All images in this site can be bought in an enlarged version. Other important structure made of carbon is graphite, which is also hexagonal. Other calculated results are also shown: the electron density distribution and also one of its vibrational modes. (these were calculated by Ricardo using the ADF - amsterdam density functional - software). If you don't remember what benzene looks like, it is shown at the bottom of the page, vibrating! Here the HOMO (highest occupied molecular orbital) and LUMO (lowest occupied molecular orbital) of benzene are pictured. The most important orbitals of a molecule are its frontier orbitals, because they govern its chemistry. Because the electrons are responsible for chemical bonding, it is very interesting to know their distributions. In summary it may be said, the HOMO/LUMO interactions in the side attack do not produce bonding interactions between the substrate and the nucleophile.They represent the spatial distribution of the electrons around the atoms forming a molecule. The interaction between HOMO and LUMO can only occur if the orbitals have the. Consequently, the overlapping of the nucleophile's HOMO with one of the small substrate's LUMO lobes causes a bonding interaction, while the overlapping with the other small LUMO lobe leads to an antibonding interaction. Let us consider an orbital diagram of two molecules which are in the. These electrons are found on the oxygen, and are equivalent to the lone pairs in the Lewis structure. In this case, that refers to the non-bonding electrons. When molecules contain pi bonding orbitals, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are typically pi. If the carbonyl is going to donate electrons, the electrons will come from the HOMO. These small lobes of the LUMO have opposite signs in the wavefunction, as the LUMO possesses a nodal plane between the carbon and the leaving goup. The LUMO in this case is the CO pi or pi antibonding orbital. In the side attack, one lobe of the nucleophile's HOMO overlaps with two small lobes of the substrates LUMO (σ*). In the side attack, the minor overlapping of the nucleophile's LUMO with the smaller orbital lobes of the substrate's LUMO (σ*), which are located between the carbon and the leaving group, leads to a weaker interaction than in the back-side attack. A side attack also has several disadvantages. Thus, the HOMO/HOMO interaction does not lead to a bonding interaction between the substrate and the nucleophile.įig.5 HOMO/LUMO interaction (n/σ*) in the side attack.ĭue to the steric hindrance between the nucleophile and the leaving group, a front-side attack that occurs right along the C-L bonding axis is particularly unfavourable. The system would then not be stabilized by this HOMO/HOMO interaction. If the nucleophile's n orbital interacts with the bonding, occupied σ orbital of the C-L bond, the new antibonding molecular orbital would also have to be occupied. Thus, the HOMO/LUMO interaction leads to a bonding interaction between the substrate and the nucleophile. If the symmetries do not match, then the HOMOLUMO overlap is symmetry forbidden and cycloaddition will not pro-ceed. Therefore, the system is stabilized by the HOMO/LUMO interaction. As only the two electrons of the nucleophile have to be distributed among the new molecular orbitals, the antibonding molecular orbital is not occupied. The bonding molecular orbital has a lower energy level than the initial n orbital of the nucleophile does. As a result, a new bonding, as well as a new antibonding molecular orbital are developed. In an S N 2 reaction, an occupied n orbital of the nucleophile ( HOMO = highest occupied moecular orbital lone electron pair) interacts with the unoccupied, antibonding σ* orbital of the substrate's C-L bond ( LUMO = lowest unoccupied molecular orbital).
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