Question

Why $BBr_{3}$ is a stronger Lewis acid than $BF_{3}$?

Answer 1

The order of acidic quality can be effectively clarified based on the inclination of the halogen molecule to back - give it's solitary - sets of electrons to the boron - particle through π(pi)- π(pi) holding.  

As the size of halogen increases from F to I , the degree of cover between 2p-orbital of B and a greater p-orbital of halogen decreases and subsequently the electron - insufficiency of B increases and hence Lewis acid  character increases from  $BF_3$ to $BI_3$. Because of π(pi)- π(pi) back gift resonance , the electron-lack of B decline and in this manner $BF_3$ is the weakest Lewis acid.

Answer 2

The order of acidic quality can be effectively clarified based on the inclination of the halogen molecule to back - give it's solitary - sets of electrons to the boron - particle through π(pi)- π(pi) holding.

As the size of halogen increases from F to I , the degree of cover between 2p-orbital of B and a greater p-orbital of halogen decreases and subsequently the electron - insufficiency of B increases and hence Lewis acid character increases from BF_3BF

3

to BI_3BI

3

. Because of π(pi)- π(pi) back gift resonance , the electron-lack of B decline and in this manner BF_3BF

3

is the weakest Lewis acid.