The tendency of $B F_{3}, B C l_{3}$ and $B B r_{3}$ to behave as Lewis acid decreases in the sequence

B F_{3} > B C l_{3} > B B r_{3}

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B C l_{3} > B F_{3} > B B r_{3}

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B B r_{3} > B C l_{3} > B F_{3}

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B B r_{3} > B F_{3} > B C l_{3}

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Solution

The correct option is C $B B r_{3} > B C l_{3} > B F_{3}$
A molecule is said to be a Lewis acid if it can accept a lone pair.

This anomaly is explained on the basis of the relative tendency of the halogen atom to back donate its unutilised electrons to vacant p-orbital of boron atom. In $B F_{3}$ , boron has a vacant 2p-orbital and each flourine has fully filled unutilised 2p-orbitals. Fluorine transfers two electrons to vacant 2p-orbital of boron, thus forming $p π - p π$ bond.

The tendency to back donate decreases from $F$ to $I$ as energy level difference between $B$ and halogen atom increases from $F$ to $I$ .

So,the order of Lewis acidity is
$B B r_{3} > B C l_{3} > B F_{3}$

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