Wikipedia:ChemSpider

ChemSpider is a service of the Royal Society of Chemistry. It is a database of data about chemicals. It generates a unique identifier for each molecule, and these are used in Wikidata and in Wikipedia's {{Chembox}} template.

Obtaining an identifier value

The ChemSpider URL for, say, Aesculin is:

http://www.chemspider.com/Chemical-Structure.4444765.html

This may appear in the browser as, for example:

http://www.chemspider.com/Chemical-Structure.4444765.html?rid=4bdb2e1a-ff9a-484e-8a91-4b7f87a39b3c

in both cases, the ChemSpider ID is 4444765

and that is the value which should be entered in Wikidata, and in the Chembox parameter |ChemSpiderID= (or one of:

in its {{Chembox Identifiers}} module sub-template).

The following Wikidata items are relevant:

Property PP661 is used for ChemSpider IDs (it may have multiple values)