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Question

The tendency of BF3,BCl3 and BBr3 to behave as Lewis acid decreases in the sequence

A
BF3>BCl3>BBr3
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B
BCl3>BF3>BBr3
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C
BBr3>BCl3>BF3
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D
BBr3>BF3>BCl3
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Solution

The correct option is C BBr3>BCl3>BF3
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 BF3, 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 pπpπ 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
BBr3>BCl3>BF3

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