Question

Which is the strongest Lewis acid?
A. $B{F_3}$
B. $BC{l_3}$
C. $BB{r_3}$
D. $B{I_3}$

Answer 1

We can define Lewis acids are chemical entities, which have empty orbitals, and could accept pairs of electrons from Lewis Bases. We can use this concept to explain chemical species with trigonal planar structure and an empty p-orbital.

Complete step by step answer:
In case of $B{F_3}$, boron contains 2p-vacant orbital and each fluorine atom contains unused 2p-orbitals that are completely filled. Fluorine transfers two electrons to the 2p-vacant orbital of boron. This leads to the formation of $p\pi - p\pi$ bond. We can call $p\pi - p\pi$ bond as back bonding (or) dative bond. The $p\pi - p\pi$ bond reduces the electron deficiency of boron, and their Lewis acid character weakens.
In $B{F_3}$, the bonding tendency is optimum, and it decreases from $B{F_3}$ to $B{I_3}$. Therefore, $B{I_3}$ acts as the strongest Lewis acid.

$\therefore$ Option (D) is correct.

Note:
We need to know that Boron triiodide is a radioactive boron and iodine compound that has a chemical formula of $B{I_3}$. The structure of boron triiodide is cellular with trigonal planar geometry. The dielectric constant of boron tri-iodide is 5.38 and it has vaporizing heat of $40.5\,kJ/mol$. It is crystalline solid.
The compound with chemical formula $B{F_3}$ is boron trifluoride. It is colorless and poisonous gas with pungent odor. It absorbs white gases in moist air.
The compound with chemical formula $BC{l_3}$ is boron trichloride. Boron trichloride is a colorless gas and is used as a reagent in organic synthesis. It is highly reactive with water.
The compound with chemical formula $BB{r_3}$ is boron tribromide. The compound is liquid, and it contains boron and bromine. Boron tribromide is decomposed by alcohol and water.