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Question
Account for the following observations:
Though fluorine is more electronegative than chlorine yet BF3 is a weaker Lewis acid than BCl3
Solution
Undoubtedly, fluorine has more electronegativity than chlorine, BF3 is stronger Lewis acid than BCl3 because of n – pπ back bonding in BF3, both the constituting atoms boron and fluorine are involved p-orbital in back bonding. On moving down the group, the size of halogen atoms increases, back bonding decreases and Lewis acid character also decreases.
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