Question #1628a

2 Answers
Jul 30, 2017

Answer:

The B-I bond is weaker than the B-Br bond.

Explanation:

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.

Oct 7, 2017

Answer:

#"BI"_3# has a longer #"B—X"# σ bond.

Explanation:

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

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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#