[madisonwi]

With isomers, there are various classifications used.  I've sketched together a hierarchy of isomers, but am having some issues getting all the pieces to fit as information from various texts and web sites conflicts.  I've created the file using OmniGraffle 3.0.

You will see in the image where my confusion lies.  I ask two things:

1) Help me fit the pieces better together (esp. under Configurational)
2) Add any missing isomers to the hierarchy. (esp. the question mark.)

Thanks for any *delete me*
Douglas Weittenhiller

[Mitch]

It looks about right, we can host the image for you if you want? Once your completely done with it.

[movies]

I see a couple of errors in your heirarchy.  I really like this way of presenting isomeric relationships though.  I found these concepts to be difficult for a long time.

Here are the errors I see (from top to bottom, left to right):

1) You might add a branch at the very top concerning whether the molecules have the same molecular formula or not.  If the don't, they aren't isomers, if the are, then you can enter the rest of the chart.

2) You may want to add "structural" as another name for "constitutional."  Constitutional is the modern term, but some older literature uses the term "structural isomer."  The meaning is the same.  Additionally, I would just omit the "?" on the left hand side and branch the arrow to the two things below.  I'm not aware of a name for that particular type of isomer.

3) Below "functional" you may want to add a branch for "tautomers."  Tautomers are defined by two or more rapidly equilibrating structural arrangements, like the keto and enol forms of a carbonyl compound.

4) I don't understand what is meant by "different bond order."  At this point in the flow chart, you should only be dealing with compounds that differ in stereochemistry, so bond order should be irrelevant.  Configurational isomers should differ only by the configuration around a chiral center, conformational isomers should differ only by a different conformation of the molecule (e.g., a ring-flip of a cyclohexane ring).  Conformational isomers are difficult to deal with because there are both conformational diastereomers (e.g., anti-butane and gauche-butane) and conformational enantiomers (e.g., the two ring-flip forms of cis-1,2-dimethylcyclohexane).  More below.

5) Diastereomers are NOT superimposable.  Diastereomers are stereoisomers that are not mirror images of one another.  Diastereomers must have more than one chiral center.

6) Enantiomers must be non-superimposable, but must also be mirror images of one another.  Also, enantiomers do not actually require a chiral carbon.  There are a handful of examples where you have an axis of chirality, but no chiral carbon (e.g., allenes).  Finally, a stereocenter can be any atom where if two of the groups attached to it are switched, you get a different compound.  This definition includes cis and trans alkene isomers.

7) Geometric isomers cannot be enantiomers.  Geometric isomers differ only by a different configuration about a double bond like in an alkene.  For example, cis and trans 2-butene are geometric isomers, but are not chiral and therefore cannot be enantiomers.  Furthermore, the presence of a pi-bond does not mean that two compounds are geometric isomers.  For example, the two enantiomers of cyclobutenol have a double bond, but are not geometric isomers.

8) Optical isomers are related by a difference in optical activity.  Optical isomers must be optically active (so meso compounds are out), but they don't necessarily have to be enantiomers.  A pair of optically active diastereomers are said to be optical isomers.  A pair of enantiomers are also optical isomers.


Sorry for all the nit-pick points, but I hope that these suggestions are useful to you.

[madisonwi]

Movies (and all),

Thank you for your very helpful reply.  i'm starting to understand why the texts necessarily conflict - there is no perfect isomer hierarchy.  But I'll strive to get as close as I can.

You'll see Version 2.0 (if you will) attached below taking into consideration the points made by movies.  Same assistance is requested of the forum - feedback and help in correctness.

Movies, in response to your 8 points, I have some follow-up.

1) I had always assumed one would recognize the difference, but hey - if it helps one more person - great!

2) In the hierarchy, I'm trying to create a flow of "either a or b or c...." for simplicity.  However, I don't wish to lose any correctness while ascending the chart.  For example, are all tautomers really functional isomers?  Are all functional isomers really constitutional isomers?  And so on.

3) Great addition!

4) Different bond order proved too confusing so the nature of the mirror reflection made more sense to use.  That was a great catch with conformational enantiomers and diastereomers and the note reflects that.  (Drawing additional lines would ugly the chart too much.)

5) What I was getting confused most on with diastereomers was the idea of one (or more) chiral center.  The phrase "chiral center" - does that refer only to a single carbon atom?  Chiral's definition is a molecule non-superimposable on its mirror image.  When regarding it in this way, are we talking about planes of chirality?  And is it acceptable to regard a chiral carbon as a carbon with four different functional groups?  By that logic, sp2 and sp hybridized carbons cannot be chiral and should be labelled "not chiral".

6) I know "non-enantiomer" is poor, but it serves it's purpose here.

7) Here again that "no chiral carbon" is tricky.  I wish to imply that the carbon isn't chiral because it's sp or sp2 hybrided and not imply "achiral".

Optical isomers are tricky to place.  In there current location, it implies that optical isomers are mostly enantiomers, some are diasteriomers and no geometric isomers can be optically active.  This lends for me to need to understand optical isomers much more in depth.  Are there examples of optically active geometric isomers?  Does there exist a methodology to look at two isomers and determine if they are or are not going to be optically active?

Don't apologize for the nit-picking.  I found it incredibly helpful and hope you can aid some more!

Thanks!
Douglas Weittenhiller

[movies]

Awesome!  I like all the changes you made.

For tautomers, I think it would be good to clarify that the two compounds being considered must be the two that are in rapid equilibrium with one another, to save confusion.

To get rid of the dashed line at the bottom right, you could branch "diastereomers" to "meso" and "non-meso" (or "chiral").  All chiral diastereomers will be optical isomers.  I would probably put enantiomers as the last thing on the list.  It's really the most specific definition.  Perhaps "mirror images" and "non-meso" should both lead to the "optical isomers" bubble, then there should be a branch for mirror images (leading to "enantiomers") and non-mirror images (leading to "diastereomers").  Any chiral molecule will be optically active, provided it isn't in a racemic mixture.

For the "conformational isomers" I think that you need to specify that the two isomers can inter-convert through a change of conformation.

Also, for note 3, the conformational enantiomers term only applies to the cis form of 1,2-dimethylcyclohexane.  The ring flips of trans-1,2-dimethylcyclohexane are conformational diastereomers.

I don't know of a good way to deal with the problem of axial chirality.  It muddies the waters a lot.  I'll let you know if I think of anything.

Finally, "diastereomers" is spelled wrong.


I think it looks really good now.  It's really a great thing to have around.  As I mentioned before, it took me a long time to figure out the relationships of all the different types of isomers.

[madisonwi]

I'm hoping this is my final version.    You can find it in the downloads section.

Thanks for all your *delete me*



http://www.surfbest.net/~madisonwi@surfbest.net/Isomers.jpg

[Mitch]

I placed the file in our downloads section. If you want a different description or you want to use your real name or you have a website you want us to link to from the download section. Let me know.

If you edit the file again let me know, so I can use the latest version.

[madisonwi]

Thanks for the props.  I hope others can use it.  I'll post it on my website as a 7000x3000 1.1MB tiff file with my name and  a copyright tag and the original 3500x1500 250KB jpeg.  Here's the links:

JPEG:  http://www.surfbest.net/~madisonwi@surfbest.net/Isomers.jpg
TIFF:  http://www.surfbest.net/~madisonwi@surfbest.net/Isomers.tiff

Incidently, I have another file for the Fed Liver processing glucose.  I'll post that in a thread in biochemistry.

Thanks for all the help,
Douglas Weittenhiller

[madisonwi]

movies -

I would much prefer that if you are to use my file that you use the more current file available from my webpage that includes the copyright information.  Also, you can list the originator as my real name and not my user name.

i have several other drawings in the works.  One is currently available in Chemical Biology forum (The Fed Liver) and the other in the General Chemistry forum (Phase Change).  After they have been review, you may put those in the download section as well.

Thanks,
Douglas Weittenhiller

[movies]

Doug, send an IM to Mitch about the correct file.  He set it up on the downloads page, not me.

[mike]

Here is an isomer hierarchy diagram I have drawn. It is only a first draft but would like to get peoples feedback/ideas. I have read the previous posts so hopefully I haven't made too many mistakes

By the way this is for first year or intro chem classes so doesn't need to be too in depth

[AWK]

Mike, use IUPAc terminology (eg: constitutional instead of structural)

[mike]

Mike, use IUPAc terminology (eg: constitutional instead of structural)

Ah, good, of course Thanks

[Yggdrasil]

I think you can classify conformational isomers and cis-/trans- isomers as diastereomers (e.g. see http://en.wikipedia.org/wiki/Diastereomers), so the hierarchy diagram you have may be somewhat misleading.  I think diastereomers may be defined as "stereoisomers which are not enantiomers," so it may make more sense to separate stereoisomers between diastereomers and enantiomers.

[mike]

This is a good point Ygg. I am wondering though, are conformational isomer opticaly active? And also it is possible to make the examples I have drawn for conformational isomers into mirror images. Just a thought? Will this diagram really be misleading, considering the audience? Thankyou, your input is greatly appreciated. Feel free to add more