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

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As the size of halogen atom increases, the acidic strength of boron halides increases. Thus, BF₃ is the weakest Lewis acid. this is because of the pπ - pπ back bonding between the fully filled unutilised 2p orbitals of F and vacant 2p orbitals of boron which makes BF₃ less electron deficient. Such back donation is not possible is case of BCl₃ or BBr₃ due to larger energy difference between their orbitals. Thus, these are more electron deficient. Since on moving down the group the energy difference increases, the Lewis acid character also increases. Thus, the tendency to behave as Lewis acid follows the order
BBr₃ > BCl₃ > BF₃

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