Question

Which of the following are Lewis acids?
(A)- $PC{l_3}$
(B)- $AlC{l_3}$
(C)- $NC{l_3}$
(D)- $B{F_3}$

Answer 1

Hint: To answer this question, we first need to understand the concept of Lewis acids and bases. Lewis' theory used electrons instead of proton transfer and it specifically states that an acid is a species that accepts an electron pair similarly a base donates an electron pair.

Complete step by step answer:
In general, Lewis acids accept an electron pair.
Lewis acids are electron deficient species. This is because of the positive charge present on them. We all know that a charged species is always an unstable species. So, to gain stability they accept an electron pair and hence, are called lewis acids.

Sometimes some neutral molecules are also called as lewis acids. This is because they have insufficient number of electrons in their valence shell and have a tendency to accept electron pairs. This is because of the presence of vacant orbitals in the valence shell.

This means Lewis Acids are electrophilic in nature, meaning that they attract electrons. Thus, all cations are Lewis acids since they are able to accept electrons.
We see that the reaction of a Lewis acid and a Lewis base, leads to the production of a coordinate covalent bond. A coordinate covalent bond is just a type of covalent bond in which one reactant donates its electron pair to another reactant. Here in this case the lewis base donates its electrons to the Lewis acid forming an additional product.
In the options provided to us $B{F_3}$ and $AlC{l_3}$ have vacant orbitals and are electron deficient, hence they can accept electrons and are Lewis acids while the other two donates electrons and are Lewis bases. So, the correct answer is “Option B and D”.

Note: We must note that Lewis acids have various application in the field of chemistry Lewis acids play an essential role as a catalyst in the Friedel-Crafts reaction where aluminium chloride accepts a lone pair of electrons belonging to the chloride ion leading to the formation of aluminium tetrachloride ion in the Friedel-Crafts alkylation process.