JEE Chemistry 2023 Question with Solution

Given: The Lewis acid character of boron trihalides is to be compared.

Find: The correct order of Lewis acidity among $BF_3$, $BCl_3$, $BBr_3$ and $BI_3$.

The extent of back bonding decreases down the group, so Lewis acidic strength increases down the group.

(extent of back bonding)

Hence the Lewis acidic nature follows the reverse order:

Therefore, the correct order is $BI_3 > BBr_3 > BCl_3 > BF_3$. The solution states that the correct option is B, but this does not match the listed options. The defensible matching option is D.

Given: Lewis acidity of boron trihalides depends strongly on back bonding.

Find: Which boron trihalide acts as the strongest Lewis acid.

In $BF_3$, effective $2p-2p$ back bonding occurs between fluorine and boron. This reduces the electron deficiency of boron and makes $BF_3$ the weakest Lewis acid among these.

As we move from $F$ to $Cl$ to $Br$ to $I$, overlap with boron becomes poorer, so back bonding decreases.

Therefore electron deficiency on boron increases in the order:

So the Lewis acid character follows:

Thus the option text that matches the extracted reasoning is D.