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.