1) Acidic strength of boron trihalideare in order of
(a) BF3 < BCl3 < BBr3 < BI3 (b) BI3 < BBr3 < BCl3 < BF3
(c) BBr3 < BCl3 < BF3 < BI3 (d) BF3 < BI3 < BCl3 < BBr3
(a) Due to effective pπ – pπ back bonding between B & F in BF3, it becomes the weakest Lewis acid. In the case of other halides, the less effective pπ–dπback bonding further decreases as the size of halogen atom increases. This makes the vacant orbital of B more and more available for receiving the lone pair of electrons.