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

Hint: Lewis Acids are the chemical entities that have empty orbitals, and can accept pairs of electrons from Lewis bases. This concept has been used conventionally to describe chemical species with a trigonal planar structure and an empty p-orbital.

Complete step-by-step answer:
For $B{F_3}$, boron has a 2p-orbital vacancy and each fluorine has unused 2p-orbitals completely filled. Fluorine passes two electrons to a 2p-orbital vacancy of boron, thereby forming a $p\pi - p\pi$ bond (dative or back bonding) between the two. This bond decreases the boron atom's electron deficiency, and its Lewis acid character diminishes. The bonding tendency in $B{F_3}$is optimum and decreases from $B{F_3}$to $B{I_3}$. Thus $B{I_3}$behaves as a Lewis acid which is strongest.
Therefore, option D is the right answer.

Note: Boron triiodide $B{I_3}$ is a radioactive boron and iodine compound with chemical formula BI3. This has a cellular, trigonal planar geometry. This is a crystalline solid containing hydroiodic acid and boric acid, which reacts vigorously with water. Its dielectric constant is 5.38, and the vaporizing heat is 40.5 kJ / mol.
The inorganic compound containing formula $B{F_3}$ is boron trifluoride. This colorless, pungent poisonous gas absorbs white gases in moist air. This is a useful Lewis acid for other boron compounds and a flexible building block.
The inorganic compound containing formula $BC{l_3}$ is boron trichloride. The colorless gas is an organic synthesis reagent. It is very reactive towards water.
Boron tribromide, $BB{r_3}$, is a liquid compound that contains boron and bromine in colourless fumes. Commercial samples are typically red / brown to black, due to poor bromine contamination. This is decomposed by alcohol and water.