peak (as often the peaks elute at the void volume so no chromatography actually took place).

In the present case we are facing an overlap of a few issues.  

I am pretty sure that every poster here understands that if stable conformers (e.g. atropisomers) and partial protonation of hindered amines are taken out of the equation, then a single enantiomer must give a single peak, either on a C-18 column (as Paresh inquired) or on a chiral column. Granted.

HOWEVER - and in a good company, excellently equipped, Bill - when you are dealing with compounds possessing more than one basicity center and having steric hindrance around one or more of these basicity centers you may notice additional peak(s) resulting from either incomplete protonation of one or more of the basic centers, or from the fact that tertiary amine with three different substituents after protonation becomes a chiral sp3 center in case when equilibration is difficult due to steric factors.

For the sake of scientific correctness, I guess, please let me repeat for the third time here that this fact is well known to people working with sterically hindered polyamines, and I mean excellent researchers e.g. with a background of decades of research in America's primary chemistry schools, followed by many years at great companies.

Due to IP limitations I cannot draw our recent series of examples, but in general terms some molecules that possess an amidine or guanidine group and two or three amino groups (tertiary, primary), positioned on asymmetric alkyl moieties, always give two very symmetrical peaks when the mobile phase is at pH ca. 2.2; these same compounds, injected from the same solution, will give one, clean peak when mobile phase buffered at pH 4.0 is used. We run many tens of these injections per day on best available equipment (not UPLC/UHPLC), and we do have extensive HPLC (C-18, chiral, etc) background also.

Coming back to Paresh's question - I am pretty sure I understand what he meant there. My direct answer to him is that yes, if you have single isomer of polyamine with 5 different centers of asymmetry you can see more than one peak. In that case run it under different pH also, and see if you change the number of peaks, however caution is necessary in how this is done (e.g. peak width vs retention time, etc). Also if you have slow to equilibrate restricted conformers, you may see more than one peak, and that can sometimes be clarified just by heating your sample and afterwards re-running the analysis. Good luck.

We do not know what his sample is. He has not responded to provide more information. I think we are all on the same page, but then again, we are the people responding, not asking the question. Did I mention that I hate to speculate? I think I did.

Jacek wrote: "you may notice additional peak(s) resulting from either incomplete protonation of one or more of the basic centers, or from the fact that tertiary amine with three different substituents after protonation becomes a chiral sp3 center in case when equilibration is difficult due to steric factors."

- - Jacek. Agree 100%, but again, we do not know what the sample is. We have no idea what the analysis conditions are. As I work with chiral samples (often with multiple chiral centers) from the Pharm industry every week, I prefer to go with the most likely explanation for now or at least until we are provided with specific details.

Lets wait for Paresh to respond with more info.

Dear Bill Letter and Jacek Martynow...

Thank you very much for your most helpful responses. I am reading all these comments and trying to see on my molecules...

Once again thank you very much to all the friends for your helpful suggestions.  

Paresh. Please share with us your actual question.

Looks like the question is formulated with some ambiguity, and leaves a lot of room for assumptions (call it speculations if you wish).

For example, "a symmetric molecule is having five chiral centers" - raises few questions for me, like what kind of symmetry is there ? Is it c2 found in tartaric acid, or a sort of meso-form ? Does this molecule even have enantiomers ? Saying "no optical rotation" is not a reliable indication for enantiomers absence or presence. Jacek's suggestions are very reasonable, but we don't know if the molecule is a polyamine. Hindered rotation or some type of molecule associations (dimers, etc) could be an explanation as well. 

Without further clarification this question has no definite answer. It caught my attention only because I was thinking: could it happen in principle ? I think the answer to this question is yes, in principle it is possible, but the reasons for this could be numerous. 

Agreed... maybe, out of the numerous options that you mentioned, somewhat obvious to me have been (a) the polyamine (or poly-base) case, and (b) the non-equilibrated conformer/atropisomer case (which I had also indicated above). Let us wait for more information.