Acid Halide

From WikidChem

When looking at the Acid Halide functional group, we notice three important features. We notice the C=O double bond, we see the two lone pairs on the Oxygen atom, and we notice three lone pairs on the Halide element. It then becomes important to identify which parts of the group would be most reactive, more specifically which bonds would be easiest to break or rearrange. The bonds of importance, then, are the C=O and the C-X bond. The C=O bond is obviously much stronger than the C-X bond, so the C-X bond is the bond of importance. When we look at the C-X bond, we notice that the C atom is using an sp2 hybrid orbital to bond and the X atom is using a p orbital. The p orbital on the X atom is roughly the same in energy as the sp2 hybrid orbital that the C is using, so when we draw the energy diagram, we see that the sigma* is an unusually low LUMO, which is the reactive part of the functional group. The low LUMO is important because the base can attack from the backside of carbon and cause the C-F bond to break. Therefore, as far as reactivity goes, an electrophile, such as a good base, would attack the Halide and replace it on the group.

Note that the X atom bonds with a p orbital so as to put more s character into its own AO, thereby stabilizing its own structure. C simply uses all of its orbitals for bonding so it uses a sp2 hybrid orbital.