QuestionAcids and bases / Lewis acidity
Which one of the following is the correct order of Lewis Acidity in Boron halides?
- (a) BF₃ < BI₃ < BBr₃ < BCl₃
- (b) BI₃ < BF₃ < BCl₃ < BBr₃
- (c) BF₃ < BCl₃ < BBr₃ < BI₃
- (d) BBr₃ < BI₃ < BF₃ < BCl₃
Answer: (c) BF₃ < BCl₃ < BBr₃ < BI₃
Expected from electronegativity: BF₃ strongest, but actual order is BF₃ < BCl₃ < BBr₃ < BI₃ → (c). Reason: strong pπ–pπ back-bonding from F to empty B 2p is lost when BF₃ becomes pyramidal on accepting a lone pair; overlap weakens with larger halogens.