Past exam of the chemistry course of the year 3 of the natural sciences course of the University of Cambridge / 2022 / Paper 4 / 67 / d

Bridgehead substitution proceeds by ionization and solvent capture; backside displacement at the adamantyl bridgehead is geometrically inaccessible. The rigid cage prevents the labeled bonds from sampling all the favorable alignments available in cyclopentyl solvolysis.

The two labeled bonds in the adjacent methylene group are poorly oriented for ordinary C-H-to-empty-orbital hyperconjugation, explaining an effect close to $1.000$. Cage C-C sigma donation and geometrical relaxation can still stabilize the developing adamantyl cation. The three remote bridgehead labels perturb that coupled framework and give the smaller normal effect $1.073$, or about $1.024$ per label if contributions are equivalent.

This is a geometrical and remote secondary isotope effect. It does not imply cleavage of the remote C-H bonds, and absence of a beta effect at one selected site does not establish absence of a cation.