Question

The Lewis acid character of boron tri halides follows the order :

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

Hint:

The strength of Lewis acids can be compared based on the extent of back bonding and the electronegativity of halides.

Answer 1

The Correct Option is B

Extent of back bonding, reduces down the group leading to more Lewis acidic strength
$\underset{(2p-2p)}{B{F}_3}>\underset{(2p-3p)}{BC{l}_3}>\underset{(2p-4p)}{BB{r}_3}>\underset{(2p-5p)}{B{I}_3}\text{(extent of back bonding)}$
$B{F}_3<BC{l}_3<BB{r}_3<B{I}_3\text{(lewis acidic nature)}$
So, the correct option is (B).

Answer 2

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

For example, \( \text{BF}_3 \) has the most extent of back bonding due to the \( 2p - 2p \) interaction, whereas \( \text{BCl}_3 \) and \( \text{BBr}_3 \) show less back bonding. 
Therefore, the Lewis acid strength increases from \( \text{BF}_3 \) to \( \text{BCl}_3 \), \( \text{BBr}_3 \), and finally \( \text{B}_1 \).

Thus, the correct order is \( \text{B}_1 > \text{BBr}_3 > \text{BCl}_3 > \text{BF}_3 \).