Question

Why BBr3 is stronger Lewis acid as compared to BF3 through F is more electronegativity than Br?

Answer 1

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Boron compounds are electron deficient because of vacant 2p orbital. It has an ability to receive electrons from electron rich compounds hence acts as Lewis acids. Strength of the Lewis acid property depends on how vacant that P orbital Boron is.

In BF3 due to effective overlapping of 2p orbital of boron and 2p filled orbital of fluorine via synergistic effect. Hence vacant p orbital became less electron deficient. Its Lewis acid strength decreases.

In BBr3 unfavourable overlapping between Vacant 2p orbital and filled 4p orbital makes Boron to retain its electron deficient property. Hence it is stronger Lewis acid.

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