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

Question

Statement-I: An element ‘X’ of P-block forms a hydride H–X, which has the longest bond length, then element ‘X’ will have the shortest covalent radius.
Statement-II: An element ‘E’ of Group 15 forms hydride EH$_3$, that has least B.P. The maximum covalency of E is 4.

Answer 1

To determine the order of Lewis acid character in boron trihalides, we need to understand the concept of Lewis acids. A Lewis acid is a compound that can accept an electron pair. In the context of boron trihalides, the central boron atom is electron-deficient and can accept electron pairs from donor molecules.

The Lewis acidity of boron trihalides ($ BX_3 $, where $ X $ is a halogen) is significantly influenced by the electronegativity and the size of the halogen atom:

The electronegativity of the halogen affects the electron-withdrawing ability of the halogen. Higher electronegativity means stronger electron withdrawal, reducing the electron deficiency of boron, and thus decreasing Lewis acidity.
The size of the halogen determines the ability of the halogen atoms to back-donate electron density to the empty p-orbital of boron. Larger halogen atoms are less effective at back donation due to poorer overlap with boron's orbitals.

Let's consider each boron trihalide:

$ \text{BF}_3 $: Fluorine is highly electronegative, which results in strong electron withdrawal from boron. However, due to extensive back-donation from the fluorine atoms, $ \text{BF}_3 $ is not as strong a Lewis acid as expected.
$ \text{BCl}_3 $: Chlorine is less electronegative compared to fluorine, with less effective back-donation, making $ \text{BCl}_3 $ a stronger Lewis acid than $ \text{BF}_3 $.
$ \text{BBr}_3 $: Bromine is less electronegative and larger than chlorine, leading to poorer back-donation, making $ \text{BBr}_3 $ a stronger Lewis acid than both $ \text{BCl}_3 $ and $ \text{BF}_3 $.
$ \text{BI}_3 $: Iodine is the least electronegative and the largest atom, resulting in the weakest back-donation. Hence, $ \text{BI}_3 $ is the strongest Lewis acid among the boron trihalides listed.

Based on the above explanations, the order of Lewis acid character of boron trihalides is: $BI_3 > BBr_3 > BCl_3 > BF_3$.

Therefore, the correct answer is:

$BI _3> BBr _3> BCl _3> BF _3$

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

Show Hint

The Correct Option is B

Solution and Explanation

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