Bürgi–Dunitz angle

The Bürgi- Dunitz trajectory ( also Bürgi- Dunitz angle; named after Hans Beat Bürgi and Jack D. Dunitz ) describes the angle of attack of a nucleophile on an electrophilic carbonyl group, this is about 107 °. The π - bond between carbon and oxygen is simultaneously achieved, while instead forms a σ - bond between the carbon and the nucleophile.

According to the molecular orbital theory there must be an overlap of the highest occupied molecular orbital ( Highest Occupied Molecular Orbital ) HOMO of the nucleophile, often a lone pair of electrons to the lowest unoccupied molecular orbital ( lowest unoccupied molecular orbital) LUMO of the electrophile, in this case, the π * bond of the carbonyl ( the antibonding orbital of the carbon double bond), take place. The nucleophile attacks linear, that is, the angle of attack depends on the electrophile. The double bond of the carbonyl group is composed of a σ - bond and a higher-energy π - bond together (see picture ), here there is only the π - bond important. The higher the energy level of a binding molecular orbital, the lower the energy level of the corresponding anti -binding molecular orbital, so the attack on the π - bonding occurs, the antibonding π * is thus the LUMO.

Because oxygen is more electronegative than carbon, π - orbital be binding is greater than that of carbon. For symmetry reasons, it behaves exactly reversed in the anti- bonding orbitals. Thus, the π * orbital has ( LUMO ) on the carbon the largest coefficient, the attack of the HOMO of the nucleophile takes place there.

The p orbitals that form the π * bond, perpendicular to the plane of the C = O function. The optimum angle of attack for the nucleophile would therefore be 90 ° to the carbonyl plane, because its LUMO and the HOMO of the nucleophile would be optimal overlap. However, since the oxygen has two lone pairs of electrons, and a significantly higher electronegativity than carbon, oxygen, the electron density is at the largest, that bearing a lone pair nucleophile is therefore rejected by the oxygen.

The angle of 107 ° and the Bürgi Dunitz trajectory is therefore a compromise between the optimum angle for the possible overlap of HOMO and LUMO (90 °) and the repulsive Coulomb force between the high electron density on the oxygen and the HOMO of the nucleophile.