core atom

in organic reaction mechanisms
https://doi.org/10.1351/goldbook.C01341
The primary @[email protected] in a mechanistic change; usually either of the two atoms of a transformed multiple bond or the single atom at which substitution occurs.
Source:
PAC, 1989, 61, 23. (System for symbolic representation of reaction mechanisms (Recommendations 1988)) on page 55 [Terms] [Paper]