The Lewis acid character of boron trihalides follows the order:

Question

Canvas Classes · Accepted Answer

Lewis acidity of boron trihalides depends on the extent of back donation (p–p):

- Greater back donation → more electron density on B → weaker Lewis acid
- : Strongest back donation (F 2p ↔ B 2p, best size match) → weakest Lewis acid
- : Less back donation (Cl 3p ↔ B 2p, poor overlap)
- : Even less back donation
- : Least back donation (I 5p ↔ B 2p, very poor overlap) → strongest Lewis acid

Order:

Answer: Option (2)

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