Wikidata:WikiProject Chemistry/Guidelines/Entities with undefined isomeric or isotopic features

Entities with undefined isomeric or isotopic features

See discussion about this topic (2018) in WikiProject Chemistry.

In the following rules structural need is defined as having at least one external identifier to a reliable database (InChI, InChIKey or SMILES are not regarded as such) or at least one valid sitelink to a page on a Wikimedia project (cf. point 1 of Wikidata:Notability).

Inclusion criteria

1.	Every chemical entity with fully defined isomerism (cis–trans isomerism; ortho, meta, para isomerism; enantiomerism; etc.) or isotopic composition can be described in a separate item (hereafter item A).	(S)-2-pentanol (Q20680358) describes a compound with fully defined stereochemistry (has one stereogenic centre and it is defined). Thus, it can be described in a separate item. 2-Bromobenzaldehyde (Q33859440) describes a compound with defined positions of two substitutents of the benzene ring (ortho position), so it can be described in a separate item.
2.	Chemical entity with undefined (item B) or partially defined (item C) isomerism or isotopic composition can be described in a separate item if it fulfils some structural need.	DL-methamphetamine (Q44909815) describes a compound with undefined stereochemistry (has one stereogenic centre and it is undefined). However, it has some external identifiers in reliable databases, like ChemSpider ID (P661) or DSSTox substance ID (P3117), so it can be described in a separate item. (2S)-homocystine (Q27161892) describes a compound with partially defined stereochemistry (has two stereogenic centres and only one is defined). It has some external identifiers like ChEBI ID (P683) or PubChem CID (P662), so it can be described in a separate item. bromobenzaldehyde (Q33859433) describes a compound with undefined position of two substitutents of the benzene ring (i.e. it is a group of three isomers: ortho, meta and para). It has an article in Wikipedia, so it can be described in a separate item.
3.	Item about a racemate (Q467717) (or more generally: about mixture of isomers) can be described in a separate item (item D) if it fulfils some structural need.	rac-nicotine (Q56697247) describes a racemic mixture of (−)-nicotine (Q28086552) and (+)-nicotine (Q27119762) and it is different from nicotine (Q12144) which describes a compound with undefined stereochemistry. It can be described in a separate item, because there is an external identifier (ChEBI ID (P683)).
4.	Chemical entity being an atropisomer (Q757764) can be described in a separate item if it can be isolated or if it fulfils some structural need.	(+)-gossypol (Q29652392) and (-)-gossypol (Q29652363) describe different atropisomers of gossypol (Q411882). These two entities can be described in a separate items, because they can be isolated and have unique external identifiers (MeSH concept ID (P6694)).

Relations between items

From the definitions above: item A is about a chemical entity with defined isomerism or isotopic composition, item B is about a chemical entity with undefined and item C about a chemical entity with partially defined isomerism or isotopic composition, item D is about a mixture of isomers (usually about a racemic mixture).

5.	Item A should always be classified using relation subclass of (P279) to item B or item C.	Both (R)-2-pentanol (Q24953060) and (S)-2-pentanol (Q20680358) are items A, 2-pentanol (Q210479) is an item B; thus, (R)-2-pentanol (Q24953060)subclass of (P279)2-pentanol (Q210479) and (S)-2-pentanol (Q20680358)subclass of (P279)2-pentanol (Q210479).
6.	Item B and item C should always be classified using relation instance of (P31) to a metaclass group of chemical entities (Q56256086) (or one of its subclasses like group of isomeric entities (Q15711994)).	2-pentanol (Q210479) is an item B, so 2-pentanol (Q210479)instance of (P31)group of stereoisomers (Q55662747). bromobenzaldehyde (Q33859433) is an item B, so bromobenzaldehyde (Q33859433)instance of (P31)group of ortho, meta, para isomers (Q55662456) (2S)-homocystine (Q27161892) is an item C, so (2S)-homocystine (Q27161892)instance of (P31)group of stereoisomers (Q59199015).
7.	If an item B and an item C exist, the latter should be a subclass of the former (subclass of (P279)).	DL-homocystine (Q58879461) is an item B, (2R)-homocystine (Q58879587) and (2S)-homocystine (Q27161892) are items C; thus (2R)-homocystine (Q58879587)subclass of (P279)DL-homocystine (Q58879461) and (2S)-homocystine (Q27161892)subclass of (P279)DL-homocystine (Q58879461).
8.	Item A with modified isotopic composition should have a relation subclass of (P279) to isotopic compound (Q22332141) or its subclass (unless it has a relation subclass of (P279) to item B or item C that has a relation subclass of (P279) to isotopic compound (Q22332141) or its subclass) and isotopically modified form of (P9977) to its isotopic unmodified form.	rhenium Re-186 sulfide (Q27272832) is an item A, thus rhenium Re-186 sulfide (Q27272832)subclass of (P279)isotopic compound (Q22332141) and rhenium Re-186 sulfide (Q27272832)isotopically modified form of (P9977)rhenium(VII) sulfide (Q4445877)
9.	Items A that are stereoisomers of each other should be linked using stereoisomer of (P3364) property.	(R)-2-pentanol (Q24953060) and (S)-2-pentanol (Q20680358) are enantiomers, so (R)-2-pentanol (Q24953060)stereoisomer of (P3364)(S)-2-pentanol (Q20680358) and (S)-2-pentanol (Q20680358)stereoisomer of (P3364)(R)-2-pentanol (Q24953060).
10.	Item D should be linked to its constituents (items A) using part of (P361) and has part(s) (P527) properties. Item D should be usually linked to item B or item C using different from (P1889) property.	rac-nicotine (Q56697247) is item D, (−)-nicotine (Q28086552) and (+)-nicotine (Q27119762) are items A, nicotine (Q12144) is item B; thus, rac-nicotine (Q56697247)has part(s) (P527)(−)-nicotine (Q28086552) and rac-nicotine (Q56697247)has part(s) (P527)(+)-nicotine (Q27119762), (−)-nicotine (Q28086552)part of (P361)rac-nicotine (Q56697247) and (+)-nicotine (Q27119762)part of (P361)rac-nicotine (Q56697247), rac-nicotine (Q56697247)different from (P1889)nicotine (Q12144) and nicotine (Q12144)different from (P1889)rac-nicotine (Q56697247).

Examples

Examples in this section are graphical versions of the examples given above and contains all statements that are required by the above rules.

Example 1: 2-pentanol (two enantiomers)

Following example has three elements: 2-pentanol (Q210479) is an item B (compound with undefined stereochemistry) and two enantiomers (items A): (R)-2-pentanol (Q24953060) and (S)-2-pentanol (Q20680358), both has defined stereochemistry.

Example 2: nicotine (racemic mixture)

Following example has four elements: nicotine (Q12144) is an item B (compound with undefined stereochemistry), (−)-nicotine (Q28086552) and (+)-nicotine (Q27119762) are items B (have defined stereochemistry), rac-nicotine (Q56697247) is a racemic mixture.

Example 3: homocystine (partially defined stereochemistry)

Following example has six elements: DL-homocystine (Q58879461) is an item B (compound with undefined stereochemistry), (2S)-homocystine (Q27161892) and (2R)-homocystine (Q58879587) are items C (in both compounds only one of two stereogenic centre is defined), (2R,2′R)-homocystine (Q58879658), (2S,2′S)-homocystine (Q24188398) and (2R,2′S)-homocystine (Q58879684) are items A (compounds with both stereogenic centres defined).

Example 4: bromobenzaldehyde (ortho, meta, para isomers)

Following example has four items: bromobenzaldehyde (Q33859433) is an item B (has undefined ortho, meta, para isomerism, i.e. exact positions of substitutents of the benzene ring are not defined), 2-Bromobenzaldehyde (Q33859440), 3-bromobenzaldehyde (Q926392) and 4-bromobenzaldehyde (Q27259983) are items A (defined isomers).