The order of acidic strength of boron trihalides is

Step-by-step solution

Boron trihalides are Lewis acids because the boron atom is electron-deficient, having only six electrons in its valence shell. The acidic strength depends on the extent of back-bonding between the lone pair of the halogen atom and the empty 2p orbital of the boron atom. The back-bonding involves the overlap of the 2p orbital of boron with the np orbital of the halogen. The extent of this overlap is maximum for BF ₃ because both boron and fluorine have 2p orbitals, leading to effective p -p back-bonding. This reduce