Question

BBr3 is a strong lewis acid than Bf3.why?

Answer 1

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Here is your answer

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.

Answer 2

Explanation:

According to lewis, acids are the species that are able to accept electrons.

For example, $BBr_{3}$ is a lewis acid.

Whereas in case of $BF_{3}$, boron undergoes back bonding with fluorine atom. As a result, boron gain electrons from fluorine atom because both boron and fluorine are similar in size.

On the other hand, boron and bromine have different atomic size. Hence, they will not undergo back bonding.

Thus, boron in $BBr_{3}$ will be available to gain electrons thus, it acts as a lewis acid.