Question:
The order of acidity of boron trihalides is:
The order of acidity of boron trihalides is:
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Think about pi back bonding between the halogen lone pair and boron's empty 2p orbital, not just electronegativity.
Updated On: Jul 3, 2026
- BF3 > BCl3 > BBr3
- BBr3 > BCl3 > BF3
- BF3 > BBr3 > BCl3
- BBr3 > BF3 > BCl3
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The Correct Option is B
Solution and Explanation
Step 1: Boron in BX3 has an empty 2p orbital, making it a Lewis acid.
Step 2: Halogens donate a lone pair via pi back bonding into this empty orbital, reducing Lewis acidity.
Step 3: Fluorine's 2p orbital matches boron's 2p size best, giving strongest back bonding and weakest acidity in BF3.
Step 4: Larger halogens (Cl, Br) overlap progressively worse, weakening back bonding.
Step 5: So acidity increases BF3 < BCl3 < BBr3.
\[\boxed{\text{BBr}_3 > \text{BCl}_3 > \text{BF}_3}\]Was this answer helpful?
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