Question #1628a

Because the bond between B and I is weaker, that means `BI_3` is easier to dissociate than `BBr_3`. Because it is easier to dissociate, `BI_3` is a stronger acid.

The boron halides are Lewis acids. Their acid strength depends on their ability to accept a pair of electrons from a Lewis base.

Thus, we might expect `"BF"_3`, with its highly electronegative `"F"` atoms, to be the strongest acid of the boron halides.

However, it is the weakest Lewis acid of the group.

It used to be thought that the reason involved overlap between the vacant `"2p"` orbital of `"B"` and the filled `"p"` orbital of the halogen (π bonding).

Recent research suggests that σ bonding, not π bonding, is the important factor.

In `"BF"_3`, the `"B—F"` bond distance is short.

The `"sp"^2` orbitals of `"B"` and the `"2p"` orbitals of `"F"` are about the same size.

Thus, there is good `"sp"^2"-2p"` overlap (σ bonding) between the `"B"` and `"F"` atoms.

This increases the electron density on the `"B"` atom and decreases its Lewis acidity.

In `"BI"_3`, the `"B—I"` bond length is longer and the `"I"` atom is using a much larger `"5p"` orbital to form the σ bond.

Thus, the `"sp"^2"-5p"` orbital overlap is quite small.

Since the σ electrons around `"B"` are much further away, the electron density is greatly reduced.

`"BI"_3` is therefore the strongest Lewis acid.

The order of acidity is

`"BI"_3 >"BBr"_3 > "BCl"_3 > "BF"_3`