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

1. BCl3 > BF3 > BBr3
2. BBr3 > BCl3 > BF3
3. BBr3 > BF3 > BCl3
4. BF3 > BCl3 > BBr3

Correct answer: BBr3 > BCl3 > BF3

Solution

The Lewis acidity of boron halides depends on the ability of the halogen atoms to back-donate electron density to the boron atom. As the size of the halogen increases (F < Cl < Br), back-donation decreases, making BBr3 the strongest Lewis acid.

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