Talk:Isomer

Question: Isomer Named Incorrectly?

Is it just me, or does the article incorectly label isomer number III (3) in the graphic-drawing as isopropanol, which is an alcohol, whereas the drawing is of an isomer that is clearly NOT an alcohol - it is an ester. I need to brush up on my IUPAC naming, but it seems that CH3-CH2-O-CH3 would be something like methyl-ethyl-ester, which I'm not sure is a stable molecule. Just looking at it, it might spontaneously dehydrogenate (CH3CH2-O-CH3 --> CH3CH2-COH + H2)to form the relatively stable aldehyde - propanal(CH3-CH2-COH), although I'm purely spectulating here. My main point/question is: isn't number III an ester, not an alcohol (specifically not isopropyl alcohol) ? And if I'm right - what is it, and what of my speculation concerning methyl-ethyl-ester (if that is indeed the proper name)? Spiral5800 (talk) 13:04, 15 January 2008 (UTC)

It calls III methoxyethane, which is correct. --Itub (talk) 09:54, 17 January 2008 (UTC)

Formula

Is there a mathematical formula for the number of constitutional isomers for in the nth element of, for example, the alkane series?

Yes. I believe there is one for the Alkane series. However, this page is speaking about a more general understanding of what an isomer is, and the formula and specific details of the alkane series is better kept with the alkane series. Glooper—Preceding unsigned comment added by 220.233.184.203 (talk • contribs)

older entries

Does the definition of "isomerism" require "the same kinds of bonds between atoms"? I would have said that, say, benzene and prismane are isomers. And is it true that "isomers typically share similar if not identical properties in most chemical contexts"? In fact the examples given in the history section seem to belie both of these claims.

Josh Cherry 22:18, 13 Oct 2003 (UTC)

I wrote both those statements and I definitely wasn't thinking of prismane, which does seem to disprove the rule. On the other hand, I think a qualifier like "typically" or "generally" covers a multiple of ills. Since you pulled the example of prismane out of the air, I suspect you know more chemistry than me. If prismane is one of a relatively narrow category of exceptions (I don't know), then to me I think "typically" still cuts the mustard. I think the priority should be to convey information and not to construct all inclusive definitions. In other words, if rules of thumb exist, I think we should tell people. I arrived at "the same kinds of bonds between atoms" by thinking of a simple ketone or alcohol or halagonated alkane. I couldn't think of an isomer of one of those where you wouldn't still have a "C" double-bonded to "O", then I thought of the electron shell filling principle, and from there I jumped to my rule of thumb. But if you give a couple not too bizarre examples besides prismane, you'll convince me that I was mentally impaired. My motive in concocting this rule-of-thumb statement about isomers was my intuition that a reader who knows even less chemistry than I is likely to assume that there's no limit to the number of ways that a given set of atoms can bond to form a stable molecule--such that isomers (defined only as sharing the same chemical forumla) are unlikely to be more than trivially related--whereas actually something closer to the reverse is true. 168... 00:27, 14 Oct 2003 (UTC)

I don't think prismane/benzene is especially bizarre. Here are a few other isomer pairs to consider:

• Cyclohexane and any acyclic hexene
• Any ketone and its enol tautomer
• Lactic acid and glyceraldehyde
• H2C=C=CH2 and methyl acetylene
• Some organic sulfate and some sulfide isomer with the oxygens put into hydroxyl groups or something

Josh Cherry 01:44, 14 Oct 2003 (UTC)

O.K.. I can't remember o-chem but I think I recognize when I see it. I just weakened the intro generalizations. Does that work for you? I think one of your examples above with a figure would be good to add somewhere below the intial one to illustrate how the intro generalizations do not always apply.168... 03:25, 14 Oct 2003 (UTC)

Just a minor point, the images would be better if they were enlarged somewhat. The lettering is a bit hard on the eyes and somewhat hard to decipher.

Cudos!

-Zidel

There appear to be at least one more type of isomers; nuclear-spin isomers, as in the para and ortho isomers of water (http://www.sciencemag.org/cgi/content/long/296/5577/2363, subscription required). I don't know enough about the subject to know where in the classification mentioned this would fit in (I'd guess another box beside structural isomers and stereoisomers), or if there are more types of isomers, so I think the article needs the attention of someone well versed in chemistry and particle physics to complete it. W 10:46, 24 April 2006 (UTC)

Tautomerism

Where does this fit in? Presumably under constitutional(structural) isomerism.I realise there is a link, but should it be linked to from this page? --Copperman 15:51, 29 May 2006 (UTC)

Question - Meso compounds?

Should a discussion about meso compounds not be mentioned?

Yes. The diastereomers section completely ignores structures with multiple stereocenters that are not enantiomers (i.e. they are not mirror images and not superimposable). For example in 2,4-pentanediol, there are three possible arrangements of the stereocenters: R,R S,S and R,S . The R,R and S,S are chiral and are enantiomers of each other. The R,S is achiral. Its mirror image (S,R) is superimposible on the R,S, so they are identical. Such achiral compounds with chiral centers are called meso. Reengler (talk) 18:52, 12 January 2012 (UTC)

Question: structures vs. structural formulas

Should isomers be defined as having different structural formulas or different structures? A minority of books claim it to be the former, while the majority seem to say that it is the latter and that stereoisomers are considered to have the same structural formula

Isomerization devices

I work in law enforcement and am reviewing the state statutes reference drug paraphernalia. I've come to notice most state statutes include "Isomerization devices" under the drug paraphernalia code. Could someone please discuss Isomerization devices as used specifically to increase the potency of species of plants.

Thank you, Anne

Has this page been hacked?

The illustration at the top contains NO "C" (Carbon) atoms, yet the descriptions clearly refer to an Oxygen atom attached to a Carbon atom. I'm no chemist but played one in college. I don't know enough to fix this, but I do believe someone qualified needs to take a peek.

Keith Adney

64.86.28.237 15:35, 25 March 2007 (UTC)

No, it has not been hacked and the illustration is correct. Carbon is present in ALL organic molecules. By convention, carbon atoms are not explicitly indicated by a letter, the way oxygen or hydrogen would be in molecular structure drawings. The carbon atoms are at the intersections or vertices of the straight lines used in drawing the chemical structure. Pzavon 16:00, 25 March 2007 (UTC)

Chemical isomers vs. Nuclear isomers

I believe this article should be renamed "Chemical Isomers". A search for Isomers should redirect here, but the page should be called Chemical Isomers.

I know it sounds pedantic, but it's just common sense I think. What does everybody else think? Glooper—Preceding unsigned comment added by 220.233.184.203 (talk • contribs)

where did you guys get your info

And the chemical formula of ferrocene does not include a six ring compound-like the one in the isomerization example figure; it has one Fe(iron) atom bounded with 2 identical 5 membered-ring(cyclopentadienyl) Have you ever consulted a organic chemistry textbook. This page has so many errors that if needs to be totally rewritten. To wit:

(1) Conformers are technically diastereomers, but are rarely considered so.

(2) Diastereomers consist of other isomers than conformers and cis/trans isomers. What about glucose and galactose? They are diastereomers.

(3) Biological systems typically discriminate between enantiomers. Other stereoisomers (i.e., diastereomers) are different chemical compounds. Get this straight.

(4) Xanthines are a very obscure example of isomers with different properties. Choose a better example.

I reworded this section and provided a second example; its now quite a bit more on target with the idea of isomerism in simple chemical medicine. Didn't change anything else tho. Feel free to be bold! Zaphraud (talk) 14:33, 24 June 2008 (UTC)

(5) Why are cis-trans isomers important in food chemistry. I don't see a connection. They are really important in organic chemistry, which isn't mentioned.

(6) Some tautomers can be separated.

(7) The only type of stereoisomer that rotates polarized light is an enantiomer.

(8) Topoisomers are incredibly important in organic chemistry, especially in catalysis. Consider BINAP.

BINAP doesn't have topoisomers (DNA does). It has atropisomers. --Itub (talk) 08:12, 26 June 2008 (UTC)

(9) Etc., etc., etc., ad nauseum. Should read ad nauseam --Slashme (talk) 08:31, 26 June 2008 (UTC)

Are you authors even chemists? PLEASE, PLEASE, PLEASE consult a textbook and get this stuff correct. It is really simple.

Have added an unreferenced template. I would have though this one would at least have a references section. Richard001 03:57, 19 August 2007 (UTC)

Template

A template linking together the different types of isomerism would be handy. Any chemistry editors willing to make one? Richard001 (talk • contribs) 03:57:14, August 19, 2007 (UTC).

valence isomer

What is a valence isomer? Is it just a synonym for structural isomer? Jeff Knaggs (talk) 11:08, 9 March 2008 (UTC)

See http://goldbook.iupac.org/V06590.html . --Itub (talk) 14:42, 8 April 2008 (UTC)

Qusetionable Redirect

Structural Isomer redirects to Isomer, there is however an article called Structural isomer. This appears a bit of a problem to me.

PiAndWhippedCream (talk) 05:55, 21 September 2009 (UTC)

Isomerism.png: wrong picture?

I'm a teacher of organic chemistry. The picture of isomers classification is wrong. It was taken also in the Italian article and it misleaded a student of mine who showed me. That picture is wrong because 1. includes conformers, and rotamers 2.it considers conformers and rotamers as a kind of stereoisomers. I have controlled in several expert books about this topic, as the one of F.D. Gunstone "Guidebook to stereochemistry" Longman, and university level textbooks. Nobody maintains that classification. This website confirms my POV: http://www.chemistryexplained.com/Hy-Kr/Isomerism.html and moreover confirms me that geometric isomery is a obsolete term. The text f the article "Diastereomerism is again subdivided into "cis-trans isomers"", which have restricted rotation whithin the molecule (typically isomers containing a double bond) and "conformational isomers" (conformers), which can rotate about one or more single bonds whithin the molecule." is wrong too. Diasteromers with two or more stereocenters remain different if you rotate groups about single bonds, whereas rotamers and conformers can be transformed one in the other and they are not different isolable substances as isomers are. So I propose to change text and to eliminate that picture or to substitute it with a correct one (taking in account what professor Eliel said and fixed under the approval of the chemical community in 1971). Alfredo Tifi —Preceding undated comment added 18:45, 14 March 2010 (UTC).

There is no doubt about the fact that conformers are diastereomers, although sometimes rather stable, sometimes very unstable and cannot "be isolated under normal conditions". This is explained in the article. Also that "geometric isomery" is an obsolete term.--Wickey-nl (talk) 10:42, 15 March 2010 (UTC)

But, as I assume there are diastereomeric as well as enantiomeric conformers (e.g. as chair or boat conformation) the picture may be regarded as wrong, indeed. The conformers then belong in both groups.--Wickey-nl (talk) 12:27, 16 March 2010 (UTC)

An example of conformers that are enantiomers is given here (scroll to the end).--Wickey-nl (talk) 13:28, 18 March 2010 (UTC)

I'm reviving this section because the OP is absolutely correct, conformers and rotamers are NOT isomers. They CAN also be isomers in very specific cases, but to imply they are "types" of isomers is highly disingenuous. Isomers by definition cannot be interconverted without an actual chemical manipulation. My left and and my right hand are isomers. My left hand and my left hand rotated 180 degrees are not isomers. This is really bad for an encyclopedia and is probably confusing a lot of chemistry students. 75.144.177.101 (talk) 22:09, 15 April 2018 (UTC)

Definition of isomer [comment moved from talk page]

Definition of isomer The definition is missing a critical attribute. The first line states, "In chemistry, isomers are compounds with . . ." It should say, "In chemistry, isomers are covalent compounds with . . ." or "In chemistry, isomers are molecules of covalent compounds with . . ." This would exclude ionic compounds, which do not have isomers or have variations that require a different term, like polymorphic crystals. Bob Widing, Department of Chemistry, University of Illinois at Chicago

Note: Above comment placed on article page today by Bwiding99, moved to talk page by Dirac66 (talk) 17:32, 3 November 2012 (UTC)

But molecular ions can also have isomers and form isomeric compounds.

Organic example: (CH₃)₄N⁺ and (C₂H₅)₂NH₂⁺ are two structurally isomeric ions, and form the two isomeric compounds (CH₃)₄N⁺ Cl^- and (C₂H₅)₂NH₂⁺ Cl^-.

Inorganic example: [Co(NH₃)₄Cl₂]⁺ has cis and trans isomers, which form two stereoisomeric chlorides cis- and trans-[Co(NH₃)₄Cl₂]Cl. Dirac66 (talk) 18:44, 3 November 2012 (UTC)

‎Isomer page missing illustration

I made a request at the Wikipedia Graphics Lab/Illustration workshop, but I can't tell if I made the request correctly so that it will be reviewed.
On the Wikipedia page for Isomer, in the section of Structural isomers, the end of the second paragraph reads:

"For example, two position isomers would be 2-fluoropropane and 1-fluoropropane, illustrated on the right."

The illustration is missing/non-existent. Christopher, Salem, OR (talk) 07:42, 3 February 2013 (UTC)

I checked the revision history for this article, and found that this illustration was part of a diagram which was in the article from 2 Aug 2011 to 27 Oct 2012. I will put the diagram back in. Dirac66 (talk) 17:46, 3 February 2013 (UTC)

Alkene cis-trans isomers are NOT diastereomers

This is a very common mistake which I've seen throughout the internet, very likely due to this mistake on Wikipedia. Cis-trans isomers are ONLY diastereomers when applied to cyclic compounds NOT alkenes. Alkenes do not have chiral centres. Please refer to the latest chemistry textbook. ★Ffgamera★ - My page! · Talk to me!· Contribs 13:52, 4 July 2013 (UTC)

IUPAC does say that alkene cis/trans are stereoisomers.[1] My copy of McMurry's Organic Chemistry (standard college-level/orgoI text) 7th ed (2008) places cis/trans as a subcategory under diastereomers and specifically mentioning both alkenes and cycloalkenes in the description of them in this categorization context. DMacks (talk) 17:37, 4 July 2013 (UTC)

DMacks is correct. An authoritative and explicit source is Stereochemistry of Organic Compounds by Ernest L. Eliel and Samuel H. Wilen (Wiley Interscience 1994), which says (p.52-53)

Diastereomers ... often contain two or more chiral centers, chiral (torsion) axes, or a combination thereof. However, this is not necessarily the case; Figures 3.6b and 3.10 illustrate cases where diastereomers are neither chiral nor contain chiral centers; alkene diastereomers (Fig. 3.11) are also of this type. Thus, diastereoisomerism is not necessarily associated with chiral centers or chiral torsion axes; ... In this book figure 3.11 (p.53) is just cis and trans-dichloroethene, which of course have no chiral center.

So we now have three sources, and it is clear that we can and should restore the statement that alkene cis-trans (or E-Z) isomers are diastereomers, and this should be shown in the diagram as well. Dirac66 (talk) 19:50, 5 July 2013 (UTC)

• Yes, alkene cis-trans isomers have always been considered diastereomers, so no User:Ffgamera you are incorrect (unless there are some new IUPAC rules now "enforced"). However I think the image now present is somewhat inadequate. and the one provided by User:Ffgamera wrong in this regard. 175.38.164.190 (talk) 11:31, 13 July 2013 (UTC)
• To be specific they are a geometric isomer subset of diastereomers 175.38.164.190 (talk) 11:47, 13 July 2013 (UTC)
• I like this one 175.38.164.190 (talk) 13:35, 13 July 2013 (UTC)

  

Clarifying sentence in intro?

For those unfamiliar with chemical concepts, the intro isn't especially helpful; I had to click on several of the links and read the intros there to figure out what the first sentence meant. Could a clarifying sentence be added after the first one, something like, "That is, isomers contain the same elements in the same proportions, but differ in the spatial layout of their component atoms."? Is that accurate? -MK 69.244.84.31 (talk) 17:13, 11 August 2013 (UTC)

Good idea, I have now added a second clarifying sentence along these lines. Your suggestion is almost accurate, except that "same elements in the same proportions" would include for example C₂H₂ and C₆H₆. These are NOT considered isomers, so I changed it to "same number of atoms of each element". Dirac66 (talk) 20:24, 11 August 2013 (UTC)

Isobaric isomers?

I'm trying to understand what an isobaric isomer is. The term appears in various places but always without a definition. Here's an example: "Δ8-THC is an isobaric isomer of Δ9-THC that differs by the position of a double bond." - Ken Tseng et al.

Here's what I've found so far, but I don't know what to make of it: "Atoms of different elements with different atomic numbers but having the same mass number are called isobars." - nextgurukul Page Notes (talk) 01:25, 12 April 2019 (UTC)

These are two totally different meanings of the term isobar. The second meaning is from nuclear physics and has a Wikipedia article at Isobar (nuclide); an example is ¹⁴C and ¹⁴N. But this does not help explain the first meaning from organic chemistry, which I have never seen. Dirac66 (talk) 01:41, 12 April 2019 (UTC)

Is the "map" on the lede incorrect? Classes are not a hierarchy, are they?

The "map" of types of isomerism currently in the head section (File:Isomerism.svg) presents isomer pairs (incompletely) as a hierarchy. Is that correct?
As I understand it, there is a fairly clear distinction between pairs that are structural(or constitutional) isomers and those that are stereoisomers. Among the former, some pairs are positional isomers, some are not. In the latter, there are a few pairs that are enantiomers, and all other pairs in that class are said to bediastereomers. So far, we have a hierarchy, okay.
However, the next division into configurational and conformational isomer pairs seems to cut across the last two categories. The distinguishing criterion seems to be whether the easiest (minimax barrier) deformation that takes one arrangement of atoms in space to the other involves breaking bonds or not. However, some enantiomer (or diastereomer) pairs that can interconvert easily (e.g. H
₂O
₂, chiral phosphine derivatives, or cyclooctene) while other pairs apparently require partial or complete bond-breaking (e.g. CHFClBr or 1,2-dichloroethene). So it is not correct to say that these two categories are subdivisions of diastereomers.
These two categories also seem to cut across the rotamer class, since some rotamer pairs (like biphenyl enantiomers) can interconvert without bond-breaking, while the atropisomers (like o-substituted biphenyls) can't. And in fact the boundary between those two classes seems fuzzy and difficult to establish.
Finally, the rotamer and enantiomer classes too are orthongonal -- there are pairs with all four yes/no combinations of the two.
--Jorge Stolfi (talk) 07:27, 26 August 2020 (UTC)

Classification Stereoisomerism

This does seem to ambiguous. Compare the diagram of this article with that of the German version. On the one hand conformers are named as a subgroup of the diastereomers and on the other hand as a delimitation to configurational isomers which are then again divided into enantiomers and diastereomers.

Professor G. Helmchen, one of the leading experts in the field of stereochemistry, speaks of two different classifications in his lectures at Heidelberg University:

1. Stereoisomerism can be divided into configurational or conformational isomerism OR 2. Stereoisomerism can be divided into enantiomerism and diastereomerism

This also seems to me to be the most logical explanation. The diagram shown here suggests that two conformers cannot be enantiomers at the same time, which they can obviously be.178.24.252.234 (talk) 15:06, 25 January 2021 (UTC)

Different spin states

Is there a reference for not referring to different spin states as isomers? I don't agree with the assertion that they always decay rapidly to the ground state (or that they are not isomers due to a short lifespan). For instance, triplet oxygen (1Δg) state can be generated as a reagent and last on the order of milliseconds in solution to be used in [4+2] cycloaddition reactions. Similarly singlet and triplet methylene have distinct bond distances, bond angles (134 vs. 102 degrees), are generated under different conditions, and exhibit different reactivity in cyclopropanation reactions. Arguably, singlet and triplet oxygen is an example of allotropy, but singlet and triplet methylene have been described as "electronic (state) isomers" or even as "spin isomers". The IUPAC defines "One of several species (or molecular entities) that have the same atomic composition (molecular formula) but different line formulae or different stereochemical formulae and hence different physical and/or chemical properties." The case of methylene (and other cases like singlet and triple HNO) clearly fits this definition.

I propose removing the first paragraph in the "Ionization and Electronic Excitation" section that describes different electronic states as not being examples of isomerism.

Alsosaid1987 (talk) 05:13, 7 April 2022 (UTC)

The thing you have to weigh in the proposed revision is that 99.99% of isomers are something to do with geometries and 99.99% of spin isomers relax super quickly, as you imply. So the issue is whether being a stickler for these special cases would damage the understandability of an already complicated subject. You could overlook the minor flaws in the general lede and tack on a section about spin isomers as an exception. Or we could insert the word "ordinarily" in the lede to signal to specialists that exceptions exist. Just a thought.--Smokefoot (talk) 14:48, 7 April 2022 (UTC)

I have today deleted both paragraphs of the section Ionization and electron excitation for now, since all they contained was unsourced statements that these are NOT examples of isomers, so why mention them in the article. However according to Alsosaid1987 who proposed deleting one paragraph, singlets and triplets are examples of isomers for CH₂ and HNO, as per his interpretation of the IUPAC definition of isomers. If someone can find a reliable source which explicitly describes singlet and triplet states CH₂ or HNO as isomers, then I would accept that description as suitable for inclusion in the article. Dirac66 (talk) 01:22, 3 May 2022 (UTC)

Allotope VS Isomer

Read the few lines of allotrope page, then read this article. Is diamond (elemental carbon) an isomer or an allotrope? Hence. "The term allotropy is used for elements only, not for compounds." Fix your site. — Preceding unsigned comment added by 61.69.238.160 (talk) 11:49, 5 March 2024 (UTC)