Term: in-out isomerism
https://doi.org/10.1351/goldbook.I03055

Definition:
isomerism found in bicyclic systems having long enough bridges to allow the bridgehead exocyclic bond or lone pair of electrons to point either inside the structure or outside. I03055-1.png I03055-2.png

Related Terms:
1) isomerism (http://doi.org/10.1351/goldbook.I03294).
2) bridgehead (http://doi.org/10.1351/goldbook.B00736).

Images:
1) Not defined (https://goldbook.iupac.org/img/inline/I03055-1.png) 
2) Not defined (https://goldbook.iupac.org/img/inline/I03055-2.png) 

Source: PAC, 1996, 68, 2193. 'Basic terminology of stereochemistry (IUPAC Recommendations 1996)' on page 2210 (https://doi.org/10.1351/pac199668122193)

Citation: 'in-out isomerism' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.I03055

License: The IUPAC Gold Book is licensed under Creative Commons Attribution-ShareAlike CC BY-SA 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) for individual terms.

Disclaimer: The International Union of Pure and Applied Chemistry (IUPAC) is continuously reviewing and, where needed, updating terms in the Compendium of Chemical Terminology (the IUPAC Gold Book). Users of these terms are encouraged to include the version of a term with its use and to check regularly for updates to term definitions that you are using.