Bürgi-Dunitz trajectory

The Bürgi-Dunitz trajectory describes the preferred approach of a nucleophile toward a carbonyl carbon, usually at an obtuse angle to the carbon-oxygen bond, often near $107^{\circ }$. It is a geometrical trend, not a fixed angle obeyed by every reacting system.

An oblique approach balances favorable interaction with the carbonyl antibonding orbital against repulsion from occupied orbitals. Crystallographic observations of close contacts motivated this picture, linking static molecular structures with a possible path of nucleophilic addition. Steric constraints and the other substituents can modify the accessible trajectory.