Question

The decreasing order of power of boron halides to act as lewis acids is.

• A. $BF_{3} > BCl_{3} > BBr_{3}$
• B. $BBr_{3} > BCl_{3} > BF_{3}$
• C. $BCl_{3} > BF_{3} > BBr_{3}$
• D. $BCl_{3} > BBr_{3} > BF_{3}$

Answer 1

Answer: (B)

$BBr_{3} > BCl_{3} > BF_{3}$

Answer 2

: This order is observed due to relative tendency of the halogen atom to back donate its electrons to vacant p- orbital of boron atom. In $BF_{3}$, fluorine transfers two electrons to vacant 2p – orbital of boron forming $p\pi - p\pi$ bonds. This reduces the electron deficiency of boron The tendency to form back bonding decreases from $BF_{3}$ to $BI_{3}$