Talk:Structural isomer

Regioisomer

"Regioisomer" redirects to this article. Is regioisomer synonymous with structural isomer? -- Sakurambo 桜ん坊 12:28, 6 November 2006 (UTC)

I don't know ([1] suggests it is) but Regioselectivity seems related. It would be helpful if the target of the redirect could define regioisomer. Rod57 (talk) 08:08, 2 June 2010 (UTC)

It's rarely used, but I think yes, it is synonymous. Mbb1056 (talk) 18:49, 8 April 2014 (UTC)

Metamerism

Metamerism is condered as one of the functional group isomerism, right? —Preceding unsigned comment added by 61.244.148.89 (talk) 09:32, 15 April 2009 (UTC)

"Isomer counting" section

I deleted it because it was factually incorrect. I don't have a ref that gives the correct total and I don't think that this specific information is required in the article, so someone who thinks it is feel free to readd with corrections, or conjure up an alternative that is correct. DMacks (talk) 18:51, 31 December 2010 (UTC)

I now see this has been reverted, along with a dozen-odd unrelated changes that also fixed various MOS, spelling, format, and factual mistakes? DMacks (talk) 18:54, 31 December 2010 (UTC)

• The table is sourced, see source. Nuking (your words in the edit summary) content is plain vandalism. Whatever happened to discussion before deletions? If everybody nukes content that they feel is not required only the IUPAC definition and link will remain V8rik (talk) 21:18, 1 January 2011 (UTC)

It is factually incorrect in numerous ways (starting with the "12" itself, and the physical properties that contradict our own pages on the compounds) and a component of the cite itself is is confusing at best (suggests the factually false statement that butane-1,4-diol has stereocenters). Notice the full history of my work before giving up in disgust that we would have so much faulty material on a science page. DMacks (talk) 21:24, 1 January 2011 (UTC)

• you are suggesting that your edit history will show the article was riddled with faulty material. I have checked your recent edits: you only seem to have a problem with the isomer counting section. Please do not misrepresent the overall quality of this article. V8rik (talk) 21:51, 1 January 2011 (UTC)

I said nothing of the sort. You first reverted without comment my explained edits to numerous sections (including the rationale for removal of the table). As a starting point, I find that offensive per Help:Reverting. Then you revived them but reinstated the table, which we are now discussing here. I notice you revived it in a form that re-includes a nomenclature mistake that I had corrected in the table and omits editorial comments about concerns I had with the intro to the table. Even the cite itself is so vague as to be useless ("somewhere among the 2–3K pages in 65th CRC"?). As I was trying to fix (with an eye towards keeping the table) I kept finding mistake after mistake. From random spot-checking the bp/mp data against other sources, I was no longer comfortable trusting it. How do these types of physical data relate to isomer-counting? And the cite is not clear enough to be able to verify even the fundamental/key idea in this section (a counting of all reasonable structural isomers of a given formula) as it relates to the article itself. But again, an Aldrich catalog also disproves it easily, and additional creative structure-searching disproves it further. Yet you removed the notes about these problems. DMacks (talk) 06:02, 2 January 2011 (UTC)

Hi DMacks,

• I have checked the bp/mp values in the CRC handbook, no errors there.
• do not expect me to include page numbers in CRC cites. A lot of work but without merit. My edition has a big table called formula index for organic compounds that lists a large number of isomers with each chemical formula. The numbers relate to another table called physical constants, just a matter of looking up up the data
• the error in nomenclature I quess you mean the ethyl diethylperoxide, that one originates with CRC (this time I will include the page number C293). I apologize for trusting CRC too much.
• the comment about the 1,4-diol not included should have been the 2,3-meso compound , that is the only mistake (and the mistake made by CRC) I have been able to detect and that is a mistake in a note and not the text itself. Do not suggest the law of physics have changed when someone mentions a 1,4-meso compound
• as to the number of structural isomers, the table presents 12 structural isomers. I stuck to the CRC list and it is nowhere suggested that the limit is 12. You mention hidden in the text (something we should not do) 2-methyl-1,3-propanediol , n-butyl hydroperoxide , formaldehyde ethyl methyl acetal and methyl propyl peroxide. With the exception of 2-methyl-1,3-propanediol these compounds are obscure, better stay away from them
• As to the why of the isomer counting section: there is more than just giving definitions of a concept, the reader should also understand the concept. Often in chemistry a dataset is presented from which the concept can be understood even by non-chemists or inexperienced chemists. The bp/mp data emphasize that the compounds are isomers but with very different physical properties. V8rik (talk) 18:12, 2 January 2011 (UTC)
  • Hi DMacks, I think I have have taken in all your critical comments and made changes, so how do we stand on this particular section? V8rik (talk) 17:35, 8 January 2011 (UTC)
    • Thanks for double-checking the physical properties! Sounds like there are two ideas being supported by this section: that there are countably-many isomers for a formula, and that different isomers can have widely different properties. I think both of these are good topics to include. But they are definitely (in my mind) different topics. If the topic is "counting isomers", then a partial listing of stable ones doesn't address that issue well (and even a complete listing of just stable ones doesn't). A common exercise in Orgo I classes is "draw all structural isomers of a given formula". Sometimes the exercise even specifies "stable", but in this case that term simply means using standard valences rather than actually considering physical properties or chemicals known in the literature. DMacks (talk) 08:40, 9 January 2011 (UTC)
• Hi DMacks, If we can agree that a table would have merit then perhaps we can agree on a more suitable set of isomers without obscure members or stereoisomers. Even a simple set of acetone isomers is too complicated V8rik (talk) 18:21, 11 January 2011 (UTC)

Sure! What formula do you have in mind? I like acetone because it does have stereoisomers--we can use that to illustrate that this feature is not structural isomerism. And it's about the simplest one that does contain both positional and functional-group isomerism, and all members are both known and important chemically. DMacks (talk) 11:48, 20 January 2011 (UTC)

• • I have constructed a new table with structural isomers of acetone, physical data based on wiki articles (some of them German) I was unable to find physical data for cyclopropanol. V8rik (talk) 18:31, 22 January 2011 (UTC)
    • Looks great! I added a note that the enols are unstable structural isomers (my whole problem here was that we only considered "stable at STP" not "structurally viable"), with a clear comment that they are not stable. Should they get separate rows instead of just parenthetical notes, with "unstable" in the properties columns. Again, they are valid structures for purposes of counting? If the goal of properties is to illustrate that different isomers have different properties, then seeing some clearly marked "unstable" helps highlight that fact. I also wrote a stub for cyclopropanol and got a bp value (scifinder lists a mp value, but I can't access the ref to verify it). DMacks (talk) 20:51, 22 January 2011 (UTC)
• I think that the current table best represents the structural isomers. There are an additional 3 not 2 tautomeric but unable structural isomers? Thanks for the MVE article, I had already started (offline) on the translation of the German article but it was not good enough V8rik (talk) 22:11, 25 January 2011 (UTC)

There are indeed three tautomers, but the two acetone enolates are not two different structural isomers (cis/trans is a stereoisomeric difference within the same structural connectivity). Glad you like the MVE! One of its related OrgSyn entries happened to be on my desk so I figured I'd write a bit:) DMacks (talk) 22:16, 25 January 2011 (UTC)

Make that "...but the two propanal enolates...". Maybe could improve the article wording to clarify this point. DMacks (talk) 23:20, 25 January 2011 (UTC)

• Thanks DMacks, the issue is almost resolved. I have gathered some info on cyclopropanol and will add some content on synthesis in the near future. V8rik (talk) 18:10, 1 February 2011 (UTC)

Positional isomerism

I was taught that structural isomerism and positional isomerism are entirely different classes of isomerism; why is positional isomerism a section under structural isomerism? My understanding was this:

Structural isomerism: same molecular formula; different structural formula

Positional isomerism: same molecular formula; different positions of functional groups — Preceding unsigned comment added by Mbb1056 (talk • contribs) 18:43, 8 April 2014 (UTC)

Mbb1056; My idea about this is that all isomers (even stereoisomers) are, strictly speaking, structural (positional) isomers - otherwise, the same atoms, in the same arrangement (structure) would be identical molecules. lifeform (talk) 05:15, 29 May 2014 (UTC)

I think the structures of some of the isomers of xylenol are incorrect. Isn't 2,5 xylenol actually a picture of 3,5 xylenol? I came here to learn about isomers so either I didn't understand it or it's wrong. If someone with actual chemistry knowledge sees this, please reply. Nerdette42 (talk) 08:05, 27 July 2016 (UTC)

Methyl positions 2,5 vs 3,5 both equally describe the facts of the structure. The difference is just which direction (counterclockwise vs clockwise, respectively) you are counting around the ring from the OH as position 1. That's a difference of your perspective on the molecule, there's no intrinsic fact of the chemical itself. You might have gotten in the habit of always going a certain direction, and even other molecules in adjacent diagrams might go a certain direction, but you have to consider each molecule on its own merits. By IUPAC_nomenclature_of_organic_chemistry, we always go in the direction that gives the substituents at the lower numbers, so 2,5 is better than 3,5. DMacks (talk) 12:40, 27 July 2016 (UTC)

Infrared spectroscopy

Coming from an astronomy background the section on spectroscopy sounds intriguing but overstated. For example: "In fact, the infrared spectrum is divided into two regions. The first part is called the functional group region." ...may be correct for a specific field of study but seems like too broad of a statement to make about the infrared spectrum in general. I can't propose any improvements because I don't know much about molecular physics, can anyone else suggest a clarification? kanoa (talk) 09:59, 26 November 2016 (UTC)

Linear isomerism

What is the definition of a linear isomer?

See https://wiki.riteme.site/wiki/Phenanthrene#Canonical_forms

Simon de Danser (talk) 02:14, 7 August 2020 (UTC)

Why "like a textbook"?

@Christian75: Hi Christian, can you be more specific about why you think the article "reads like a textbook"? Thanks... --Jorge Stolfi (talk) 04:33, 29 August 2020 (UTC)

From WP:TEXTBOOK: "Wikipedia is an encyclopedic reference, not a textbook. The purpose of Wikipedia is to present facts, not to teach subject matter.". We present facts. This article reads like a textbook, e.g. "There are only three pentanols, not five or 12, because replacing each of the 12 hydrogens by hydroxyls yields many molecules with the same structure. " Christian75 (talk) 20:34, 29 August 2020 (UTC)

@Christian75:I see. However, the next sentence explains that a textbook is characterized by "leading questions and systematic problem solutions as examples." (emphasis mine). While the article was (and may still be, after my last reorganization) a bit "textbookish", it is still far from "reading like a textbook": there are no leading questions, the examples are not quite "problem solutions", and they are very far from covering the whole subject, as one would expect of a textbook.
And note that the authors of the MOS say that Wikipedia is an "encyclopedic reference", not a technical reference handbook (like the Rubber Bible or the Gold Book).
Again there is the issue of whether each article should be written for a specialist in the topic, of for a general reader who has the minimum knowledge needed to understand the parts of the contents that he may want to understand. To me, the second view is practically the definition of wikipedia, or of any encyclopedia. Articles on abstract but fundamental concepts like isomerism should definitely have examples -- and indeed most of them do, in chemistry and in other areas.
Besides, Wikipedia will always be useless to specialists, who generally have subscriptions to CAS and journals, $1000 handbooks, etc. Articles written for those readers will be wasted effort...
I hope we can find a text that we both are comfortable with. If you think that some examples or explanations are excessive, please do trim them.
All the best, --Jorge Stolfi (talk) 10:05, 30 August 2020 (UTC)

Heteroatoms and double bonds in skeletal isomerism

For the purposes of skeletal isomerism, are heteroatoms considered part of the skeleton? E.g., can one say that methylpropyl ether and diethyl are skeletal isomers? What about 1,3-dioxolane and 1,2-dioxolane? Or 1,3-disilacyclohexane and 1,4-disilacyclohexane?
And what about unsaturated compounds: are 1-butene and 2-butene skeletal isomers, or are they considered to have "the same skeleton"?
--Jorge Stolfi (talk) 02:50, 30 August 2020 (UTC)

"Geometrically impossible" structural isomers?

I tend to agree that the section "Counting isomers" does not make much sense,except perhaps as an illustration of the difficulty of doing that.
If it is to remain: should one count structural isomers that have the right valences, but are impossible for geometric reasons? Like, one of the isomers of C
₃H
₂O would be 1,3-epoxy allene; but that seems impossible because the C=C=C backbone is rigid and the C-O bonds would be stretched to much. (Right?)
On the other hand, could H–C⁻=C=C=O⁺–H or H–C⁻=C=C⁺(=O)–H be counted as isomers? --Jorge Stolfi (talk) 09:24, 30 August 2020 (UTC)