Question

The Lewis acid character of boron tri halides follows the order 

(1) BBr₃ > BI₃ > BCI₃ > BF₃

(2) BCl₃ > BF₃ > BBr₃ >BI₃

(3) BF₃ > BCl₃ > BBr₃ > BI₃

(4) BI₃ > BBr₃ > BCl₃ > BF₃

Answer 1

(4) BI₃ > BBr₃ > BCl₃ > BF₃

Extent of back bonding, reduces down the group leading to more Lewis acidic strength

BI₃ > BBr₃ > BCl₃ > BF₃  (extent of back bonding)

(2p 2p) (2p 3p) (2p 4p) (2p 5p)

∴ Lewis acid character of boron halides is  BI₃ > BBr₃ > BCl₃ > BF₃