Gauche Effect & SN2 Reactions: Sigma-Sigma* Orbital Interactions

[Titan97]

Recently, I read two chapters from March's advanced organic chemistry. I came across gauche effect and SN2 reactions. In both phenomena, $\sigma-\sigma^*$ orbital interactions is involved. In gauche effect present in 1,2-diflouroethane, the C-F bonding orbital becomes an antibonding orbital so that C-H $\sigma$ orbital can overlap with the empty C-F $\sigma$ orbital.

In SN2 mechanism, C-X bonding becomes antibonding orbital so that the nucleophile can attack the empty $\sigma$ orbital of Carbon

Am I correct? Is there any flaw in my understanding?

 

[DrDu]

No, it is just that electron density from a near by orbital (e.g. the filled orbital on Nu) is brought into the anti-bonding orbital (C-X). This compensates the bonding by the filled C-X sigma orbital. It is not so that the latter one is emptied somehow.

 

[Titan97]

C-X bond has a bonding and an antibonding orbital. Initially 2 electrons are filled in BO. That is, if the reactant was CH₃Cl, then a +sp³ orbital of C and a p-orbital of Cl would overlap. This forms BMO. But how can C-Nu bond form when C-Cl ABMO is filled?

Also, how can BMO and ABMO be formed simultaneously? Its given everywhere that 2 orbitals overlap to give two molecular orbitals. But each MO requires a different configuration of orbital.

 

[DrDu]

O-chem books are not a good place to learn about MO-theory.