Question

The order of stability of boron trihalide is

• A. BF$_3$ < BCl$_3$ < BBr$_3$ < BI$_3$
• B. BI$_3$ < BBr$_3$ < BCl$_3$ < BF$_3$
• C. BCl$_3$ < BBr$_3$ < BI$_3$ < BF$_3$
• D. BBr$_3$ < BCl$_3$ < BF$_3$ < BI$_3$

Answer 1

Answer: (B)

BI$_3$ < BBr$_3$ < BCl$_3$ < BF$_3$

Answer 2

The stability of boron trihalides ($BX_3$) is primarily determined by the extent of pπ-pπ backbonding from the halogen atom (X) to the boron atom (B).

1. Backbonding: Boron in $BX_3$ compounds is sp$^2$ hybridized and has a vacant 2p orbital. Halogen atoms have lone pairs of electrons in their p-orbitals. These lone pairs can be donated into the vacant 2p orbital of boron, forming a partial π-bond. This phenomenon is called backbonding.

2. Effectiveness of Backbonding: The effectiveness of backbonding depends on the size and energy match between the orbitals involved.

   • In $BF_3$, the overlap is between B(2p) and F(2p) orbitals. This 2p-2p overlap is very effective due to the similar size and energy of the orbitals. This strong backbonding significantly reduces the electron deficiency of boron.
   • As we move down the group from fluorine to iodine, the size of the halogen's p-orbital increases (Cl: 3p, Br: 4p, I: 5p). The overlap between boron's 2p orbital and the halogen's np orbital becomes progressively less effective due to increasing size and energy mismatch.
   • Therefore, the extent of backbonding decreases in the order: $BF_3 > BCl_3 > BBr_3 > BI_3$.

3. Stability and Backbonding: Stronger backbonding leads to greater delocalization of electron density and a more stable molecule, as it reduces the electron deficiency of boron and imparts partial double bond character to the B-X bond.

   • Since $BF_3$ has the most effective backbonding, it is the most stable among the boron trihalides.
   • $BI_3$ has the least effective backbonding, making it the least stable.

4. Order of Stability: Based on the extent of backbonding, the order of stability is:

$BF_3 > BCl_3 > BBr_3 > BI_3$

In increasing order of stability, this is:

$BI_3 < BBr_3 < BCl_3 < BF_3$